Comments on: IS-IS and fast convergence ongoing tricks

By: Kamel

Kamel — Tue, 19 Jan 2010 14:26:46 +0000

For my last project I had to study this comparison
ISIS vs OSPF. I had the chance to compare two different famous network equipments vendor. Finally I have understood that it’s really dependant of what you are building and for who (SP, Corpo etc …). In addition to that, note that it’s also interesting to focus on the services you have to setup L3vpn, L2vpn, TE, etc… . In our case ISIS was more adapted since we have to setup L2vpn and that TRILL (a new protocol that will be very usefull) is developped for ISIS.

Kamel

By: islam

islam — Wed, 09 Sep 2009 21:53:11 +0000

i want really . why is is-is more flexible and stable than ospf ? give me examples?

By: mmahmoud

mmahmoud — Wed, 15 Jul 2009 09:51:45 +0000

Hi,

I must admit that you are raising very interesting points, I really enjoy this discussion.

Like I’ve said in my first comment you’ll have to evaluate your needs, tools and resources to reach what best fits your design, there is no one answer to the question of which is the best, tuning IGP convergence might be the best approach in some circumstances/networks while in others it’s not. Generally yes relying on MPLS-TE should be the best approach for large scale networks and in such case tuning the IGP would be of minor need, but again I don’t admire generalizing.

For the second point, after doing some testing in the lab, what really happens is that this command stops IS-IS from exchanging the IP reachability information, but this doesn’t intervene by any mean with the exchange of the traffic engineering extensions, and thus the shortest unconstrained path calculation is not affected – you can do a simple lab to test this using three routers in-line and use “debug isis mpls traffic-eng advertisements” and “debug isis mpls traffic-eng events”.

I hope that I’ve been informative.

BR,
Mohammed Mahmoud.

By: wwwzen

wwwzen — Tue, 14 Jul 2009 23:41:10 +0000

Hi,
I was just
pointing out the fact that IGP convergence mechanism
is no longer relevant and networks will have to rely
on pure MPLS-TE schemes. Having said that, how would it impact CSPF and the TED database if ISIS does not provide
link information.

regards

By: mmahmoud

mmahmoud — Fri, 03 Jul 2009 20:16:24 +0000

Hi,

I am not sure that I’ve understood you thoroughly. Actually when IS-IS stops sending the connected IP prefixes in the LSPs, the network routers won’t be having these IP addresses in their routing table, and thus their label binding won’t be populated into the LFIB, thus we are now left only with MPLS traffic between the loopback IP addresses, which is enough for a MPLS based Service Provider (if any other traffic flow requires to be label switched for any reason, you can still manipulate things and make it work).

Regarding MPLS re-route, I believe that using MPLS TE and FRR is a crucial tool for a MPLS based Service Provider, it provides a flexible, deterministic and well controlled network – If your point was the interaction of the described feature with MPLS TE, I haven’t tried this out (needs to be labed), but from a higher design level, generally when using MPLS TE don’t bother tuning the IGP.

For an overall good design you’ll have to evaluate all the tools you have, which of them is supported on the platforms and IOS codes you are running, the pros and cons for each of them in your particular network topology and finally lab things until you completely feel the behavior.

I hope that I’ve been informative.

BR,
Mohammed Mahmoud.

By: wwwzen

wwwzen — Tue, 30 Jun 2009 00:57:08 +0000

Could you discuss the convergence alternatives
in this case i.e when connected information is not
advertised. Is it going to be purely MPLS re-route
schemes. It would also impose restrictions on
usagee of MPLS forwarding adjacencies.
THanks