Routing Principles - Angelfire

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BSCI Cram session


1

of
22

Keith Huggins

BSCI 640
-
901

CRAMSESSION

Routing Principles

Links for Further Reading



http://www.scit.wlv.ac.uk/~jphb/comms/iproute.html




http://www.nwnetsmart.com/ns/books/ciscopress/

samples/1587200015.pdf

Routing Definition

Routing consists of moving information across a network from a source to destination. There are intermediate steps along the

way that
require “intelligent” path determination and optimization of packet flow.

Routing Components

There are two basic concepts to the routing of information:



Path Determination


Metrics are used to evaluate all paths for a packet to reach its dest
ination. Metrics
(bandwidth, delay, etc.) are used by the routing algorithm to choose the optimal path to the destination.
Algorithms process path information and metrics, and then populate a routing table with the results. Routing
tables contain a variety

of information, but the most important piece is the “next hop” required to reach a
destination.



Switching


This is the moving of packets through a network and is simple compared to path determination. The
router forwards packets to the destination addre
ss and, if the destination is unknown, the packets are dropped.
Packets are always forwarded to the next hop. The IP address, or protocol address, will always remain the same,
but the physical (MAC address) will change with each hop.


Routing Algorithms

R
outing algorithms process network information, and produce routing tables. Algorithms can be defined by several key traits (t
aken
from Cisco.com):

Design Goals



Optimality


Refers to the methodology and capability of the algorithm to choose the best route
. This is dependent
on the use of one or more metrics.



Simplicity


The algorithm needs to be simple and have minimal impact on the network and resources. Efficiency
is key on platforms with limited processing power or memory.



Stability


The algorithm n
eeds to be efficient and correct in a wide variety of circumstances.



Convergence


This is the “state of agreement” of all routers on the network. Rapid convergence is key to
ensuring the stability of a network and its routing functionality. It also preve
nts the dissemination of erroneous
information and routing loops.



Flexibility


Adaptation is a key trait in routing functionality. The algorithm must be quick to adapt, and needs to
work under a variety of circumstances and on a wide range of platforms.


Algorithm Types



Static vs. Dynamic


Static routing are mappings placed on devices by a network engineer. They should only be
used in very simple environments where change is unlikely. Dynamic routing algorithms continuously process
network information an
d metrics to quickly adapt to environmental changes and ensure routing integrity. Good
network design principles utilize both static and dynamic routing for backup.


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Single
-
Path vs. Multipath


Single
-
path algorithms only support one route to a destination
. Multipath algorithms
utilize many paths to a destination, and even “load share” packets to ensure traffic distribution.



Flat vs. Hierarchical


When all routers are peers, a flat system is in place. Hierarchical systems segment portions
in the network t
o form a backbone. Routers that live in these segments are referred to as backbone routers.
Hierarchical systems can also have additional levels of hierarchy, depending on the network size. The main
advantage of hierarchical routing is that functionality c
an be divided into layers. The layers ensure that routers
perform duties specific only to their layers (i.e., core, distribution and access).



Host
-
Intelligent vs. Router
-
Intelligent


There are algorithms that assume the source node (host) has the
knowled
ge to determine the route to the destination. These source routing systems store and forward packets,
with no knowledge of the transit path. Router
-
Intelligent systems rely on the routing device to determine the
appropriate path.



Intradomain vs. Interdoma
in


Intradomain algorithms are optimized for the movement of packets within a routing
domain. Interdomain algorithms move packets between separate routing domains.



Link
-
State vs. Distance Vector


Link state algorithms rely on individual routers to suppl
y the state of their own
links. These updates are flooded throughout the network, and each router uses them to build a map of the entire
network. Distance vector algorithms usually send their entire routing table to their own neighbors, who in turn
send in
formation on to their neighbors. Link
-
state algorithms converge quickly and rarely create routing loops, but
require far more resources to process routing information. Distance vector algorithms are less scalable than link
-
state systems.

Metrics

Routing t
ables are built through the use of metrics. These metrics describe the many different attributes that can help an algorithm
determine the best path to a destination. Some routing algorithms use single metrics, while others use multiple metrics to bu
ild the
ir
routing table. Below are metrics that are used:



Reliability


Can also be viewed as the dependability of a link. This is usually based on the error rates, along with
the number of interface transitions. Reliability can include any number of factors.



P
ath length


This is the most commonly used metric. Path length can be the hop count from one destination to
another, or it can be sum of costs throughout the network.



Delay


This is the amount of time it takes a packet to traverse a network (source to d
estination). This metric
depends on many different factors including: load, bandwidth, queuing, and total distance to be traveled. Because
it is a composite of several factors, this metric can be very useful in optimizing routing paths.



Bandwidth


This i
s the total traffic capacity available on a link. Bandwidth is not a standalone metric, because it
usually does not take current usage into account. A faster link will not always provide the optimal path, dependent
on the loading.



Load


Load is the curre
nt usage of the link or network resource.



Cost


Cost can be an administrative cost that the network engineer assigns to a link, or the actual monetary cost
to use a specific link. Cost metrics can be manipulated to ensure traffic takes preferred, cheaper

paths.

Routing and Routed Protocols

Routing protocols apply rules to a network to evaluate the topology and determine the optimal path to a destination. Routers
use this
info to build a routing table, and to communicate to other routers throughout the ne
twork. Routing protocols include OSPF, RIP, EIGRP
and BGP.

Routed protocols contain layer 3 address information that allows communication and packet movement from source to destination
. An
example would be IP. There are 3 types of routing protocols:



Dist
ance Vector


These protocols determine the best path to a destination by using hop count as a metric. The
hop count is incremented for each router
-
to
-
router “hop”. Distance vector protocols are usually slow to converge,
and use periodic updates to dissemi
nate information. RIP is an example of a distance vector protocol.



Link State


These protocols create a network map, or topology, by receiving all the link state information sent by
routers on a network. Examples include OSPF and IS
-
IS. A shortest path f
irst (SPF) algorithm is used to
determine paths to remote networks. Hello packets are generated for periodic communication between routers.
Topology changes trigger link state updates, which routers then use to rebuild their topology databases. Heavy
resou
rce utilization (CPU) is required to run the SPF algorithm, especially in an unstable network. There are five
basic steps required to build the topology:

o

Link state packets are sent to describe the links

o

All these packets are combined to create the link
state database. Then,



The SPF algorithm is run


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A tree is created with the router as the root, and this maps the network



The routing table is built from this information



Hybrid
-

These are combinations of distance vector and link state protocols that us
e the best traits of both. Cisco’s
Enhanced Interior Gateway Routing Protocol (EIGRP) is an example of a hybrid protocol.

