Hi Lucy,
It is certainly true that the BGP route distribution mechanism is not
optimal for multicast signaling. The advantages and disadvantages of
using BGP for multicast signaling were discussed extensively in the WG
when RFCs 6513 and 6514 were being written. But the entire mechanism is
built around the standard BGP route distribution procedures, and one has
to be very cautious about making changes.
> [Lucy] What you say is that, in BGP distribution mechanism, BGP
> (child here) chooses the bestpath (i.e. parent here) for a particular
> NLRI, then distributes the NLRI to all BGP peers (including the
> parent node).
Right. But ...
One thing you have to remember is that the child might not have a BGP
session directly with the parent. For instance, one can have a
deployment where ingress replication is used in a non-segmented manner,
such that the ingress PE makes a copy for each egress PE. In this case,
when an egress PE originates a Leaf A-D route, the RT will identify the
ingress PE. But ingress and egress PEs generally don't have BGP
sessions to each other. In many deployments, the ingress and egress PEs
are clients of a route reflector; the egress PE sends the Leaf A-D route
to the RR, and the RR redistributes it to the ingress PE. In other
deployments, the Leaf A-D route from a given egress PE may have to
travel over several BGP sessions before it reaches the ingress PE.
> Since the NLRI is only stored by the parent node and may be removed
> by old parent node, such distribution mechanism has no advance for
> such purpose and causes a scaling issue. To reduce the distribution
> symptom, it should explicitly require that, if a node receiving a
> leaf A-D route is not the parent node including old parent node, the
> node should not redistribute the leaf A-D route; in other words, only
> the parent node is allowed to readvertise the leaf A-D route.
This would break the deployment scenarios described above.
> One question, if one ASBR or ABR node that is the parent for a set of
> downstream neighbors fails, what is the procedure for the downstream
> neighbors to select a new parent?
If a node fails, its BGP sessions go down, and the routes it originated
are considered to have been withdrawn. If parent and child are
separated by a RR, failure of the parent will cause its session to the
RR to go down. The RR will then send withdraws to the child for all of
the routes that were originated by the parent and redistributed to the
child.
If there are two potential parents for a given flow, each will have
originated an S-PMSI A-D route with an NLRI identifying the flow. One
of these S-PMSI A-D routes is the bestpath for that NLRI, and the next
hop (or P2MP Inter-Area Segmented Next Hop Extended Community) of that
route determines the parent that is chosen by a given child. If that
parent fails, the route from the other potential parent becomes the best
path, and the child will then rehome itself by changing the RT on the
Leaf A-D route.
> If a child fails, the parent should
> update multicast state as if the child is withdrawn.
If a child fails, its Leaf A-D route appears to the parent to have been
withdrawn, and this causes the change in multicast state.
Note that the procedures for dealing with withdrawn S-PMSI or Leaf A-D
routes are not specific to Ingress Replication.
Eric
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