Where's my IS-IS folks?

Hey guys..so looking through the forums..I find plenty of EIGRP and OSPF convo's..not much on IS-IS. I just have to say, IS-IS has some weird characteristics. Aside from the wild addressing and terminology, it seems like the key to IS-IS lies in understanding how L1, L2, and L1/2 routers function in theory, because that comes into play a lot...it's huge.

I've been experimenting with a 6 router topology, trying to tweak this stuff to get a lock on it. I must say, the BSCI self-study guide from Cisco press is lacking a LOT of key IS-IS information it seems. I'm using Doyle's Volume I TCP/IP for most of the good stuff...

So what are some things that may be nice to know while screwing with IS-IS? I've already learned that depending on the topology, the L1/2 router may or may not send a default route into an L1 router, so route leaking may be required in some cases..leaving a full route table at one end of the topology, but the L1 unable to find routes back the other way, etc...

Just looking to share a bit of info as there's definitely a lot of weird stuff going on in IS-IS, especially in NBMA environments.

Find more posts tagged with

SAVE $250 on Your CISCO Boot Camp — Use Code "EXAM26"

Exclusively for TechExams members for Infosec Boot Camps starting before April 30, 2026

View CISCO Boot Camps

Comments

lildeezul

I always wanting to know, Whos still runs ISIS..

according to the cisco press, it is kinda popular.. I just never seen ( then again, i dont have much experience , due to age) it in a company

Mrock4

I have yet to see it in a live network..but my brother used to work at a service provider who ran IS-IS. That's about it though. It's a pretty slick protocol..just very different from the norm.

Trying to get it working properly over NBMA now. This will be fun.

GT-Rob

I have yet to see or hear of a company that runs it. Although I also have not seen ipv6 run more than a test environment either.

I really think IS-IS should be taken off of the CCNP.

Mrock4

I think that if IS-IS isn't covered on the CCIE..it's kind of ridiculous to have it on the BSCI, but oh well. I'll learn it.

As a side note..our network is very close to implementing IPv6 foundry gear. Pretty cool..scary at the same time.

JohnDouglas

I've seen it in large networks. Think it was just used intrasite though.

dynamik

GT-Rob wrote:

I have yet to see or hear of a company that runs it.

I'm sure this is a stupid question for you Cisco geniuses, but why is that? I haven't come across it in my studies and am just curious. TIA.

Mrock4

I'm nowhere near a "genius", but I'll give my $.02...

I think the prevailing reason most companies opt to use OSPF as opposed to IS-IS is the lack of trained personnel with IS-IS, and the lack of documentation versus the available resources on OSPF. It really is quite insane how many people are pretty good with OSPF, but don't know a thing on IS-IS.

There's really not a big enough benefit of IS-IS over OSPF to search for network engineers who know IS-IS, when you could just use OSPF, and have near the same performance, but with more support. When employed properly, both will converge very quickly.

lildeezul

There's really not a big enough benefit of IS-IS over OSPF to search for network engineers who know IS-IS,

I sorta agree, but ISIS is definitely has benefits over ospf... although OSPF can be tuned to reach these specs, ISIS has its benefits..

it supports TLV which can easily spand to cover newer technology that is implement, instead with OSPF you have to create new LSA, with isis you can just as a TLV at the end..

also ISIS runs right into the data link layer, and can perform PRC which can be easier on the cpu than the SPF algorithm. Also ISIS uses the hexadecimal addressing scheme, and can be effecivly planned and designed.

also the ISIS backbone is spread across the L2 routers ( L2 of the L1/2 routers also), this is a better design than having a single backbone, and everything has to connect to area 0. with isis, the backbone is spread through all areas. very flexible.

Although these are just some of the benefits ospf can be tuned to reach these heights

but yeah i agree, ISIS is lacking the technical support personal and documents.