EIGRP - Hop Count
When configuring EIGRP, the hop count for a particular route plays a role in how EIGRP functions.
Unlike RIP, EIGRP does not use hop count as a parameter that directly affects the calculated metric. This is true for both classic and wide-metrics EIGRP. However, hop count still plays a role in EIGRP, but it is used differently.
It serves as a limit to prevent routing loops and to control the scale of routing domains. EIGRP has a default maximum hop count limit of 100, which can be configured up to a maximum of 255. This limit ensures that routes with hop counts exceeding this threshold are considered unreachable, thus helping to maintain network stability and manageability. If the hop count exceeds this limit, the route is considered unreachable and is given a metric of infinity (i.e. 4294967295 for classic EIGRP).
When issuing the show ip protocols command, you can see the maximum hop count that is used by EIGRP. The following is an example of this output:
R1#show ip protocols Routing Protocol is "eigrp 12" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: eigrp 12 EIGRP NSF-aware route hold timer is 240s Automatic network summarization is in effect Maximum path: 4 Routing for Networks: 192.168.12.0 Routing Information Sources: Gateway Distance Last Update Distance: internal 90 external 170
You can see the maximum hop count indicated to be 100. For most implementations this would remain unchanged, however, for extensively large EIGRP networks, you can change this value using the following EIGRP router mode command:
R1(config)#router eigrp 1 R1(config-router)#metric maximum-hops ? <1-255> Hop count R1(config-router)#metric maximum-hops 200 R1(config-router)# *May 7 07:36:25.802: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 192.168.12.2 (GigabitEthernet0/1) is down: Max hopcount changed *May 7 07:36:29.422: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 192.168.12.2 (GigabitEthernet0/1) is up: new adjacency
The above command set the maximum hops value for EIGRP for the local router to 200. Note that this resulted in the adjacency going down and being renegotiated.
The maximum hop count is not something that is propagated throughout the network. However, it does influence how a router treats routes in terms of their validity based on the hop count—whether to consider a route as reachable or unreachable. If a route exceeds the configured maximum hop count on the local router, the local router deems it invalid and it is not used for routing decisions.
Links
https://forum.networklessons.com/t/eigrp-wide-metrics/2114/26?u=lagapidis