[184873] in North American Network Operators' Group
Re: IGP choice
daemon@ATHENA.MIT.EDU (Mikael Abrahamsson)
Fri Oct 23 17:07:15 2015
X-Original-To: nanog@nanog.org
Date: Fri, 23 Oct 2015 23:07:11 +0200 (CEST)
From: Mikael Abrahamsson <swmike@swm.pp.se>
To: Pablo Lucena <plucena@coopergeneral.com>
In-Reply-To: <CAH+2GVJNxeydYyy8797oNA-fhwppg48etp03htrsCfjvC_1LNg@mail.gmail.com>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
On Fri, 23 Oct 2015, Pablo Lucena wrote:
>> A lot of carriers use ISIS in the core so they can make use of the'
>> overload bit' with a 'set-overload-bit on-startup wait-for-bgp". Keeps
>> them from black holing Traffic while BGP reconverges., when you have
>> millions of routes to converge it can take forever. It's also a really
>> handy tool when you're troubleshooting or repairing a link, set the OL
>> bit, and traffic gracefully moves, then when you're done it gracefully
>> moves back. You can do the same thing with the Metric, and Cost in OSPF,
>> just not quite as elegant.
>>
>
> That feature is also present in OSPF. 'max metric router-lsa'.
This is not exactly the same thing as overload-bit set, but it can be
argued that setting max-metric actually makes more sense than what the
overload bit does.
The choice between IS-IS and OSPF depends more on soft than hard factors.
OSPF support is more widespread amongst smaller equipment vendors, IS-IS
is the traditional choice for large ISP core IGP, mostly due to the Cisco
codebase for IS-IS happened to be more stable than OSPF around 1995, and
that's when a lot of larger ISPs started running these protocols, and that
stuck.
There is no right or wrong IGP to run, both protocols have their quirks
and pro:s and con:s.
--
Mikael Abrahamsson email: swmike@swm.pp.se
