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

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ROUTING PROTOCOLS

PART III

ET4187/ET5187 Advanced Telecommunication Network

Enhanced Interior Gateway Routing Protocol

(EIGRP)

2

Enhanced Interior Gateway
Routing Protocol (EIGRP)


Really just an enhanced version of IGRP


A Cisco proprietary routing protocol


Called a hybrid protocol, but really just an advanced distance vector
protocol.


Fast convergence


Variable length subnet masks


Partial updates
-

only when the metric for a route changes


Multiple network layer support
-

IP, IPX, and AppleTalk


A router running EIGRP stores all its neighbor’s routing tables so that it
can quickly adapt or alternate routes.

EIGRP Concepts


Every EIGRP router maintains a topology
table for each configured network protocol.


All learned routes to a destination are
maintained in the topology table.


Features of EIGRP


Classless
Routing Protocol (VLSM, CIDR)


Faster convergence

times and improved scalability


Multiprotocol

support: TCP/IP, IPX/SPX, Appletalk



Rapid Convergence

and
Better handling of routing loops



(
DUAL
)


Efficient Use of Bandwidth



Partial, bounded updates
: Incremental updates only to the
routers that need them.



Minimal bandwidth consumption
: Hello packets and by
default uses no more that 50% of link’s bandwidth EIGRP
packets.


PDM (Protocol Dependent Module)



Keeps EIGRP modular



Different PDMs can be added to EIGRP as new routed protocols
are enhanced or developed: IPv4, IPv6, IPX, and AppleTalk

IGRP & EIGRP

They work together and routes are redistributed
automatically

RTB(config)# router igrp 2446

RTB(config
-
router)#network 192.168.1.0

RTB(config)#router eigrp 2446

RTB(config
-
router)# network 10.1.1.0

RTB(config
-
router)# network 172.16.1.0

Displaying Interface Values

shows reliability as a fraction of 255,
for example (higher is better):


rely 190/255 (or 74% reliability)


rely 234/255 (or 92% reliability)


rely 255/255 (or 100%
reliability)

Router> show interface s0/0

Serial0/0 is up, line protocol is up


Hardware is QUICC Serial


Description: Out to VERIO


Internet address is
207.21.113.186/30


MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,


rely 255/255, load 246/255


Encapsulation PPP, loopback not set


Keepalive set (10 sec)

<output omitted>

Bandwidth

Delay

Reliability

Load

shows load as a fraction of 255, for
example (lower is better):


load 10/255 (or 3% loaded link)


load 40/255 (or 16% loaded link)


load 255/255 (or 100% loaded link)

EIGRP Terminology

Term

Definition

Neighbor table

Lists adjacent routers (like
adjacency DB in OSPF

Topology table

Each router has one for each
network protocol routed

Routing table

Chooses routes from topology
table


maintains one for each
network protocol

Successor

Route selected as the primary
route to use to reach a
destination

Feasible successor

A backup route to above


multiple feasible successors
for a destination can be kept in
the topology table

Improvements from IGRP

1.
Neighbor discovery & recovery


Use small “hello” packets to estab. adjacencies


sent every 5 seconds


Dynamically learn routes that way

2.
Reliable Transport Protocol (RTP)


A transport layer protocol that guarantees
delivery order


EIGRP is protocol independent, so has its own
guarantee (compare to TCP)

Improvements from IGRP

3.
Dual finite
-
state machine


An algorithm that EIGRP uses to calculate routes


Tracks all routes advertised by neighbors and uses a
composite metric of each route to compare them

4.
Protocol
-
dependent modules


Each module is responsible for all functions
related to its specific routed protocol


In other words, there is an IP PDM, an IPX PDM,
an AppleTalk PDM, etc.

EIGRP Technologies


Neighbor discovery and recovery


Reliable Transport Protocol


DUAL finite
-
state machine algorithm


Protocol
-
dependent modules


By forming adjacencies, EIGRP routers:


Dynamically learn of new routes that join their
network


Identify routers that become either unreachable or
inoperable


Rediscover routers that had previously been
unreachable

EIGRP Neighbor Table


This table is the basis for all EIGRP routing
updates & convergence


Has info about the neighbor routers


Smooth Round Trip Timer (SRTT)

The average
time it takes to send and receive packets from
a neighbor
.


Queue count

The number of packets waiting in
queue to be sent
.

EIGRP Topology Table

Each EIGRP router maintains a topology table for each configured
network protocol.

This table includes the current routes (
successors
) and back
-
up
routes (
feasible successors
).

