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: swm...@swm.pp.se
