Chapter 4 (Module 3) - PDF

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IST 203 –Chapter 4 – EIGRP
EIGRP
– Enhanced Interior Gateway Routing Protocol
EIGRP has four basic components:
1.
Neighbor Discovery/Recovery
- the process that routers use to learn about other

routers that are directly connected to them. This includes finding out when a router

(neighbor) goes down for some reason. This is achieved by sending very small hello

packets at periodic intervals.
2.
Reliable Transport Protocol (RTP)
- refers to the fact that EIGRP is TCP based and

therefore has the ability to use TCP based transmission when it is necessary to guarantee

that a neighbor received a communication.
3.
DUAL Finite State Machine
- stands for Diffusing Update Algorithm and differs from

other routing protocols in that routing calculations are shared among multiple routers. A

router only sends routing updates as distance vectors of directly connected routes, rather

than every route that is in the network. Also, the router only sends an update if a particular

topology change has occurred to that specific route. In addition, this update is only sent to

relevant neighbor routers, not to all routers. This makes EIGRP a bandwidth-efficient

routing protocol.
4.
Protocol Dependent Modules
- handle network layer, protocol specific requirements,

sich as IP or IPX. EIGRP maintains separate tables for each layer 3 protocol used in the

network, just as almost all routing protocols do.
Comparisons between EIGRP and IGRP fall into the following major categories:
1.
Compatibility mode
2.
Metric calculation
3.
Hop count
4.
Automatic protocol redistribution
5.
Route tagging
EIGRP can be used with IGRP
. EIGRP and IGRP automatically redistribute routes between

Autonomous systems with the same number… i.e. EIGRP 2446 and IGRP 2446
Both EIGRP and IGRP use the same metric for calculating their cost only EIGRP mulitplies it by 256
Metric=((K1*Bw) + (K2*Bw)/(256-Load) + (K3*Delay)*(K5/(Reliability + K4)))*256
The default administrative distance for EIGRP is 90.
EIGRP stores and maintains 3 tables:
1.
Neighbor table
a.
Network layer addresses of the neighbor routers
b.
Number of EIGRP packets waiting to be sent
2.
Topology table
a.
All learned routes to a destination are maintained in the topology table.
b.
maintains a topology table for each configured network protocol.
c.
EIGRP DUAL algorithm stores the primary rout to a destination
d.
Also stores the following:
i.
The routing protocol
ii.
The feasible distance of the route
iii.
The route cost as advertised by the neighboring router
3.
Routing table
a.
EIGRP DUAL algorithm stores the primary rout to a destination
You should configure your bandwidth on your interfaces when using EIGRP because the network may

be unable to converge if the actual bandwidth doesn’t match the actual bandwidth, or because a

worse path may be chosen if the bandwidth isn’t configured properly.
There are 5 EIGRP packet types:
1.
Hello
2.
Acknowledgement
3.
Update
4.
Query
5.
Reply
Feasible Successors
A destination entry is moved from the topology table to the routing table when there is a feasible

successor. All minimum cost paths to the destination form a set. From this set, the neighbors that

have an advertised metric less than the current routing table metric are considered feasible

successors.
Feasible successors are viewed by a router as neighbors that are downstream with respect to the

destination. These neighbors and the associated metrics are placed in the forwarding table.
When a neighbor changes the metric it has been advertising or a topology change occurs in the

network, the set of feasible successors may have to be re-evaluated.
Active
– when a router is flagged as FAILED it is considered Active.
EX
shown in the EIGRP Routing table shows that a route was either discovered from another

autonomous system or from another routing protocol.
To enable EIGRP on the router you simply need to enable eigrp and define a network number. This is

done as follows:
Router# conf t
Router(config)# router eigrp
Router(config-router)# network 192.168.0.0
Optionally, but not normal, you can disable auto-summarization by adding:
Router(config-router)# no auto-summary
(Typically used in Point-to-Point)
You can also enable authentication, change the hello interval and hold times, and change split-
horizon if you want from the eigrp configuration.
There are a few commands you will want to use to verify EIGRP is running correctly:
*
show ip interface brief
- Used to verify your interface status.
*
show ip route
- It is useful to see the results of EIGRP in your actual routing table.
*
show ip route eigrp
- This lets you view the routes that EIGRP is handling.
*
show ip eigrp neighbors
- Verify that all of your neighbors are coming up and being seen. If

your neighbors aren't here, don't bother troubleshooting the routes you are supposed to be

transmitting because you aren't at that stage yet.
*
show ip eigrp traffic
- This is useful to see that EIGRP traffic is behing passed back and forth

between neighbors. Often with EIGRP, the problems that occur are related to other things besides

EIGRP.
*
show ip eigrp topology all-links
– used to view all routes that are known by the router.