EIGRP Hello Packets & Hold Timers

[jk865]

Hi All. 

I have a simple topology with three routers connected by serial interfaces with loopback interfaces and two switches connected by Gigabit interfaces , I have the option to modify the hello and hold timers , in a production network what value would I change them to and for what reason? 

Thanks in advance. 

[Cristian Matei]

Hi,

    First off, keep in mind that EIGRP does not require hello/holdtime values to match for establishing adjacency, however you'll always have it matching to avoid adjacency flaps.

   The sole reason of lowering the values is to get faster initial convergence (how long does it take for adjacency to come up) and faster convergence during network events to avoid potential traffic blackholing (how long does it take to detect adjacency failure).

    What values you'll end up using, it depends on two factors:

1. scale: how many EIGRP adjacencies do you have on same router, cause if you have thousands, the lower the timers, the higher the impact on CPU
2. link stability: if links are unstable and you use lower timers, you'll have constant adjacency flaps

Usually, nowadays, you'll leave EIGRP default timers (will affect initial convergence, unless you have strict requirement in which case you'll move to lowest available values, except on unstable links where you leave the defaults) and use BFD to detect fast adjacency failure (at ms level, except for unstable links where you don't use BFD but rely on EIGRP timers).

Best,

Cristian.

[MHM Cisco World]

Slow link need long intervals hello timer

So long link length' or multi hops link (using vpn) need long intervals hello timer.

MHM

Faster link can use defualt hello timer.