Routing Protocol Features

Routing protocols need to completely discover a network, converge quickly, and prevent router loops. There

are several features that
allow this to happen:



Holddown


Used to ensure that invalid routes are not placed in the routing table. Forces the router to ignore
information about a network for a certain time period when a network is deemed unreachable. Thi
s allows all
routers in a network to receive the route information.



Triggered Updates


These are updates that are sent outside the normal update period. They prevent erroneous
information from remaining in a routing table when a network is down. They thu
s speed up convergence and
ensure the routing table is as up to date as possible.



Split Horizon


When a router learns about a network, it does not advertise that network back to the originating
device. This is key in the prevention of routing loops.



Poi
son Reverse


This is utilized with split horizon to prevent routing loops. When a route is received on an
interface, it is advertised back to the originating routing as “poisoned” (hop count of 16, or unreachable). This
removes the route from the routing
table and allows correct routing information to propagate throughout the
network.


The Routing Table

Router>sh ip route


Codes:

C
-

connected, S
-

static, I
-

IGRP, R
-

RIP, M
-

mobile, B
-

BGP

D
-

EIGRP, EX
-

EIGRP external, O
-

OSPF, IA
-

OSPF inter
ar
ea

N1
-

OSPF NSSA external type 1, N2
-

OSPF NSSA external
type 2

E1
-

OSPF external type 1, E2
-

OSPF external type 2, E
-

EGP

i
-

IS
-
IS, L1
-

IS
-
IS level
-
1, L2
-

IS
-
IS level
-
2, ia
-

IS
-
IS inter area

*
-

candidate default, U
-

per
-
user static route, o
-

O
DR

P
-

periodic downloaded static route


Gateway of last resort is 172.16.1.1 to network 0.0.0.0
1


172.16.0.0/24 is subnetted, 2 subnets
2

C 172.16.1.0 is directly connected, FastEthernet2/0

C 172.16.2.0 is directly connected, FastEthernet4/0

10.0.0.0/8 i
s variably subnetted, 13 subnets, 2 masks

O IA 10.5.8.0/24 [110/65] via 192.168.1.18, 5d10h, Serial1/1:1
3

[110/65] via 192.168.1.22, 5d10h, Serial1/0:2

O 10.250.1.32/32 [110/3] via 192.168.1.26, 5d10h,
FastEthernet0/0

O 10.250.1.31/32 [110/3] via 192.168.
1.26, 5d10h,
FastEthernet0/0

O 10.250.1.30/32 [110/3] via 192.168.1.26, 5d10h,
FastEthernet0/0

C 10.250.1.1/32 is directly connected, Loopback0

192.168.1.0/30 is subnetted, 4 subnets

C 192.168.1.24 is directly connected, FastEthernet0/0

C 192.168.1.16 is d
irectly connected, Serial1/1:1

C 192.168.1.20 is directly connected, Serial1/0:2

O 192.168.1.192 [110/3] via 192.168.1.26, 5d10h,

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FastEthernet0/0

S* 0.0.0.0/0 [1/0] via 172.16.1.1

1.

The gateway of last resort identifies the default gateway the router will u
se for all destinations that do not exist
within its routing table.

2.

The routing table will identify networks by a network heading, and how that network has been subnetted.

3.

Individual routes are listed as follows:

o

The source of the route. This can be any

of the codes listed at the beginning of the “show ip route” output.
In this case it is
O

for OSPF, and additional information that indicates this is an Inter
-
Area route (
IA
).

o

Next is the network, and mask:
10.5.8.0/24.


o

[110/65]
Indicates an administrati
ve distance of 110, and a metric of 3 (In the case of OSPF, the cost).

o

via 192.168.1.18
The source of the route.

o

5d10h
The age of the route.

o

Serial1/1:1
The interface the route was learned on.

Extending IP Addresses

Links for Further Reading



http://www.learntosubnet.com/




http://www.cisco.com/warp/public/701/3.html




http://www.bergen.org/ATC/Course/InfoTech/Cooli p/

Addressing Review

An IP address is a 32 bit address with a 32 bit subnet mask that divides the address into a network and host portion. With th
e depletion
of addresses from the “public” add
ress pool, the networking community has begun to implement several measures to use the space
more efficiently, and move away from “classfull” addressing.



Classfull Address Space


The original IP address space was divided into the classes defined below:

Class

Range

Subnet Mask

Beginning of
First Octet

A

1
-
126

255.0.0.0

0

B

128
-
191

255.255.0.0

10

C

192
-
223

255.255.255.0

110

D
(Multicast)

224
-
239

255.255.255.240

1110

E

240
-
255

Reserved

11110



Routing protocols that use the above address definitions are

considered to be classful. RIP v1 is a classful
protocol. Classless routing protocols take advantage of variable subnet masks, and ignore the classfull
boundaries above.



Subnetting


The default IP address space is very inefficient and, in most cases, ad
dresses are wasted, or not
used. Subnetting allows “borrowing” of bits from the host portion of the address to define additional networks.
Addresses have three sections: network, subnet and host. Variable Length Subnet Mask (VLSM) is a technique in
which t
he subnet mask is adjusted in order to segment a given network. An example is below:



Network


195.1.1.0



Original Mask
255.255.255.0 (/24)






The network administrator has a network with 5 hosts, and does



not want to waste addresses. To save on address spa
ce,

the mask is



adjusted:






New Network

195.1.1.0



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New Mask


255.255.255.248 (/29)



New Mask Binary

11111111.11111111.11111111.

11111000






This leaves 3 bits for the hosts, and using the equation



2
n
-
2, this gives us 6 host addresses to use.



2
3
-
2 = 6



We subtr
act 2 because the first address is used as the network



address, and the last for the broadcast address (in this case 195.1.1.7).



Completely dividing the network with this scheme will give us

32 subnets with 6



hosts each. How do you get the 32 subnets?



2
5
= 32 The five signifies the host bits



borrowed.



Route Summarization


This is used to summarize routes, and reduce the overall size of a routing table. As seen
in the above example, a single network can represent many subnetworks. Smaller routing tabl
es require less
resources to process, and less bandwidth to transmit.


The above example shows how summarization works within a network. All the networks on the left can be summarized into one
network (10.1.1.0/24), which is then ad
vertised into the core. Some benefits of summarization include:



Smaller routing table size
-

As you can see above, 4 network are summarized into one net.



Stability


Summarization has a great affect on stability. In the above example, if one of the netwo
rks goes down,
routers beyond the gateway are not affected, they only see the summarization.

