intro_routing algorithms

gascitytankNetworking and Communications

Oct 28, 2013 (3 years and 9 months ago)

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1

Introducing Routing

1.
Dynamic routing
-

information is learned from other routers,
and routing protocols adjust routes automatically.

2.
Static routing

-

network administrator configures information
about remote networks manually. They are used to reduce
overhead and for security.


Because of the extra administrative requirements, static routing
does not have the
scalability

of dynamic routing.

In most networks static routes are often used in conjunction with a
dynamic routing protocol.

In order to forward packets correctly, routers must learn the
direction to remote networks.


Two types of routing:

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2

Routing and Routed Protocols

Routing Protocols

allow the routers to communicate with

other routers to update and maintain tables.

Examples:

RIP, IGRP, EIGRP and OSPF




Routed Protocols

provide enough information in their network

layer address to allow packets to be forwarded from one

host to another host based on the addressing scheme.

Examples:


IP, IPX, AppleTalk


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3

Routing Protocols


Path Dertemination

Routing consists of two basic mechanisms:



1.
Path Determination

Router uses the routing table to determine the best path
.

2.
Switching

(forwarding)

Accept a packet on one interface and forward it to a second interface

Routing protocols create and maintain routing tables:

Simplified routing table.

The Default router entry
sends packets for any
other destinations out S1.

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4


This command sets a
default route

on a router:

Router(config)#

Static Routes

Static routes between networks are manually configured by an administrator.

Static routes are added with the following command:




Router(config)#

ip route 192.168.2.0 255.255.255.0 E0

ip route 0.0.0.0 0.0.0.0 192.168.1.1

Network Address

Subnet Mask

Gateway

Static routes out
interfaces

have an
administrative distance

of
0
.

Static routes to
next hop addresses

have
administrative distance

of
1
.


You can specify a non
-
default administrative distance for a static route:

Router(config)#

ip route 0.0.0.0 0.0.0.0 192.168.1.1 130

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5

Terms to Remember


default route



Routing table entry that is used to direct frames for which a next hop is not
explicitly listed in the routing table.



static route


A route that is explicitly configured and entered into the routing table. Static
routes take precedence over routes chosen by dynamic routing protocols.



hop



Term describing the passage of a data packet between two network nodes
(for example, between two routers). See also
hop count
.



hop count


Routing metric used to measure the distance between a source and a
destination. RIP uses hop count as its sole metric. See also
hop

and
RIP
.

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6

Dynamic Routing


Distance Vector and Link
-
State


The success of dynamic routing depends on two basic router
functions:


1.
Maintenance of a routing table

2.
Timely distribution of knowledge, in the form of routing
updates, to other routers.

Dynamic routing relies on the routing protocol.

Routing Protocols can be
Distant Vector
or

Link
-
State.

Hybrid protocols (like EIGRP) contain some elements of both.

Different routing protocols use different
metrics

to determine
the best route to a network.

Administrative Distances

are used to rate the trustworthiness
of the various routing protocols.

7

Distance Vector Routing Protocols

RIP is a distance vector routing protocol:


Uses hop count as its metric


Each router the packet goes through is 1 hop

1

2

A

B

2

3

1

Router(config)#


Configuration Example
:

router rip

Router(config
-
router)#

network 172.16.0.0

The distance
-
vector routing algorithm passes
complete routing
tables

to neighbor routers.

The neighbor routers combine the received routing table with their
own routing tables.

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8

Link State Routing Protocols

Link
-
state routing algorithms (Shortest Path First

algorithms),
maintain a complex database of topology information.

Link
-
state routing uses:

Link
-
state routing requires
more memory

and
processing

power
than distance vector, and
bandwidth

requirements are often
higher as well.

OSPF is the most commonly used Link
-
State Protocol.


Link
-
state advertisements

(LSAs)



A
topological database




The
SPF algorithm
, and the resulting
SPF tree




A
routing table

of paths and ports to each network

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9

Distance Vector or Link State?

1.
Also known as Bellman
-
Ford algorithms

2.
Flood routing information to all routers


3.
Requests routing information from directly connected neighbors

4.
Complete view of the internetwork topology


5.
Decisions based upon information provided by neighbors



6.
Use fewer system resources

7.
When a network link changes state LSA are flooded through network

8.

Less errors, but they use more system resources

9.
Calculate the shortest path to all known sites on the network

10.
Small update packets contain only changes


11.
Slower convergence

12.
OSPF and IS
-
IS

13.
Do not scale well to larger systems.

14.
Because they converge more quickly less prone to routing loops


15.
Event
-
triggered updates, so convergence is fast

16.
Based on finding the number of hops and direction to a link


17.
Passes copies of complete routing table on a periodic basis

18.
Each router simply inform its neighbors of its routing table

19.
RIP and IGRP

20.
more reliable, easier to debug, and less bandwidth
-
intensive

DV

LS

DV

LS

DV

DV

LS

LS

LS

LS

DV

LS

DV

LS

LS

LS

DV

DV

DV

DV

10

Routing Protocols

Protocol

Features

Distance vector, hop count metric, maximum 15 hops,
broadcasts updates every 30 secs.

Cisco proprietary distance vector, bandwidth / load / delay /
reliability composite metric, broadcast updates every 90 secs.

Cisco proprietary, enhanced distance vector (hybrid), load
balancing, uses DUAL to calculate shortest path.


Routing updates are triggered by topology changes.

Link
-
state, open standard, Uses SPF algorithm. Routing
updates are sent as topology changes occur.


Distance vector
exterior routing protocol
, used between
ISPs, used to route traffic between ASs
.

RIP

IGRP

EIGRP

OSPF

BGP

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11

Autonomous Systems

AS is a collection of networks under a
common administration

and sharing a common routing strategy.

ARIN, ISP, or an administrator assigns the
16 bit AS number
.

IGRP, EIGRP and BGP require assignment of a unique AS
number.


AS 10

AS 20

Each AS has its own set
of rules and policies.

The AS number uniquely
distinguish it from other
ASs around the world.


ASs divide the global internetwork into smaller, more
manageable networks.

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