The MPLS-OPS Archive

[John Smith <jsmith4112003@yahoo.co.uk>]

Sunil,

Please find my comments inline prefixed with J>

[snipped]
I work for a large ISP  which currently has OSPF for IGP,running MPLS VPN and are headed
for implementing QOS and Traffic Engineering and moving towards migrating to IPV6 and
becoming a Tier 1 ISP.Since most Tier 1 ISPs are running IS IS as their IGP,I have been
working on looking at IS IS in relation to OSPF to deciede if IS IS holds any significant
advantages keeping mind two key aspects of convergence,scalability and support as the
network grows.I have a few questions with regards to the same.

J> I dont see any difference between the key aspects that you mention (scalability,
convergence, etc) when comparing ISIS and OSPF. For all sane topologies (and sometimes
for the ones which are not too sane also) both protocols work fine!

There has been a draft submitted recently in IETF which specifically talks of all this. I
dont remember the exact name but google for "difference isis ospf ietf" and you would
find one.

coming to your specific questions .. 

1.Does IS IS have the flexibility of supporting both v4 and V6 implementations?Is the
same available with OSPFv2 aswell?

J> Adding a couple of TLVs makes ISIS support IPv6. You cant support IPv6 with OSPFv2.
YOu need to run a separate copy of OSPFv3 to do that. If that really *hurts* you, then
you can think of using ISIS.

2.Does IS IS hold an edge over OSPF in anyway for Traffic Engineering implementations?

J> Not really.

3.With regards to the basic routing functionality ....can it be safely said of IS IS as
being more scalable with its support for larger areas and a more strict hierarchy model?

J> Refer to that draft. I remember they had a very good discussion on this.

4.What in your opinion are the key benefits would a Tier 1 ISP stand to
derieve from IS IS implementation of the two level hierarchial Link 
state protocol.

J> OSPF is also 2 level hierarchial link state protocol (just in case you missed that!).
Its was eons back when you really benefitted by running ISIS, since it never leaked your
L2 routes into L1. The downside being that you suffered from non optimal routing. To
solve the latter, folks started leaking L2 into L1, which brought ISIS more closer to
OSPF! 

The upside with OSPF is that its a much more mature protoocol with a lot of stable
implementations running around. Am not so sure about ISIS!

Just my 0.02$

Smith



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