OSPF

Links for Further Reading



http://www.cisco.com/warp/public/104/1.html




http://www.faqs.org/rfcs/rfc2328.html




http://www.geocities.com/Heartland/4394/work/ospf.html


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Configuring OSPF in a Single

Area

Open Shortest Path First (OSPF) is a link
-
state routing protocol defined in RFC 2328. It was designed to take large networks, and
divide them into smaller networks called areas. These areas are used to bring down the resources (CPU and Memory) requir
ed on
routers to process large network information, and also to divide the network into administrative units. The main feature of O
SPF are:



VLSM support and classless behavior



Equal cost load
-
balancing over many paths (up to six)



Cost
-
based metric. Cost

= 10
8
/Bandwidth. Bandwidth is in Bits Per Second



Reduction of bandwidth requirements, due to the usage of hello packets to discover neighbors and maintain link
information



Authentication support



Fast Convergence



Network summarization support



Dial net
work support

OSPF Concepts

OSPF Adjacency and States


an adjacency occurs between two routers when they have discovered each other and have exchanged
information. Below is the output of the command to show adjacencies:



Router#show ip ospf neighbor

Neigh
bor ID Pri State Dead Time Address Interface

10.250.1.33 1 FULL/DR 00:00:35 192.168.1.26 FastEthernet0/0

10.250.1.2 1 FULL/
-

00:00:34 192.168.1.18 Serial1/1:1

This output is described below:



Neighbor ID


The neighbor’s router ID



Pri


The IP OSPF ro
uter priority



State


The state of the relationship between the two routers. In this case, there is a full adjacency. Other states
are listed below:

o

Down


No information is available

o

Attempt


The information from the neighbor is not current.

o

Init


T
he hello process has been initiated between the two routers, but two
-
way communications are not
established

o

Two
-
way


This type of communication has been established between neighbors

o

Exstart


Sequence numbers are being established between the two route
r.

o

Exchange


The link state database is being exchanged

o

Loading


The exchange of information is being finalized

o

Full


The exchange is complete, databases are synchronized, and an adjacency has been formed

The hierarchical nature of OSPF breaks down
router responsibilities:



Designated Router (DR)


The DR is the central point of all adjacencies. It coordinates, disseminates and
synchronizes advertisements. While each router sends out its own link
-
state, the DR sends the network link
advertisement for

the entire network. When no FDR exists, one is elected (the one with the highest priority). If
there is a tie, the one with the highest IP address wins. Note: setting a router’s priority to 0 will prevent it from
being a DR or BDR.



Backup Designated Rout
er (BDR)


The BDR is also elected during the election process. It will take over the DR’s
job if it fails.



Types of routers within an OSPF network:

o

Internal Router


A router with all interfaces in the same area. Runs a single copy of the basic routing
a
lgorithm.


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o

Area Border Router (ABR)
-

Router that lives in multiple areas. Runs a copy of the routing algorithm for
each area. These routers compile their routing tables for distribution into the backbone area. The
backbone then passes on the information t
o other areas.

o

Backbone Router


A router with an interface in area 0 (the backbone area). Note: a router can be more
than one type (i.e., a router with an interface in area 0 and area 1 is an ABR and a Backbone router).

o

Autonomous System Border Router (
ASBR)


A router that exchanges information with another
Autonomous System. Once again, it is possible for a router to be classified as more than one type.

OSPF Operations (summary from rfc 2328):



Router starts and initializes the protocol, then waits fo
r an indication that all the interfaces are up and operational.



Uses OSPF Hello Protocol to discover neighbors. Sends and receives hello packets. On broadcast and point
-
to
-
point networks, hello packets are sent via multicast AllSPFRouters 224.0.0.5. Non
-
b
roadcast networks need
neighbor configuration in order to form a proper adjacency.



A designated router (DR) is elected to determine which routers should be adjacent.



Routers form adjacencies with neighbors, and then synchronize their link
-
state databases
. Routing updates are
only sent to adjacent neighbors, and routers send state updates, also known as Link State Advertisements
(LSAs).



Flooding of LSAs throughout the area ensures that all link
-
state databases are identical. This database is used to
const
ruct the shortest
-
path tree, and ultimately, the routing table.

LSA Types:

LS Type

LSA Name

LSA Description

1

Router LSAs

Originated by all routers, and
flooded within a single area.
Includes the states of all
interfaces.

2

Network LSAs

Designated Route
r originates
these. Contains a list of all the
routers, and flooded within a
single area only.

3,4

Summary LSAs

ABR originated. Describe Inter
-
area routes. Type 3 describe
routes to networks, type 4 are
routes to ASBRs.

5

AS External LSAs

ASBR Originated
. Routes to
networks in other Autonomous
Systems.

Commands for single area configuration:



Router(config)#
router ospf

1

o

Enables OSPF on the router. The one specifies the OSPF process ID.



Router(config
-
router)#
network

10.1.1.0 0.0.0.255
area
0

o

This defin
es an interface on which OSPF runs, and assigns it to an area. Area 0 is referred to as the
backbone area, and is the only area within a single
-
area implementation.



Router(config)#
interface

loopback 0

o

Router(config
-
if)#
ip address

10.250.1.1 255.255.255.2
55

o

Using loopback addresses adds stability to the OSPF network. The loopback is a virtual address that is
assigned to a router (using a single address, as above). This address is then advertised over the OSPF
network (when enabled with the network command
). Unlike physical addresses interfaces that can go
down, the loopback will always be reachable as long as one interface is up on the router.

o

Using a loopback will force the router to use this IP as its router ID. This gives the network engineer more
cont
rol over the network.

Special Media

OSPF has some specialized functionality for certain configurations:


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Demand circuits


The Hello protocol sends and receives packets on set intervals. If Hello packets are not
received within 4 times of the hello interv
al (dead interval), the link will be torn down. This can cause issues over
ISDN links, because OSPF will keep the link up trying to form an adjacency. The solution to the problem is the
following command
ip ospf demand
-
circuit.
This stops router to router
communication once their databases
have been exchanged.



Broadcast Media


OSPF relies on multicast to function, and if it cannot, problems will result. Manual
configuration is required to ensure proper adjacencies over non
-
broacast media. The
neighbor
<ip

address>

command will ensure proper communications take place.

Interface configuration commands:



Router(config
-
if)#
ip ospf

cost ( 1
-
65535)


Allows overriding of the default interface cost. Adjusting this value can
help control traffic flow through a net
work. The end result will be an altered routing table (remember that cost is
OSPFs metric, and the sum of all the costs determine the best path.).



Router(config
-
if)#
ip ospf priority

(0
-
255)



This sets the priority to help OSPF determine the designated ro
uter
(DR) for the network.



Router(config
-
if)#
ip ospf authentication
-
key

key
-

A
ssigns a password to be used by adjacent routers on a
network. For simple authentication.



