EIGRP – The Basics
Today marked the start of my CCNP studying. The first topic I found on the Blue Print was EIGRP. While I have used EIGRP for years now I have to admit did not fully understand how the protocol worked and how to get the most out of it. Like I did with my CCVP I will be making posts of useful information.
Neighbor Table
Each router is responsible for keeping state information about their adjacent neighbors. When a neighbor is discovered the address and interface of the neighbor is recorded. Each of the protocol dependent modules has their own neighbor table. During the Hello phase a HoldTime is advertised. This time is the amount of time a router waits between Hellos before marking the neighbor as unreachable and non-operational. If this occurs DUAL is informed of the topology change. Along with this information sequence numbers are stored to help detect out of order packets. A transmission list is used to queue packets for retransmission on a per neighbor basis.
Topology Table
The Topology Table is managed by the protocol dependent modules and accessed by the DUAL finite state machine. It contains the destinations advertised by neighboring routers. Each entry is contains the destination address and a list of neighbors that advertise the destination. For each neighbor the advertised metric is recorded. This is the metric that the neighbor stores in its routing table.
The router then takes the sum of the best advertised metric from all neighbors plus the link cost to the best neighbor. This is the metric that the router uses in its own routing table.
Feasible Successors
Feasible Successors are routes that can be installed into the routing table if a route goes away. As long as the neighbor is considered downstream it will consider it a feasible successor. This allows a new route to be selected without having to re-compute the routing table.
Viewing Route Details
To view the detailed information regarding the routes in the router you will want to issue the following command. If you have two routes installed in the route table to the same destination it will tell you which route will be used.
Routing entry for 192.168.1.0/24
Known via “eigrp 1″, distance 90, metric 3321856, type internal
Redistributing via eigrp 1
Last update from 172.16.1.17 on Serial1, 00:25:41 ago
Routing Descriptor Blocks:
172.16.1.17, from 172.16.1.17, 00:25:41 ago, via Serial1
Route metric is 3321856, traffic share count is 1
Total delay is 65000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 3
* 172.16.1.13, from 172.16.1.13, 00:25:41 ago, via Serial0
Route metric is 3321856, traffic share count is 1
Total delay is 65000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 3
R4#
The route with the * next to it will be selected for the traffic at that destination. By default the route selection is determined by Delay and Bandwidth. If they metric isn’t the same for two routes you will only see a single route installed. You can influence these selections by various means which I will cover in a later post.