Re: [Idr] Review Updates to draft-ietf-idr-bgp-prefix-sid

[Robert Raszuk <[e-mail]>]

Hi Acee,

Since we are at the open surgery of this draft there is one more point
which did not got resolved during our offline discussion. Feedback from WG
is welcome here.

The draft RECOMMENDS the following procedure in case of a conflict:

... it is RECOMMENDED that the first prefix using the label index is selected.


In BGP there is no guarantee about ordered delivery of BGP prefixes.  It
may work such in some specific topologies but not in any arbitrary one. So
if we recommend to pick one prefix on R1 and different prefix on R2 and
install in forwarding the same SID for those I am afraid the data plane
will get quite severely messed up and even finding it during
troubleshooting will be quite hard.

Also as pointed out already MPLS anycast SID may on purpose use the same
label across nodes according to draft-ietf-spring-mpls-anycast-segments-02.
Now if we want to accomplish the same in BGP we would need to advertise
different prefixes with the same SID (in MPLS-SR index). the above
paragraph does prevents that. Note that adding and advertising the _same_
prefix from different nodes will not work as BGP speakers may pick only one
(best path) and use it for forwarding. IBGP or EBGP multipath may not
always be enabled/use same set.

The crux of the issue when advertising SIDs in BGP is not so much about
prefixes, but their next hops which unfortunately the draft does not even
mention once.

Kind regards,
Robert.