Router(config
-
if)#
ip ospf message
-
digest
-
key
keyid

md5

key



Enables OSPF MD5 authenti
cation.



Router(config
-
if)#
ip ospf authentication

[message
-
digest | null]


Specifies the authentication for the interface.

Interconnecting Multiple OSPF Areas

Commands to configure OSPF with Multiple Areas:



Router(config)#
router ospf

1


Same as in the s
ingle area.



Router(config
-
router)#
network

10.1.1.0 0.0.0.255
area

area



Along with configuring the backbone area (area 0),
you will divide your network into multiple areas. Note that the backbone area is responsible for disseminating all
inter
-
area route
s. All areas must touch the backbone!! (there is one exception we will talk about in minute


Virtual
Links).

Virtual Links (VL)

All areas must have at least one router that is connected to the backbone. In some rare instances, you might have a router th
a
t needs
to cross another area to get to the backbone. To do this you need to create a virtual link. The virtual link is not recommend
ed, and is
usually used during a migration. VLs have two main purposes:



Linking an area that does not have a physical conn
ection to area 0.



As a patch, in the event the ABR that connects an area to the backbone fails.

VLs must be configured on both routers, and cannot be configured through stub areas. Below are the commands for VLs:



Router(config
-
router)#
area

area
-
id
virtua
l
-
link

router id



This is the most basic form of the command. To display
information about VLs on the router, use
sh ip ospf virtual
-
links.

Other Area Types

Stub Area


Areas that have a single entrance and exit. A stub area reduces advertisements into th
e backbone. These areas reject
Type 5 LSAs. This reduces the link
-
state database size, and required resources on the routers. Stub areas requires advertised default
routes. Stub areas are configured with the
area
area
-
id

stub [no summary]
command
.




Not So
Stubby Area (NSSA)


Like stub areas, but allows external routes to be advertised within (special Type 7
LSA). They are configured with the
area
area
-
id

nssa

command.

External Routes


Two types. The difference is in how the metric (cost) is calculated. No
te: external routes will show up whenever
redistribution takes place into OSPF.



External Type 1 (E1)
-

Both external and internal cost are factored into the overall cost. Preferred over type two.


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External Type 2 (E2)


Only the external cost is used.

Con
figuring Default Routes


usually you will want to redistribute a default route into your network. This is done on an ASBR with the
following command:



Router(config
-
router)#
default information originate [always]


This command will redistribute its defaul
t route
into the network. When you add the
always

option, it will advertise a default, even in the absence of a route to
0.0.0.0 0.0.0.0.

Summarization:



Router(config
-
router)#
area

area
-
id

range

address mask

[advertise | not
-
advertise]


This summarizes ro
utes at
the ABR.

Commands for the verification of OSPF operation:



Show ip route


This command will show the ip routing table. Use
show ip route ospf

to just view OSPF routes.



Show ip ospf


This shows a general summary of the ospf configuration, along w
ith the areas.



Show ip ospf interface



Used to show interface configuration for OSPF (hello time, cost, etc.). This will also
show the type of media for which each interface is configured.



Show ip ospf database


This displays the OSPF topological datab
ase.



Show ip protocol


This shows information about all routing protocols configured on the router.



Show ip ospf neighbor



This shows the adjacencies extablished by the router.

IS
-
IS

Links for Further Reading



http://www.cisco.com/univercd/cc/td/doc/product/software/

ios113ed/113ed_cr/np1_c/1cisis.htm




http://w
ww.ietf.org/rfc/rfc1142.txt?number=1142




http://www.cisco.com/univercd/cc/td/doc/cisintwk/

ito_doc/osi_prot.htm

Features and Operation

IS
-
IS Features

IS
-
IS
(Intermediate System to Intermediate System) is a link state protocol with widespread usage by Internet Service Providers. It

is an
Open System Interconnection (OSI) dynamic routing protocol designed to be used in the OSI Connectionless Network Service (CL
NS).
The following are features of the protocol:



Classless behavior



Rapid flooding of info/Fast Convergence



Largely scalable



Hierarchical routing



Support of Cisco IOS route
-
leaking, multi
-
area routing and overload
-
bit

Basic Operation



Hello packets are

sent out all IS_IS interfaces, neighbors are discovered and adjacencies are established.



Adjacencies are formed when three main criteria are matched: authentication parameters, IS
-
type and MTU.



Link
-
state packets (LSPs) are built for active interfaces,
along with information from adjacent routers. Flooding
generally occurs to all adjacent neighbors.



Each router constructs a link
-
state database form these LSPs.



Each IS constructs a shortest
-
path tree, and uses this to build a routing table.


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Metrics



The
default metric for IS
-
IS is cost. The cost is assigned per interface and has a value of between 1 and 63, with
a default of 10. The total cost of a path is determined by adding all the costs en route.



Originally, 1023 was the highest path cost. Cisco adde
d the use of a 24
-
bit metric, deemed a “wide metric”.

IS IS on NBMA

ISIS allows control of link state packet (LSP) flooding. This is vitally important on meshed point
-
to
-
point links over NMBA. There are two
ways to reduce LSP flooding:



Block flooding at
the interface level.



Configuration of mesh groups


Mesh groups allow grouping of interfaces. When an LSP is received on an
interface that is a member of a mesh group, the LSP is not forwarded to interfaces that are members of the group
(normally it would

be forwarded out all interfaces).

Connectionless Network Service (CLNS)

CLNS is a network layer service that is used for peer communication. In this system, routers are Intermediate Systems (IS) an
d hosts
are called Host Systems (HS). Below is a descript
ion of the operation:



ESs do not have routing info, they discover routers through Intermediate System Hellos (ISHs). ESs also send
hellos (ESHs), and the protocol determines how to optimally route traffic.



There is no ARP or ICMP for CLNS. There is an ES
-
IS protocol that provides these services for CLNS. IS
-
IS is
the protocol for routing OSI, and operates at the data
-
link layer.

Integrated/Dual IS
-
IS

Integrated IS
-
IS can be used to support both OSI and IP routing. (RFC 1195). There are two ways of suppor
ting integrated systems:
through separate routing protocols, or a single integrated protocol servicing both IP and OSI.

Areas and the Domain

A routing domain is a group of areas under the same administrative authority and subject to the same routing poli
cies. The backbone is
a collection of Level 2 routers.
There is no backbone like with OSPF.




A router is only in a single area, and an area border consists of two routers, each within a distinct area.
This is
different than OSPF, where the ABR is a member
of both areas.

