IS-IS's relationship with CLNS

CriscoSystems · ‎10-31-2009

I know I posted about this before, and Giuseppe was very helpful. But still...

What really is the interplay between IS-IS and CLNS? There are claims in my books that IS-IS uses CLNS to transport its own routing info. In the real world, I know that IS-IS is its own Layer 3 protocol, separate from CLNP; and I also set up a tiny IS-IS environment in my lab in which two routers were configured with IS-IS routing and "ip router isis" on their interfaces. There were some loopbacks on one of the routers, and those loopback networks appeared in its neighbor's ip table as IS-IS routes (lowercase "i" on the left).

And when I did "show clns neighbors" I got a "%CLNS not running" message.

So it seems to me like IS-IS can get along fine without using CLNS at all. Or, did it work in the above case just because the routers were on the same broadcast medium? Do Level 2 routers need CLNS, since they do a fair amount of remote routing?

IS-IS is one of the scariest parts of the exam for me (along with IPv6, multicast routing, BGP prefix lists, route reflectors, regular expression, EIGRP query scoping, 6to4 tunneling...)

Giuseppe Larosa · ‎10-31-2009

Hello Stuey,

as we have discussed in previous threads IS-IS has its own encapsulation over OSI Layer2 (an 802.2 encapsulation and uses 3 bytes more), and it is not an upper layer protocol of CLNP.(comparison with OSPF is quite natural OSPFv2 uses IPV4 protocol type 89)

CLNP/CLNS has been the first routed protocol for which IS-IS has provided routing services.

Later support for IPv4 routing has been added.

the IS-IS net that we configure inside IS-IS process is actually a CLNS/CLNP address.

So we can say that ISIS uses addressing services of CLNP/CLNS to identify each node.

The need of CLNS routing enabled can come in some older IOS versions and it is a question of implementation.

Current IOS releases don't need the command

clns routing

in global config

to be able to route IP both at L1 and at L2.

There have been IOS releases where this command was needed to be able to use IS-IS to route IP.

This is not your case as you see isis routes in routing table.

About show commands:

just to give an example in IOS XR:

sh clns neighors

not supported

sh isis neighbors

supported provides the same info.

so about show commands I can only say that in real life be prepared for all these possible variations.

For the exam your understanding of ISIS should be enough.

Hope to help

Giuseppe