[j-nsp] Segment Routing Real World Deployment (was: VPC mc-lag)

[Mark Tinka]

On 7/Jul/18 18:26, Saku Ytti wrote:

> You can (and should) run your iBGP inside LSP too, so there is no
> difference, except iBGP can be redundant.

I just let iBGP sessions form normally over IGP-mapped paths. Or am I
missing something.

While I know an IGP path can follow an LSP, short of IGP Shortcuts
(Autoroute Announce, as it was known in Cisco land), I think that would
be too much layering.


> Consider this
>
> Edge1----Core1----Edge2
> |          |       |
> +--------Core2-----+
>
> Now imagine you have L2 transport between Edge1 and Edge2. With iBGP
> Edge[12] would receive information redundantly from both core
> interfaces, to lose signalling information either end needs to lose
> state to both BGP sessions.
> With LDP it's single session, if that becomes sufficiently lossy that
> it flaps, it's done, you lose state.
>
> So BGP just provides more signalling redundancy.

Hmmh - not sure I understand your use-case, Saku.

LDP forms sessions over the IGP. Whatever path is available via IGP is
what LDP will follow, which covers the signaling redundancy concern.

In most cases, LDP instability will be due to IGP instability.  IGP
instability will bring everything down. Worrying about LDP or BGP
signaling will be a luxury, at that point.


> What I was trying to tell, your example is implementation detail, not
> fundamental problem of BGP signalled pseudowire.

I think there are several times when the best of intentions in a
protocol have not materialized in practice.

What I would like to hear, though, is how you would overcome the
problems that Alexandre faced with how he, specifically, deployed
BGP-signaled VPLS.


>
> Here we are not comparing same things, of course if you have L2
> redundancy it's vulnerable to loops. The debate isn't should you do
> ELAN or EPIPE, the debate it should you signal EPIPE on LDP or BGP,
> and the examples you detail making the BGP case poorer are
> implementation detail, not fundamental problem in BGP signalled
> pseudowires.

How differently would you do it?

Mark.