IGP choice

Mikael Abrahamsson swmike at swm.pp.se
Fri Oct 23 21:07:11 UTC 2015

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 

There is no right or wrong IGP to run, both protocols have their quirks 
and pro:s and con:s.

Mikael Abrahamsson    email: swmike at swm.pp.se

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