EIGRP Hello & Hold Timers

Lets take a brief look at EIGRP Hello and Hold Timers.

So, R1 and R2 want to form a Neighbor relationship.

Q. Do the timers need to match?
A. No, but if a hello timer is longer than it's hold timer, or slow links cause that you will have flapping links.


Q. How do I change the Hello and Hold Timers?
A. R1:
interface fa0/1
ip hello-interval eigrp 55 2 (where 55 is the AS number, 2 is the timer value in seconds)
ip hold-time eigrp 55 6 (cisco does not force you to make this 3 times the hello, but it is strongly recommended)


Q. So does this mean I need to make sure R2 uses a hello timer less than 6 to keep my links from flapping?
A. No, the hold time of six is sent to R2 to use. The value set on R2 will be sent to R1 to tell R1 how long to hold waiting for hello messages. The hold time set is the value actually used on the neighboring router.
If multiple routers are on this segment, they will all use this hold time while waiting for R1's next hello.

So you can interpret ip hold-time eigrp 55 6 as "tell all my neighbors with eigrp 55 to use a 6 second hold time when they listen for me".

R2 could use 10/30 as it's hello/hold-timers and the nieghborship would stay up.


Q. How can you verify your hello times?
A. show ip eigrp interface detail interface

Q. Does that show the hold timer as well?
A. No, you need to look at the show run or use show ip eigrp neighbors command and watch the countdown until it resets to it's highest value.

Q. How does the router send Hellos?
A. Routers using EIGRP send Hellos to multicast 224.0.0.10 attempting to find potential neighbors.


For general hello packet and neighboring information, read my hello packet post from last year