[afnog] afnog Digest, Vol 54, Issue 23

[Mark Tinka]

On Wednesday 24 September 2008 18:07:04 Peter Nyamukusa 
wrote:

>     *  IS-IS does not use level 3 routers.

Well, L3 routing is inter-domain. OSI's approach was IDRP.

> * Unlike OSPF, IS-IS routers are not required
> to be connected to a contiguous backbone area. In fact,
> the backbone area can also be segmented in IS-IS.

A major advantage when expanding your network beyond 
multiple regions.

>     * With IS-IS, there is no restriction that all
> backbone routers (level 2 routers) be contiguous
>       such as the backbone area of OSPF.

Actually, there is... 

If you deploy a hierarchical IS-IS network (L1 routers 
within the PoP's, L2 [or rather, L1/L2] routers in the 
core), your L2 backbone MUST be contiguous.

The merit is that L1 routers don't need to be contiguous 
between PoP's. Route leaking ensures optimal routing 
(longest match) between PoP's.

>     * In OSPF all areas must be directly linked to area
> 0, and the backbone area must also not be segmented.

A problem for networks that build "wide".

>     * With IS-IS, the backbone area can be more easily
> extended since all L2 routers need not be linked directly
> together.

Actually, it's the other way around; L1 routers do not need 
to be linked directly.

However, L2 or L1/L2 routers should be contiguously 
deployed. L2 or L1/L2 "snake" their course in between L1 
routers across the various regions an ISP may deploy its 
network.

>     * With regard to CPU use and the processing of
> routing updates, IS-IS is more efficient
>       than OSPF.

IS-IS is fundamentally less chatty than OSPF on the wire. 
Both IGP's have several knobs and switches that can be 
manipulated to make the deployment even more scalable, 
e.g., the LSP lifetime can be configured up to 18.2hrs in 
IS-IS, which makes it quite suitable for large deployments, 
e.t.c.

It's also good to note that IS-IS was easily extendable for 
the inclusion of new features, e.g., IPv6 routing, as it 
utilizes TLV's, rather than OSPFv2 which uses the "Options" 
field (already depleted) - hence the transition to a new 
OSPF protocol, OSPFv3.

Suffice it to say that 'draft-ietf-ospf-af-alt-07.txt' 
(expired the last time I checked) attempts to extend OSPFv3 
by adding multiple address families, one of which is IPv4 
(Unicast and Multicast). Of course, it runs over IPv6, so 
an IPv6 network would be a requirement for running IPv4 
over OSPFv3. Vendor support has already emerged from 
Juniper... but I digress.

And then of course, IS-IS runs directly over the data link 
layer.

> Try to design your network so it can run in a single
> area/level which makes things like TE/FRR...

I believe a hierarchical network design provides better 
scalability.

Besides, I'd design my IS-IS backbone with IP in mind. Other 
features like MPLS-TE and FRR are secondary (as they depend 
on a stable IP foundation).

However, it is true that inter-area CSPF for MPLS-TE does 
not exist today (major vendors have planned to support it, 
though). LSR's have to be 'loosely' defined in the path 
list for RSVP to successfully signal an inter-area LSP.

We, generally, treat MPLS-TE as a tactical response to a 
potential congestion problem, rather than a lasting 
solution, especially since very few points would be 
affected in the grand scheme of things... so manual 
inter-area MPLS-TE isn't an issue.

Cheers,

Mark.
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