Understanding EIGRP #1

prepared by SURAJ (imeansuraj@gmail.com)
  • Replaced the Interior Gateway Routing Protocol
  • It was Cisco proprietary till mid-2013.
  • Doesn’t use UDP or TCP since it doesn’t need the port numbers to work with. It uses Cisco’s Reliable Transport Protocol (protocol number 88).
  • Administrative Distance of a routing information from an EIGRP enabled router outside the Autonomous System is 170 and inside the Autonomous System is 90.
  • Creates an autonomous system including all the routers that it wants to transfer data within, this autonomous system is of 16 bits.
  • New metric of EIGRP is of 64 bits, the earlier composite metric of EIGRP was of 48 bits while IGRP had a metric of 24 bits
Working
-  Discovers neighbours by sending hello packets
-  Exchanges information with hellos via 224.0.0.10 (multicast address for EIGRP)
-  As soon as the router doesn’t receive them in the hold time it will drop the neighbour relationship called adjacency and EIGRP might have to look for another path for certain destinations.
-  If the hello packet are received, data is updated with query message on topology table.
-  Diffusing algorithm is run to see the best path called the feasible distance (also detects second best path, feasible successor)
- Total distance is called: reported distance/ advertised distance (distance shared by the neighbouring router).

Three main tables maintained by the EIGRP protocol in the Routers are:
  • Neighbour Table: Information about all adjacent routers running EIGRP are stored here. This information includes sequence numbers and protocol timers.
  • Topology Table: All destination networks that neighbour routers have reported knowing about are stored in this table. This table would include the metrics for every route reported, as some network ID may have multiple routes and the best route would be evaluated by the cost of the metrics.
  • Routing Table: In addition to least cost routes, EIGRP evaluates secondary routes to each network and creates a list of feasible successors that are added to the routing table. A feasible successor is a route that would be used if the primary route to a network fails. This is saved in routing table.
The information that EIGRP receives in its updates go into these three tables.

Features:
  • Support for Classless Inter-Domain Routing (CIDR) and variable length subnet masking. Routes are not summarized at the classful network boundary unless auto summary is enabled.
  • Support for load balancing on parallel links between sites.
  • The ability to use different authentication passwords at different times.
  • MD5 authentication between two routers.
  • Sends topology changes, rather than sending the entire routing table when a route is changed.
  • Periodically checks if a route is available and propagates routing changes to neighbouring routers if any changes have occurred.
·  Router# configure terminal
·   Router(config)# router eigrp 1 [autonomous system number]
·   Router(config-router)# network 12.0.0.0 [network the router is attached to]
·   Router(config-router)# no auto-summary
·   Router(config-router)# exit

Router1# show ip eigrp topology 10.0.0.1 255.255.255.255
 IP-EIGRP topology entry for 10.0.0.1/32
   State is Passive, Query origin flag is 1, 1 Successor(s), FD is 40640000
   Routing Descriptor Blocks:
   10.0.0.1 (Serial0/0/0), from 10.0.0.1, Send flag is 0x0
       Composite metric is (40640000/128256), Route is Internal
       Vector metric:
         Minimum bandwidth is 64 Kbit
         Total delay is 25000 microseconds
         Reliability is 255/255
         Load is 197/255
         Minimum MTU is 576
         Hop count is 2
(notes compiled and collected from various sources)

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