Router Types

Large routing domains use a two
-
level hierarchy. A large domain will be divided into several areas, with each system residing in its own
area. Routing within a single area is referred to as Level 1 routing. Routing between area
s is called Level 2 routing. Routers can be
Level 1, Level 2, or support both functions (L1/L2).



Level 1 Intermediate Systems track routing within their areas. If a packet’s destination is outside the area, Level 1
IS sends the packet to the Level 2 IS ne
arest to it.



On local area networks, the protocol uses a Designated Intermediate System (DIS) to conduct flooding (the DIS is
elected). The DIS is elected by priority and can be compared to the DR in OSPF. If there is a priority tie, the
highest MAC is us
ed.

Addressing

The protocol conveys both OSI network layer information, along with subnetwork addresses. The address identifies either:



The Network Service Access Point (NSAP)


Which ids the interface between layer 3 and 4.



Or, the Network Entity Title

(NET)


Which is the network layer entity on OSI IS.


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Subnetwork addresses, also called Subnetwork Point
-
of
-
Attachment Addresses (SNPAs), are the physical attachment points, and
uniquely identify each system on the network. The SNPA is the 48 bit MAC addre
ss. Systems transmit NSAP and NET to SNPA
mapping information to help define the network.


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IS IS Configuration

IS
-
IS configuration (This is an IP only implementation):



Router(config)#
router isis [area tag]



Enables IS
-
IS on the router. The area tag is
used if you are configuring
more than one area. The first instance configured is a Level1/2 router by default.



Router(config)#
net

network
-
entity
-
title


Configures a NET for a specific routing process. It is specified for each
routing process for multi
-
ar
ea implementations.



Router(config
-
if)#
ip router isis [area tag]



This configures an interface to run an IS
-
IS process.

To verify operation, do this:



Router#
show clns neighbor


This will display adjacency information



Router#
show clns interface eth2


G
ives interface level configuration, along with neighbor info, metrics, and
update status.



Router#
show isis database


Displays the database contents.

EIGRP

Links for Further Reading



ht
tp://www.cisco.com/warp/public/459/7.html




http://www.cisco.com/univercd/cc/td/doc/product/software/

ios122/122cgcr/fipr_c/ipcpr
t2/1cfei grp.htm

Features and Operation

Enhanced Interior Gateway Routing Protocol (EIGRP) Features


EIGRP is an enhanced version of the Interior Gateway Routing
Protocol (IGRP). EIGRP is a Cisco proprietary hybrid routing protocol that combines both link
-
state and distance
-
vector capabilities.
The heart of EIGRP is the Diffusing Update Algorithm (DUAL) that enables EIGRP routers to prevent routing loops, and also to
find
routes to networks quickly and efficiently.

Additional features of EIGRP include:



Pr
ovides compatibility and interoperability with IGRP routers. EIGRP also provides for the automatic redistribution
of routes to and from each protocol



Supplies extremely fast convergence


Stores all neighbor routes in its database so it can quickly reeval
uate the
network



Supports VLSM


Carries subnet information in updates, and allows for automatic network summarization



Does efficient updates


EIGRP does not send periodic updates. Updates are only sent when a metric has
changed, and only to specific ro
uters. This allows minimal bandwidth usage



Supports AppleTalk, Novell, and IP



Uses a composite metric


It does this by using several elements: bandwidth, delay, load and reliability



Supports load sharing


Up to six paths



Supports authentication

EIGRP

Operation

The protocol relies on several technologies to provide reliable routing:



Neighbor Discovery/Recovery


This allows routers to learn about routers and their directly attached networks
dynamically. This is achieved through the use of small hello
packets. The hello process allows routers to ensure
neighbor operation, along with facilitating the exchange of routing information.



Reliable Transport Protocol (RTP)


This provides for guaranteed delivery of EIGRP information to neighbors.
EIGRP packets

are sent using a mix of both unicast and multicast packets. Multicast packets are sent to
224.0.0.10.


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DUAL Finite State Machine


This is the “decision maker” for the protocol. It uses distance information to select
routes and insert them into the routin
g table. This is done with the concept of a
feasible successor,
or the
neighboring router to which packets are forwarded for a least cost path to the destination which is loop free. When
a topology change occurs, DUAL looks for feasible successors and, if
none exist, it recomputes.



Protocol Dependent Modules


EIGRP uses separate protocols modules to perform functions for their respective
protocols. This allows each network layer to have its own subprocess, and makes the protocol very efficient.

EIGRP Conc
epts

There are four fundamental concepts to the protocol:



Neighbor Tables


This is responsible for all neighbor information. It holds the neighbor’s address and interface,
along with information required by RTP (sequence numbers and a transmission list)
. The table also keeps round
-
trip information to dynamically adjust transmission intervals. Each protocol module has its own neighbor table.



Topology Table


Contains all the destinations advertised by neighbors. Each entry in the table includes the
desti
nation, and a list of all the neighbors that can reach it. Metric information is also included, along with the link
cost.



Route States


There are two states for a destination within the topology table: active and passive.


When a feasible successor is a
vailable, the router will not need to recompute, and the destination will be passive.
If the router is performing a recomputation, the destination will be active. Recomputation is performed when no
feasible successor exists. This is initiated by sending qu
eries to all neighbors and, once they have all replied (if
they have a route to the destination), the router can select the successor.



Route Tagging


There are two types of routes within EIGRP: internal and external. Internal are originated within
the AS
. External are learned from the outside (redistribution). External routes are “tagged” with the below info:

o

Router ID of the router that performed the route injection

o

AS number of the destination

o

An administrator tag

o

External protocol ID

o

Metric (exter
nal protocol)

o

Default Routing bit flag information

EIGRP Packet types

EIGRP packet types include:



Hello/Acks


Used for neighbor discovery and recovery.



Updates


Relate the reachability of destinations. These are used to build the topology table.



Qu
eries/Replies


Sent when destinations are in an active state.



Request


Used to get neighbor specific information.

EIGRP Configuration

Basic Configuration



Router(conf)#
router eigrp

autonomous system



This enables the routing process for the specified AS
.



Router(config
-
router)#
network

network
-
number


This associates networks with the router process. EIGRP
sends updates to the interfaces specified with the network statement. If you do not specify an interface’s network,
it will not be advertised.

Advance
d Configuration



Router(config
-
router)#
auto
-
summary



This enable auto summarization.



Router(config
-
router)#
maximum
-
paths


Sets the maximum paths (4 is the default).



Router(config
-
if)#
ip summary
-
address eigrp

as
-
number address mask


This interface level

command enables
summarization.



Router(config
-
router)#
passive
-
interface

interface



This prevents EIGRP updates and hello packets from being
sent on the named interface.


