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Re: OSPF vs IS-IS
From: Vinny Abello <vinny () abellohome net>
Date: Sat, 13 Aug 2011 21:11:05 -0400

On 8/11/2011 10:19 AM, Jason Duerstock wrote:
On Thu, Aug 11, 2011 at 8:57 AM, CJ <cjinfantino () gmail com> wrote:

Hey all,
 Is there any reason to run IS-IS over OSPF in the SP core? Currently, we
are running IS-IS but we are redesigning our core and now would be a good
time to switch. I would like to switch to OSPF, mostly because of
familiarity with OSPF over IS-IS.
 What does everyone think?

--
CJ

http://convergingontheedge.com <http://www.convergingontheedge.com>

Granted, we're not a service provider, so we operate on a different scale
here, but one interesting trick that can be done with ISIS (at least on
Cisco) is this:

router a
-----------
router isis
advertise passive-only

interface loopback0
ip address 10.1.1.1 255.255.255.255

interface vlan2
ip unnumbered loopback0
ip router isis
isis network point-to-point


router b
-----------
(copy router isis definition from router a)

interface loopback0
ip address 10.1.1.2 255.255.255.255

(copy vlan2 definition from router a)

-----------

This removes the associated headaches with /30s or /31s in having to keep
track of their allocation, as well as having them clog the your routing
table.

-waits for replies stating why this is a bad idea-

Now, if I could just get isis-per-vrf-instance support on the Catalyst 6500.

Jason
One of my favorite features in IS-IS is the ability to set the overload
bit during maintenance. The effect is the router on which you set it
isn't seen by any other devices in the topology as a transit path, but
you can still reach the router itself. I'm not as familiar with OSPF so
I'm unsure if there is a similar feature, but I thought it was exclusive
to IS-IS. Being able to easily limit the IGP size via the above
technique is also a great benefit. You can basically get away with just
your loopbacks.

-Vinny



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