OSPF vs IS-IS

Vinny Abello vinny at abellohome.net
Sat Aug 13 20:11:05 CDT 2011


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