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IS-IS tuning

Written by Arnaud no comments

Here is a sample configuration of ISIS for Cisco routers. This example is a level2 only zone (like a backbone area 0 in OSPF).

Note : Loopback addresses are (or should be) always configured with /32 netmask.
It's a good practice to dedicate a contiguous range for all your loopback addresses, this range is most of time the first of your supernet (i.e. your "big" network allocation), or the one with a zero.

Let's begin to create an loopback interface and annonce this IP (In this example we will use the first /24 of our "big" range for loopbacks) :

interface Loopback100
 ip address
 ip router isis

We configure the physical interface, annonce the connected range in ISIS and lower hello interval to 1 second :
interface gi0/0/0

 ip address
 ip router isis
 isis network point-to-point
 isis metric 20 level-2
 isis hello-interval 1


And finally the configuration of the protocol :
router isis
 no hello padding
 spf-interval 1 1 100
 lsp-gen-interval 1 1 100
 is-type level-2-only
 metric-style wide level-2
 net 49.0000.0000.0008.00
 passive-interface loopback 100

Each node in ISIS must have a different "net" (= ID), it's a good practice to begin with 49 and end with 00 (see links below).
It is really important to lower sfp-interval to get a faster convergence (~1sec) , lsp-gen-interval helps also but only a few compared to sfp-interval.
Padding is useless (come on, MTU to 9000 for everybody!), so we deactivate it. And we don't need to generate hello packets on the loopback interface, never.

Here are a few commands really important to troubleshoot :
sh isis neighbors
sh isis database
sh isis topology
sh ip route isis

And if you need to announce/originate a default route (make sure your routeur knows how to reach "THE Internet", blackhole spotted otherwise) :
router isis
 default-information originate always

I definitelly recommand reading :
and, if you have a lot of time, and some Juniper knowledge, is wonderful post :

Classified in : cisco Tags : none

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