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Router(config
-
router)#
variance

metric
-
variance
-
multiplier


Use this to allow load b
alancing over unequal cost
paths. This includes routes with a metric less than or equal to the multiplier times the minimum metric for the route
to the destination.

Verifying Operations



Router#
show ip protocols



Shows routing protocol information.



Router
#
show ip eigrp neighbors


Displays neighbor information within the same AS.



Router#
show ip eigrp interfaces


Shows interfaces that are participating in the EIGRP process.



Router#
show ip eigrp topology


Shows the topology database.

Border Gateway (BGP)

Protocol

Links for Further Reading



http://joe.lindsay.net/bgp.html




http
://www.cisco.com/univercd/cc/td/doc/product/software/

ios121/121cgcr/ip_c/ipcprt2/1cdbgp.htm




http://www.ietf.org/rfc/rfc1771.txt

BGP Features and Operation

BGP Overview

BGP is the routing p
rotocol of the Internet. It allows inter
-
autonomous system routing on a large scale (the internet routing table is over
100,000 routes!!).

BGP:



Is a path
-
vector protocol. This describes the fact that BGP carries the sequence of AS numbers that it has
tra
versed



Runs over TCP port 179



Supports CIDR



Uses a complex algorithm based on several attributes

External BGP is used to route between different ASes, and Internal BGP is used to route within an AS. AS numbers are managed
by
the American Registry for In
ternet Numbers (ARIN), and are unique to prevent routing loops.

BGP Attributes

BGP routes have properties, or attributes, that are used to determine the best route to a destination.

These properties include:



Weight


A Cisco defined attribute that is k
nown only to the local router. If more than one route exists to a
destination, the one with the highest weight will be preferred.



Local preference


Used to prefer an exit from the local AS. The local preference is known throughout the AS.
The higher loca
l preference will be the chosen exit point.



AS_Path


The ordered list of ASs through which an advertisement has passed. BGP uses this to prevent loops,
as it will never accept an advertisement that includes its own AS in the path.



Multi
-
exit Discriminat
or (MED)


Allows an AS to advertise a preferred entry point to a neighbor AS.



Origin


This describes how BGP has learned a route. There are three possible values:

o

IGP


The route was learned within the AS. These are routes advertised via the
network

co
mmand.

o

EGP


Routes learned via the External Gateway Protocol.

o

Incomplete
-

The route was redistributed into BGP.



Next Hop


For EBGP, it is the ip address that is used to reach the advertising router. For EBGP, this is the peer.
Note that this informat
ion is passed throughout an AS using IBGP.



Community


A group of routers to which a set of specific rules can be applied. BGP uses route maps to apply the
community attribute, and there are three predefined attributes:


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o

No
-
export


Do not advertise throu
gh EBGP.

o

No
-
advertise


Do not advertise to any peer.

o

Internet


Advertise to all.

BGP Path Selection

When many paths to a destination exist, BGP uses a structured selection process to choose the best one. Once the path is sele
cted, it
is entered into
the routing table, and advertised to peers. The ten step process is below:

1.

If the next hop cannot be reached, drop the update.

2.

Prefer the largest weight.

3.

If the weights are equal, prefer the largest local preference.

4.

Prefer locally originated routes.

5.

Prefer the shortest AS_PATH..

6.

If AS_PATH length is equal, prefer the lowest origin (IGP<EGP<Incomplete).

7.

If origins are the same, prefer the lowest MED.

8.

IF MEDS are equal, prefer EBGP over IBGP paths.

9.

If all the above are equal, take the shortest inter
nal path.

10.

The tiebreaker is the router ID (loopback), and the lowest is the winner.

Synchronization

This is the feature of BGP that ensures networks are reachable before it advertises a route. The rule states that BGP will no
t advertise
routes to externa
l neighbors learned from IBGP unless the IGP has knowledge of the destination. This feature prevents routing
blackholes, by ensuring consistency throughout the routing domain and can be turned off with the
no synchronization

command.

Peering

Routers runn
ing BGP are also known as BGP speakers. Two speakers form a TCP connection between each other for the purpose of
exchanging routing information. This relationship is known as peering, and the routers are deemed peers or neighbors.

The peering process is o
utlined below:



Initial Exchange


The OPEN message passes the BGP version number, the AS of the sending router, an
identifier, the hold time, and a set of options. The option is authentication data.



Updates


Once the initial exchange is complete, the ro
uters then send UPDATE messages. The initial update is
the entire routing table. Once the peers have passed all their routes, the updates are only done as needed. These
messages contain path information, along with attributes.



Keepalives


BGP routers con
stantly ensure that all neighbors are reachable. This is done with a KEEPALIVE
message.



Notifications


The NOTIFICATION message is sent when there are errors between the peers. This message
either terminates the negotiation, or gracefully closes the conn
ection.

Peer Groups and Communities



Peer Groups


A group of BGP neighbors that share the same update policies. This allows the administrator to
apply policies to a group, and those policies are distributed across routers within that group. This saves time
, but
also allows the router to conserve resources by only processing updates once.



Community


A community is a group of destinations that share common properties. Communities can span
Ases. The use of community attributes allow the BGP speaker to contro
l route acceptance and preferences, and
pass those on to neighbors.


BGP Configuration

Basic Configuration



Router(config)#
router bgp

autonomous system


This command enables the bgp process on a router, and
assigns an AS number.


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Router(config
-
router)#
neig
hbor

ip address

remote
-
as

autonomous
-
system


This designates the neighbor with
its ip address, along with the AS of the peer. Note that this statement is how BGP determines whether the peer is
an Internal or External peer. An internal peer is configured w
ith the same AS as the router itself. An external peer
has another AS.



Router(config
-
router)#
network

net
-
address
mask

netmask


This command tells BGP to advertise this route to
neighbors. Note that BGP will only advertise this route if it knows how to re
ach it (if it is in its routing table). If the
destination cannot be reached by the router, this will not be advertised.

Verifying Operations



Show ip bgp neighbor


This command will show information about the BGP neighbors, and the current state.
An ESTAB
LISHED state indicates that peer relationship is established, and routes are being exchanged.



Show ip bgp


This gives information about the BGP process, Network Layer Reachability Information (NLRI),
attributes, and path information.

Implementing BGP in
Scalable Networks

BGP and Scalability

Route Reflectors

When using a meshed network design with BGP, large
-
scale networks can become unmanageable because of the number of peer
relationships that need to be configured. In a network with just 10 routers, 45 r
elationships would need to be established for a full mesh.
A mesh would be required, because of the following BGP rule:



IBGP speakers cannot advertise routes learned from a neighbor to a third IBGP speaker.

