In the first part of this series, we subdivided the processes of EIGRP into four discrete steps, and detailed troubleshooting the first two. This is taken from the 5-Day CCNP bootcamp:
- Discovery of neighbors
- Exchange of topology information
- Best path selection
- Neighbor and topology table maintenance
Let us now discuss path selection and maintenance troubleshooting.
We should all remember that we can view the topology table of EIGRP with the command show ip eigrp topology. Here we can see the successor routes (these are the best routes that are placed in the routing table) and we can see the second best routes, the feasible successor routes. These feasible successor routes are the key to the lightening fast convergence that EIGRP can offer us. When a speaker loses its successor, it can quickly install a feasible successor route in its place.
We need to remember the important rule of feasible successors. The advertised distance of the proposed feasible successor must be less than the feasible distance of the current successor route. This is actually a loop prevention mechanism.
Another big gotcha when it comes to path selection in EIGRP is the configuration of variance to unequal cost load balance. I can remember fighting with this in an INE practice lab long ago when I was preparing for the exam. Something I had no idea of back then...in order to be considered for the unequal load balancing, the alternate paths must be feasible successors! Older editions of CCNP courses never thought to tell us that little nugget!
We should be careful when modifying bandwidth to effect path selection. Cisco gave us delay for this purpose. Modifying the bandwidth can starve EIGRP updates of bandwidth to use. Remember, by default, EIGRP will only use 50% of an interface's bandwidth. We can control this with the command ip bandwidth percent eigrp.
For table maintenance, show ip eigrp topology is critical. Note that in this table, passive is what we want to see. Active indicates there is not a feasible successor and neighbors are being queried for an alternative path. SIA log messages indicate a Stuck in Active issue. Here the router is not receiving a reply to queries. The most common reasons this can occur:
- Bad link
- Congested link
- The query scope if too big (too many routers involved)
- Excessive redundancy is built into the network
- The router CPU is overloaded
- There is a shortage of memory on the router
- There are software defects
When it comes to table maintenance, another excellent troubleshooting command is show ip eigrp topology summary. This command displays the total number of routes in the topology table and the total number of queries the router is waiting on responses for. It also shows a quiescent interface field that shows which interface have no outstanding packets to be sent or acknowledged.
Some of our favorite EIGRP verification commands:
- show ip route eigrp
- show ip protocol
- show ip eigrp neighbor
- show ip eigrp topology
- show ip eigrp topology all-links
- show ip eigrp topology summary
- debug eigrp packet hello
- debug eigrp packet query reply