EIGRP Metric Hop Filtering

I’m actually a little embarrassed to admit that I never knew about this feature.  I work with EIGRP on a daily basis at work and somehow never ran across it during the INE workbooks either.  Then again, it’s not something you’d probably ever see in a production environment.

We can (soft of) mimic how RIP deals with maximum hop routes.  As you are well aware, RIP’s maximum hop count is 15.  Any route with a hop count greater than 15 will not be installed in the routing table.  We can do the same with EIGRP and what’s cool is that we can set the metric maximum-hops to any number we like (up to 255).

First, we can see that EIGRP has a default maximum-hops of 100:

Rack1R3#sh ip protocols | i hop
EIGRP maximum hopcount 100

Let’s check our topology table to see the actual hop-count for a route.

Rack1R3#sh ip ei top 192.10.1.0/24
IP-EIGRP (AS 10): Topology entry for 192.10.1.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2561024256
Routing Descriptor Blocks:
167.1.13.3 (Serial0/1), from 167.1.13.3, Send flag is 0x0
Composite metric is (2561024256/2560512256), Route is External
Vector metric:
Minimum bandwidth is 1 Kbit
Total delay is 40010 microseconds
Reliability is 1/255
Load is 1/255
Minimum MTU is 1
Hop count is 2
Next, I’ll make the max hop-count 1 and check if that route is filtered.  I *like* RIP and wanted to make an EIGRP-RIP love-child with a maximum-hop of 15, but my topology isn’t big enough.  🙂
Rack1R3(config-router)#metric maximum-hops 1
Let’s verify the topology table and make sure the command took effect:
Rack1R3#sh ip ei top 192.10.1.0/24
% IP-EIGRP (AS 10): Route not in topology table
Rack1R1#sh ip prot | i hop
EIGRP maximum hopcount 1

It worked! Forget distribute-lists, we all know hop counts are where it’s at! 😉

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