IGRP Versus EIGRP


IGRP was developed in the mid 1980s as an improvement for RIP. Although RIP is still a good routing protocol for smaller networks (up to 15 hops), EIGRP may be a more efficient protocol in a large network. IGRP has some improvements, such as hop counts of 1 to 255, and uses an autonomous system (AS). IGRP is a distance-vector protocol that was designed to be more stable than RIP and to be used on larger network environments. IGRP is a classful routing protocol, meaning it does not include subnets in the route field information. Only three types of routes are recognized by IGRP: interior, system, and exterior. IGRP also uses routing stability, which includes hold-down timers, split horizon, and poison -reverse updates. After IGRP was used for a while, Cisco released another proprietary protocol, the Enhanced Interior Gateway Routing Protocol (EIGRP).

Benefits of EIGRP

One of the problems resolved by EIGRP was that IGRP, when running, would send its whole route table if some change occurred in the network. EIGRP is a hybrid, because it uses both distance-vector and link-state routing algorithms. Enabling EIGRP to support both routing algorithms means less opportunity exists for route failure. EIGRP is much better than IGRP because it uses equal-cost load balancing, formal relationships, and incremental routing updates. EIGRP, like IGRP, is very scalable and stable. EIGRP also uses an autonomous system to distinguish a route. You can even have multiple sessions with EIGRP in order for EIGRP to calculate the best route and load sharing. It uses the route database, the topology database, and a neighbor table. EIGRP converges more quickly than IGRP because it calculates only when a change in the network directly affects the routing table.

EIGRP Features

EIGRP was created by Cisco to help make up for IGRP's inherent problems. Being a hybrid protocol enables EIGRP to be a much more efficient routing protocol. EIGRP is also very easy to configure. One of the nicest features about using this protocol is that you are not limited by hop counts, and you also have the use of timing, clocking, and load balancing. The following are some of the prominent features of EIGRP:

  • Route tagging ” Makes a distinction between routes learned via a different EIGRP session.

  • Neighbor relationships ” Uses hello packets to establish peering.

  • Incremental routing updates ” Only changes are advertised, not the whole routing table.

  • Classless routing ” Supports VLSM and subnets.

  • Configurable networks ” Information can be set through configuration commands.

  • Equal-cost load balancing ” Traffic is sent equally across multiple connections to be able to calculate the best path for load sharing. EIGRP uses a database structure to store information for the routed information. All the databases are the same for IP-EIGRP, IPX-EIGRP, AT-EIGRP, and Apple-Talk-EIGRP. EIGRP uses hello packets to update its routing table, and then allows the exchange of route information. EIGRP sends hello packets every 5 seconds on a high-bandwidth level, and every 60 seconds on a low-bandwidth level.

When EIGRP sends a hello packet continuously at a configured time on the router, the configured time is called a hello interval . The hello interval can be adjusted using the ip eigrp hello-interval command. EIGRP also uses a hold time , which is the amount of time used before the router will consider the neighbor alive and start to accept hello packets. The hold time is usually set at three times the hold-time value, normally 15 seconds to 180 seconds, but you can change this range by issuing the command ip eigrp hold-down interface . If you are going to adjust the hold-time values, you must be careful calculating the values. If the interval is too long, the updates to the routing tables may not be made soon enough to add downed routes or additions. If the rate is too quick, you may use a lot of unnecessary bandwidth for updates.

graphics/alert_icon.gif

For the exam you should know that the default timers for EIGRP are as follows :

  • Update timer used to set the frequency of routing updates is 90 seconds.

  • Invalid timer used before declaring a route invalid is 270 seconds.

  • Hold-down timer used before announcing route updates is 280 seconds.

  • Flush timer used before a route is flushed from the routing table is set to 630 seconds.


EIGRP Troubleshooting Commands

The show commands are helpful in troubleshooting any type of protocol problem. Knowing the commands that will return the information you are looking for is very helpful. The following are a few commonly used commands to aid in troubleshooting EIGRP.