P = Passive

(good),
A = Active

(not ready, DUAL running)

EIGRP uses its topology table to store all the information it needs
to calculate a set of distances and vectors to all reachable
destinations.

RouterB#show ip eigrp topology

IP
-
EIGRP Topology Table for process 44

Codes: P
-

Passive, A
-

Active, U
-

Update, Q
-

Query, R
-

Reply, r
-

Reply status

P 206.202.17.0/24, 1 successors, FD is 2195456


via 206.202.16.1 (2195456/2169856), Ethernet0

P 206.202.18.0/24, 2 successors, FD is 2198016


via 192.168.0.2 (2198016/284160), Serial0


via 206.202.16.1 (2198016/2172416), Ethernet0

Successor



Current Route


A successor is a route selected as the primary route to use to reach a
destination.


Successors are the entries kept in the routing table.

Feasible Successor

-

A backup route


These routes are selected at the same time the successors are
identified, but they are kept in the topology table.


Multiple feasible successors for a destination can be retained in the
topology table.

FD=15

FD=15

FD=20

RD=5

RD=5

RD=6

10

10

6

14

Few
Terms

15

15

20

Feasible distance (FD)

is the minimum distance (metric) along a
path to a destination network.
(“This Router’s Distance”)

Reported distance (RD)

is the distance (metric) towards a
destination as advertised by an upstream neighbor.
(“The
Neighbor Router’s Distance”)

FD=15

FD=15

FD=20

RD=5

RD=5

RD=6

10

10

6

14

Few
Terms

15

15

20

The Feasible Distance to a network is sent to other
routers, as this router’s Reported Distance.

RouterX’s
FD = 30

to 172.30.1.0/24 (Sent as
RD

to RouterA)

FD = 31
172.30.1
.0 is 31
via
RouterY

RouterX

RouterY

RouterY’s
FD = 21

to 172.30.1.0/24 (Sent as
RD

to RouterA)

RouterZ’s
FD =220

to 172.30.1.0/24 (Sent as
RD

to RouterA)

Best Route

RouterZ

A
neighbor meets the F
easible condition (FC)

if the reported distance by the
neighbor is smaller than the current feasible distance (FD) of this router.


A distance
-
vector routing protocol not allowing possible paths with loops
paths.


"If a neighbors metric is less than mine, then I know the neighbor
doesn't have a loop going through me."

A
feasible successor (FS)

is a neighbor whose reported distance (RD) is less
than the current feasible distance (FD).


Feasible successor is one who
meets the feasible condition (FC).

(Current) Successor: RD= 21

Feasible Successor: RD= 30, FC: RD30 < FD31

NOT

a Feasible Successor: RD = 220, FC not met: RD220 > FD31

FD = 31
172.30.1.0 is 31
via RouterY

RouterX

RouterY

RouterZ

Select Routes


If a link goes down, DUAL looks for an alternative
route path, or feasible successor, in the topology
table.


If a feasible successor is not found, the route is
flagged as Active, or unusable at present.


Query packets are sent to neighboring routers
requesting topology information.


DUAL uses this information to recalculate successor
and feasible successor routes to the destination.

DUAL


Diffusing Update Algorithm

The centerpiece of EIGRP is DUAL fsm (finite state
machine), the EIGRP route
-
calculation engine.

DUAL selects alternate routes quickly by using the
information in the EIGRP neighbor and topology table.

If a link goes down, DUAL looks for a feasible successor
in its topology table.

Feasible successors provide the next lowest
-
cost path
without introducing routing loops.

All other routers to the same destination, that also meet
the
feasible condition
, meaning they are also loop
-
free, become
feasible successors
, or back
-
up routes.


Router#
debug eigrp fsm

EIGRP Packet Types

Packet

Definition

Hello
-

multicast

Discovers, verifies, rediscovers
neighbor routers

Acknowledgement
(unicast)

Indicates receipt of an EIGRP
during a reliable packet

Update

When a router discovers a new
neighbor or a topology change

Query

Needing specific info from a
neighbor

Reply

Response to a query

Data Structures

Looking for a New Route

X

Queries

Replies

RtrF

RtrC

RtrE

RtrD

RtrB

RtrA

RtrG


If there are
no Feasible Successors
, the router must ask neighbors for help in hope of
finding a new, loop
-
free path to the destination.


Neighbor routers are compelled to reply to this query.

If a neighbor has a route, it will reply with information about the successor(s).

If not, the neighbor notifies the sender that it doesn’t have a route to the destination either.

EIGRP Routing Table


Contains routes installed by DUAL FSM as the
best loop
-
free paths


Can maintain up to 4 routes per destination


Maintains a separate routing table for each
protocol