For large networks, such as ISPs, that require a
n internal BGP mesh, route reflectors are recommended. A
route reflector(RR)

is a
router that acts as a focal point for IBGP sessions. In this concept there are clients and route reflectors.



Route reflectors peer with clients, and then the route reflector
s peer with one another. The reflectors “reflect”
routes, and alleviate the need for full mesh configuration.



Clients peer with the local route reflector, and exchange routing information with the router. The route reflector
then passes the routing inform
ation between clients, and to other BGP peers.

Definitions



Route reflector


The router that passes on routes, or “reflects” them.



Cluster


The route reflector and its clients.



Non
-
client


Peers of the route reflector that are not part of the cluster.

Rules



Non
-
clients need to be fully meshed with one another, and also with the route reflector. They follow the standard
IBGP advertisement rule. Note that they do not need to peer with clients (this is why we use the RR!).



Route reflection is only configu
red on the RR, and the clients and non
-
clients are normally configured BGP
speakers.



The standard BGP decision process is used within this process to choose the optimal path.



The best path is propagated within the AS based on the following rules:

o

Non
-
cl
ient routes are reflected to clients only.

o

Client routes are reflected to clients, as well as non
-
clients.

o

EBGP routes are reflected to all peers (clients and non
-
clients).



EBGP speakers are considered non
-
clients.

Configuration


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Router(config
-
router)#
ne
ighbor
address

route
-
reflector
-
client


Note that this is only configured on the RR
itself, and all clients must be configured.


Confederations

Confederations are also used to combat the IBGP mesh issue within large networks. In a nutshell, BGP confederati
ons chop an AS up
into many smaller sub
-
ASs. Within each sub
-
AS, all IBGP rules apply, and EBGP is required between them because they have different
AS numbers.

Advantages of confederations:



Even though EBGP is run between the sub ASs, the confederation
preserves information that crosses routing
boundaries (MED, next hop, and local preference).



Routing loops are easily detected. EBGP uses the AS path list to ensure that routes are dropped that include their
own AS (discussed in the BGP attributes section

above).

Disadvantages of confederations:



Major reconfiguration of the routers and logical topology are required.



Manual configuration is sometimes required for optimal path routing. Note that sub
-
ASs do not have an effect on
AS path length. The confeder
ation is viewed as a single AS from the outside, and the path through the
Confederation is not known.

Configuration:



Router(config
-
router)#
bgp confederation identifier

autonomous
-
system



configures the confederation.



Router(config
-
router)#
bgp confederat
ion peers

autonomous
-
system [sutonomous
-
system...]



configures which
ASes belong to the confederation.

BGP Filtering

BGP policies are implemented through the use of route filtering and attribute manipulation. The following are filtering techn
iques:


BGP
Route Maps

Route maps are used to modify and control the sending and receiving of routing information, and to define the way the BGP pro
cess
handles information. A basic example of route map usage is below:

Router bgp 1

Neighbor 10.1.1.1 route
-
map BGPMAP

in

!

route
-
map BGPMAP permit 10

match ip address 1

set metric 10

!

route
-
map BGPMAP permit 20

set local preference 150

!

ip access
-
list 1 permit 10.1.1.0 0.0.0.255

The route map command is:

route
-
map

map
-
tag

[permit | deny]


sequence number

The map tag
is simply a name for our route map (BGPMAP), and the sequence number is the order in which the statements are
processed (10,20,30,40). The route map above is applied to the BGP neighbor 10.1.1.1 using the
Neighbor 10.1.1.1 route
-
map
BGPMAP in
command. If a

route is received from 10.1.1.1 that matches access list 1, the metric is set to 10. This match and set
combination is how most of the route map configurations will be applied. The second route map statement sets the local prefer
ence for
all routes from n
eighbor 10.1.1.1 to 150.


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Prefix Lists

This method of filtering BGP is much more intuitive, and behaves like access
-
lists. The lists can be updated incrementally, and each
entry is identified by a sequence number. The lists also allow you to filter on exa
ct matches, or ranges. The basic syntax is:

Ip prefix
-
list

list
-
name

seq

seq
-
value

deny | permit

network
[
ge

ge
-
value][
le

le
-
value]

List
-
name defines a name for the list, seq
-
value is the sequence

number, network defines the network number, and ge/le valu
e


would be a less than

or equal to operator.


A sample list would be:

ip prefix
-
list test seq 5 permit 10.1.1.0/24

ip prefix
-
list test seq 10 permit 10.1.2.0/24

ip prefix list test seq 15 deny 0.0.0.0/0 le 32

This would permit routes from the 10.1.1.0 and

10.1.2.0

networks, and deny all other routes.


To apply this filter to a neighbor, the following would be

used:

Router(config
-
router)#
neighbor 10.5.1.1 prefix
-
list test

in

This would filter all inbound routes.

AS_PATH Filtering

This is an excellent metho
d of filtering when you want to control routes based on the AS number. Instead of entering multiple routes,
you can use regular expressions to control and filter. Below is a basic example that allows the advertisement of all routes o
riginated
from the rout
er’s AS:

Router bgp 1

Neighbor 10.1.1.1 remote as 1

Neighbor 10.2.1.1 remote as 2

Neighbor 10.2.1.1 filter
-
list 1 out


Ip as
-
path access
-
list 1 permit ^$

The ^$ indicates what we call an “empty” AS path. The ^ regular expression notation is for the beginn
ing of the
AS_PATH, and $ is for the end. Nothing is between the operators.

Note that BGP routes have an empty path list until they are received by an EBGP peer, so this would allow the
advertisement of all AS3 originated routes. Also note that regular ex
pressions are well beyond the scope of this
guide, but you learn about them through the following link:

http://www.cisco.com/univercd/cc/td/do
c/product/

atm/c8540/12_0/13_19/cmd_ref/appc.htm


Multi
-
Homing BGP

When you run BGP while connected to multiple ISPs, this is called multi
-
homing. There are several ways to implement multi
-
homing:



Receiving full Internet routes (requires a large memory s
pace and processing power).



Receiving directly connected routes (this gives you only your ISP networks, along with a default route. This is
much easier on the router resources).



Receiving default routes only (this is preferable when your router has minim
al memory and processing power, and
usually gets the job done if you are just looking for redundancy).

The above implementations are accomplished through the use of the filtering methods discussed in the previous section. Rather

than
reiterate a great Cisc
o document, I will just give you the link to a great reference on this topic:

http://www.cisco.com/warp/public/459/27.html



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Verifying a Multi
-
homed Environment

The following command
s can be used to verify the operation of a multi
-
homed BGP environment:



Show ip bgp neighbor


This command will show information about the BGP neighbors, and the current state.
An ESTABLISHED state indicates that peer relationship is established, and rou
tes are being exchanged.



