Devil's Advocate - Segment Routing, Why?

Mark Tinka mark.tinka at
Thu Jun 18 10:42:34 UTC 2020

On 18/Jun/20 12:28, Robert Raszuk wrote:

> To your IGP point let me observe that OSPF runs over IP and ISIS does
> not. That is first fundamental difference. There are customers using
> both all over the world and therefore any suggestion to just use
> OSPFv3 is IMHO quite unrealistic.

Are you saying that OSPF houses that want IPv6 should just move to
IS-IS. Don't get me wrong, I support that very much as I think IS-IS is
a great IGP. That said, while it's good to convince OSPF operators to
consider IS-IS, it's not our place to force them to use it.

Also, OSPFv3-only for your dual-stack IGP needs is a supported
capability. Last time I tested it in Juniper in 2010/2011, it worked
well. I don't know if anyone is actually running IPv4 and IPv6 on OSPFv3
only, but it does work.

> Keep in mind that OSPF hierarchy is 2 (or 3 with super area) while in
> IETF there is ongoing work to extend ISIS to 8 levels. There is a lot
> of fundamental differences between those two (or three) IGPs and I am
> sure many folks on the lists know them.

15+ years ago, I'd have said that one protocol may have been suited to a
specific task than another due to the control plane limitations of the day.

In 2020, with the state-of-the-art of control planes today, it near as
makes no difference, IMHO.

> Last there is a lot of enterprise networks happily using IPv4 RFC1918
> all over their global WAN and DCs infrastructure and have no reason to
> deploy IPv6 there any time soon.

No wonder the vendors aren't seeing any LDPv6, SR-ISISv6 or SR-OSPFv3
demand :-).


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