Using show ip route eigrp displays only EIGRP routes:

 DCSRTR# DCSRTR#  show ip route eigrp  172.68.0.0/16 is variably subnetted, 2 subnets, 2 masks D       172.68.0.0/16 is a summary, 00:07:33, Null0      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks D       10.0.0.0/8 is a summary, 00:07:34, Null0 DCSRTR# 

Using show ip eigrp neighbors displays the connected neighbor and the route summary for the neighbor:

 DCSRTR# DCSRTR#  show ip eigrp neighbors  IP-EIGRP neighbors for process 100 H   Address                 Interface   Hold Uptime   SRTT   RTO  Q  Seq                                         (sec)         (ms)       Cnt Num 1   10.10.20.1              Et0           11 00:08:10 1576  5000  0  11 3   172.68.16.3             Se1           12 00:08:15    0  4500  0  10 2   172.68.16.1             Se0           12 00:08:15    0  4500  0  9 0   10.10.20.3              Et0           13 00:09:07   13   200  0  8 DCSRTR# 

Using show ip eigrp topology displays the topology table for EIGRP:

 DCSRTR# DCSRTR#  show ip eigrp topology  IP-EIGRP Topology Table for process 100 Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,        r - Reply status P 10.0.0.0/8, 1 successors, FD is 281600          via Summary (281600/0), Null0 P 10.10.20.0/24, 1 successors, FD is 281600          via Connected, Ethernet0 P 172.68.0.0/16, 1 successors, FD is 2169856          via Summary (2169856/0), Null0 P 172.68.16.0/24, 1 successors, FD is 2169856          via Connected, Serial0          via Connected, Serial1 DCSRTR# 

The above output shows the members for the entire EIGRP 100 network, and the interfaces on the router to reach each network. Using show ip eigrp traffic displays the hello and routing updates:

 DCSRTR# DCSRTR#  show ip eigrp traffic  IP-EIGRP Traffic Statistics for process 100   Hellos sent/received: 434/544   Updates sent/received: 14/19   Queries sent/received: 0/2   Replies sent/received: 2/0   Acks sent/received: 11/5   Input queue high water mark 3, 0 drops DCSRTR# 

Using show ip eigrp events displays a log of the most recent EIGRP information pertaining to what is happening on the network:

 DCSRTR# DCSRTR#  show ip eigrp events  Event information for AS 100: 1    00:52:19.879 Poison squashed: 172.68.0.0/16 reverse 2    00:52:18.807 Metric set: 172.68.0.0/16 2169856 3    00:52:18.807 Route install: 172.68.0.0/16 0.0.0.0 4    00:52:18.807 FC sat rdbmet/succmet: 2169856 0 5    00:52:18.807 FC sat nh/ndbmet: 0.0.0.0 2169856 6    00:52:18.807 Find FS: 172.68.0.0/16 2169856 7    00:52:18.807 Rcv update met/succmet: 2707456 2195456 8    00:52:18.807 Rcv update dest/nh: 172.68.0.0/16 172.68.16.3 9    00:52:18.799 Metric set: 172.68.0.0/16 2169856 10   00:52:18.799 Route install: 172.68.0.0/16 0.0.0.0 11   00:52:18.799 FC sat rdbmet/succmet: 2169856 0 12   00:52:18.799 FC sat nh/ndbmet: 0.0.0.0 2169856 13   00:52:18.799 Find FS: 172.68.0.0/16 2169856 14   00:52:18.799 Rcv update met/succmet: 2707456 2195456 15   00:52:18.799 Rcv update dest/nh: 172.68.0.0/16 172.68.16.1 16   00:52:18.279 Peer up: 10.10.20.1 Ethernet0 17   00:52:18.079 Metric set: 172.68.16.0/24 2169856 18   00:52:18.079 FC sat rdbmet/succmet: 2169856 0 19   00:52:18.079 FC sat nh/ndbmet: 0.0.0.0 2169856 20   00:52:18.079 Find FS: 172.68.16.0/24 2169856 21   00:52:18.079 Rcv update met/succmet: 2681856 2169856 22   00:52:18.079 Rcv update dest/nh: 172.68.16.0/24 172.68.16.3 DCSRTR# 

In the above output, you see each event that takes place on the router running the EIGRP process. You see the metric set, a routing installed, updates received, and the router finding a network.



CCNP CIT Exam Cram 2 (642-831)
CCNP CIT Exam Cram 2 (Exam Cram 642-831)
ISBN: 0789730219
EAN: 2147483647
Year: 2003
Pages: 213
Authors: Sean Odom

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net