Devil's Advocate - Segment Routing, Why?

Mark Tinka mark.tinka at
Wed Jun 17 22:17:20 UTC 2020

On 17/Jun/20 19:38, Saku Ytti wrote:

> I don't like this, SR-MPLS and SRv6 are just utterly different things
> to me, and no answer meaningfully applies to both.

I know they are different, but that was on purpose, because even with
SR-MPLS, there are a couple of things to consider:

  * IOS XR does not appear to support SR-OSPFv3.
  * IOS XE does not appear to support SR-ISISv6.
  * IOS XE does not appear to support SR-OSPFv3.
  * Junos does not appear to support SR-OSPFv3.
  * MPLS/VPN service signaling in IPv6-only networks also has gaps in SR.

So for networks that run OSPF and don't run Juniper, they'd need to move
to IS-IS in order to have SR forward IPv6 traffic in an MPLS
encapsulation. Seems like a bit of an ask. Yes, code needs to be
written, which is fine by me, as it also does for LDPv6.

> I would ask, why do we need LDP, why not use IGP to carry labels?
> Less state, protocols, SLOC, cost, bug surface

I'd be curious to understand what bugs you've suffered with LDP in the
last 10 or so years, that likely still have open tickets.

Yes, we all love less state, I won't argue that. But it's the same
question that is being asked less and less with each passing year - what
scales better in 2020, OSPF or IS-IS. That is becoming less relevant as
control planes keep getting faster and cheaper.

I'm not saying that if you are dealing with 100,000 T-LDP sessions you
should not consider SR, but if you're not, and SR still requires a bit
more development (never mind deployment experience), what's wrong with
having LDPv6? If it makes near-as-no-difference to your control plane in
2020 or 2030 as to whether your 10,000-node network is running LDP or
SR, why not have the choice?

> And we get more features to boot, with LDP if you want LFA, you need
> to form tLDP to every Q-space node, on top of your normal LDP, because
> you don't know label view from anyone else but yourself. With SR by
> nature you know the label view for everyone, thus you have full LFA
> coverage for free, by-design.
> Also by-design IGP/LDP Sync.
> So no need to justify it by any magic new things, it's just a lot
> simpler than LDP, you don't need to need new things to justify
> SR-MPLS, you need to want to do existing things while reducing
> complexity and state.

Again, it's a question of scale and requirements. Some large networks
don't run any RSVP, while some small networks do.

I'm not saying let's not do SR; but for those who want something mature,
and for those who want something new, I don't see a reason why the
choice can't be left up to the operator.

Routers, in 2020, still ship with RIPv2. If anyone wants to use it (as I
am sure there are some that do), who are we to stand in their way, if it
makes sense for them?

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