Troubleshooting EIGRP

What it is

EIGRP is an advanced distance vector protocol that uses the DUAL algorithm to find loop free paths quickly.

It runs in classic mode, where each setting is spread across the router and interface configuration, and named mode, where everything sits under one router eigrp NAME block. Both run for IPv4 and IPv6 address families, in the global table or inside a VRF.

EIGRP sends hellos to neighbours, builds a topology table, and installs the best routes. It sends partial updates only when something changes.

How it works

Troubleshoot in this order: neighbours, then the topology table, then the routing table.

Neighbour relationships

Two routers become neighbours only if several things match.

• The same autonomous system number, or the same named mode address family AS.
• The same K values, which are the metric weights.
• An address on the same subnet on the connecting interfaces.
• Matching authentication, if used.
• The interface is not passive.

Hellos use multicast 224.0.0.10 for IPv4 and FF02::A for IPv6. Hello and hold timers do not have to match, although mismatched timers can cause flapping.

R1# show ip eigrp neighbors
R1# show ip eigrp interfaces
R1# show ip protocols

An IPv6 only router has no IPv4 address to borrow, so set a 32 bit router ID with eigrp router-id.

Authentication

Classic mode uses a key chain on the interface. Named mode sets authentication inside the address family interface settings.

R1(config)# router eigrp ENARSI
R1(config-router)# address-family ipv4 unicast autonomous-system 100
R1(config-router-af)# network 10.0.0.0 0.255.255.255
R1(config-router-af)# af-interface GigabitEthernet0/0
R1(config-router-af-interface)# authentication mode hmac-sha-256 0 MYPASSWORD
R1(config-router-af-interface)# exit-af-interface
R1(config-router-af)# topology base
R1(config-router-af-topology)# variance 2

Classic MD5 uses ip authentication mode eigrp 100 md5 and ip authentication key-chain eigrp 100 KEYS on the interface. A key mismatch, or a key chain whose lifetime has expired, stops the adjacency.

DUAL terms

DUAL uses these terms to choose paths without loops.

If the successor fails and a feasible successor exists, EIGRP switches immediately. If not, the route goes active and the router sends queries to neighbours.

A route is stuck in active (SIA) when a query goes unanswered for too long. The neighbour is then reset. Stubs and summaries limit how far queries travel, which reduces SIA events.

R1# show ip eigrp topology
R1# show ip eigrp topology all-links

The first command shows only successors and feasible successors. The all-links form also shows paths that fail the FC.

Metrics

The classic composite metric uses bandwidth and delay by default. With default K values, the metric is 256 times the sum of the scaled bandwidth and the total delay.

The bandwidth part comes from the slowest link on the path. The delay part is the sum of the delays along the path. Named mode uses wide metrics, which suit very fast links better than the classic metric.

Changing bandwidth or delay on an interface changes the metric. Change delay rather than bandwidth when steering traffic, because delay affects only EIGRP path choice.

Stubs

A stub router tells neighbours it is not a transit path, so they do not query it.

R2(config-router-af)# eigrp stub connected summary

The default stub options are connected and summary routes. A stub that needs to advertise redistributed or static routes has to add those options.

Load balancing

EIGRP load balances equally across paths with the same metric. The maximum-paths command controls the number of equal paths.

Unequal cost balancing uses the variance command. A path is used if it is a feasible successor and its metric is less than the variance multiplied by the best FD.