Show ip bgp


This gives information about the BGP process, Network Layer Reachability Information (NLRI),
attributes, and path information.



Show ip bgp neighbor x.x.x.x advertised



This command will show the routes being advert
ised to neighbors,
and can be used to verify your outbound filters.



Show ip route



This command will show the bgp routes, and their source.

Optimizing Routing Update Operation

Links for Further Reading



http://www.cisco.com/warp/public/cc/techno/protocol/

tech/plicy_wp.htm




http://www.cisco.com/univercd/cc/
td/doc/product/ong/

15304/15304sfw/cindep.htm




http://www.cisco.com/warp/public/105/redist.html




http://www.cisco.com/univercd/cc/td/doc/cisintwk/

ics/cs001.htm




http://www.cisco.com/warp/public/104/

bgp
-
ospf
-
redis.html

Controlling Update Traffic

There are s
everal ways to control routing updates and operations:



Redistribution is the process of allowing routing protocols to learn routes from one another. The processes allow
communication between different routing protocols.



The passive interface command allo
ws the administrator to prevent advertisements from being sent out an
interface. This would restrict updates from spreading to certain parts of a network.



Route filtering goes hand
-
in
-
hand with redistribution, and allows granular control of routing update
s.

Passive Interface Configuration

The passive interface command is configured within the routing protocol, as seen below:



Router(config
-
router)#passi ve interface Ethernet 0

This would prevent all updates from being sent out the E0 interface.

Route Filt
ering

Distribute lists are configured under the routing protocol, and allow the filtering of updates in both directions (in and out
). Below is an
example of a distribute list:

Distribute
-
list 1 in serial0

Distribute
-
list 2 out ethernet0


Access
-
list 1 pe
rmit 10.1.1.0 0.0.0.255

Access
-
list 2 deny 192.168.1.0 0.0.0.255

Access
-
list 2 permit any



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The distribute list above is applied to the routing process, and direction and interfaces are named. Access lists are
then used to specify permitted and denied route
s. Note that using distribute lists with OSPF should be avoided, as
they will not stop link state advertisements.

Managing Redistribution

In a perfect world, there would be a single routing protocol, but we do not live in a perfect world, and inter
-
proces
s communication is
almost always required.


Configuration

In the above example, router A is running both OSPF and RIP, while router
B runs RIP only. The following configures the redistribution
of OSPF routes into RIP on router A:

router rip

redistribute ospf 1 metric 3

This command redistributes routes from the ospf process 1 into RIP. The metric command allows you to “normalize” metr
ics by
specifying a native metric (note that because OSPF and RIP have different metrics, you must specify a RIP metric for the OSPF

routes).

To redistribute RIP routes into OSPF, the command on router A would be:

Router ospf 1

Redistribute rip metric 50

metric
-
type 1 subnets

This command can be broken down into the following:



Redistribute rip



We specify that we are injecting RIP routes into the OSPF process.



Metric 50



Because the two protocols use different metrics, we need to specify a default con
st for the RIP
routes, here it is 50.



Metric
-
type 1



Remember that ospf has two types of external routes, type 1 and type 2. This is how you specify
the type (remember that the cost is calculated differently for the 2 types; see the ospf section for deta
ils).



Subnets



In order for subnets to be redistributed into the process, you need to include this keyword. If this is
forgotten, only major nets will be redistributed (common error).


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Administrative Distance

When using more than one routing protocol, i
t is imperative that you understand how a router chooses the priority of routes. Which
protocol takes precedence if each has a route to a destination? This is where administrative distance comes in, and it is esp
ecially
important with redistribution. Below

is a table with the default administrative distances:

Type of Route

Administrative
Distance

Connected interface

0

Static Route

1

EIGRP Summary

5

External BGP

20

EIGRP

90

IGRP

100

OSPF

110

IS
-
IS

115

RIP

120

External EIGRP

170

Internal BGP

200

Unknown

255

Redundant Networks


In large redundant networks, redistribution can cause some serious routing issues (loops). Because it is a manually configure
d process,
special care needs to be taken to ensure routes do not get redistributed into the core,

and are then relearned by the original process.
This can be prevented by using the route filters described above.

Distribute lists are used to block the “relearning” of routes through redundant links. PLAN your redistribution carefully.

Verifying Route
Redistribution

Verification of redistribution is performed by ensuring the correct operation of the routing protocols as individual entities
, and then
ensuring that routes are being properly communicated to protocols:

Show ip route



This will be your pr
imary verification command with redistribution. Note that this can be tricky if multiple routes exist
and administrative distances come into play.

Show ip route summary


This command will show statistics for individual protocols, along with information o
n how many routes have
been learned.

Show ip protocols


This will show information on the individual protocols, along with applied filters.

Policy Routing

Policy routing enables you to change the default behavior of routing. This is done with filters ca
lled route maps. Route maps enable you
to identify traffic, and then perform a specific action. Some reasons for utilizing policy based routing include:



The need for packets to utilize a specific WAN link even though it would not normally be chosen as the

preferred
path (maybe cost is an issue).



Certain packets need to travel a specific path for security reasons.



Different paths must be used by different departments for billing purposes.

Command syntax:

Route
-
map

name

permit | deny

sequence
-
number

In th
e above:



Name is the name given to the route map.



Traffic is either permitted, or denied.



The sequence number is the order in which the statements will be processed for a particular route map (spread
these out so you have room to insert statements if re
quired).

The match statement is used to identify the traffic you wish to apply changes to. This statement compares traffic to an acces
s list and
looks for a match. Note that the flexibility of access lists allow you to direct traffic based on source, desti
nation, or port, or any
combination of the three.

Command syntax:


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Match ip address

access
-
list


Set ip next
-
hop

next
-
hop address




This is the action performed on the matched traffic. In this case it is sent to the next hop address specified. Note
that ro
ute maps are like static routes on steroids. Another command that is useful is the
set interface
command.
This allows you to direct traffic to a router interface.

Ip policy route
-
map

name


Ip route
-
cache policy




These two commands are applied under the spe
cific interface on which filtering is to occur. The first command
applies the route map to the interface. The second command enables fast
-
switched policy routing and will greatly
improve the performance of the router through caching information.

Verifying
Policy Routing

The following commands are used to verify policy routing:



Show ip cache policy



This command will show the type of policy routing, along with the cache entries. This is
only available when you use both the interface commands shown above.



Debug ip policy
access
-
list


This command will allow you to see the packets crossing the interface, and those
that match your policy. You need to turn off caching before this will work (no ip route
-
cache policy).

Special thanks to

Stephen Boals

for
contributing this
Cramsession.

Please visit his site at

http://www.bitcraft.net