Dell BigIron RX Series, Configuration Manual

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628 BigIron RX Series Configuration Guide
53-1002253-01
Configuring Multicast Source Discovery Protocol (MSDP)
23
Peer Reverse Path Forwarding (RPF) flooding
When the MSDP router (also the RP) in domain 2 receives the Source Active message from its peer
in domain 1, the MSDP router in domain 2 forwards the message to all its other peers. The
propagation process is sometimes called “peer Reverse Path Forwarding (RPF) flooding”. This term
refers to the fact that the MSDP router uses its PIM Sparse RPF tree to send the message to its
peers within the tree. In
Figure 93 , the MSDP router floods the Source Active message it receives
from its peer in domain 1 to its other peers, in domains 3 and 4.
Note that the MSDP router in domain 2 does not forward the Source Active back to its peer in
domain 1, because that is the peer from which the router received the message. An MSDP router
never sends a Source Active message back to the peer that sent it. The peer that sent the message
is sometimes called the “RPF peer”. The MSDP router uses the unicast routing table for its Exterior
Gateway Protocol (EGP) to identify the RPF peer by looking for the route entry that is the next hop
toward the source. Often, the EGP protocol is Border Gateway Protocol (BGP) version 4.
NOTE
MSDP depends on BGP and MBGP for interdomain operations.
The MSDP routers in domains 3 and 4 also forward the Source Active message to all their peers
except the ones that sent them the message.
Figure 93 does not show additional peers.
Source active caching
When an MSDP router that is also an RP receives a Source Active message, the RP checks its PIM
Sparse multicast group table for receivers for the group. If the DR has a receiver for the group being
advertised in the Source Active message, the DR sends a Join message for that receiver back to
the DR in the domain from which the Source Active message came. Usually, the DR is also the
MSDP router that sent the Source Active message.
In Figure 93 , if the MSDP router and RP in domain 4 has a table entry for the receiver, the RP sends
a Join message on behalf of the receiver back through the RPF tree to the RP for the source, in this
case the RP in domain 1.
Some MSDP routers that are also RPs can cache Source Active messages. If the RP is not caching
Source Active messages, the RP does not send a Join message unless it already has a receiver that
wants to join the group. Otherwise, the RP does not send a Join message and does not remember
the information in the Source Active message after forwarding it. If the RP receives a request from
a receiver for the group, the RP and receiver must wait for the next Source Active message for that
group before the RP can send a Join message for the receiver.
However, if Source Active caching is enabled on the MSDP and RP router, the RP caches the Source
Active messages it receives. In this case, even if the RP does not have a receiver for a group when
the RP receives the Source Active message for the group, the RP can immediately send a Join for a
new receiver that wants to join the group, without waiting for the next Source Active message from
the RP in the source’s domain.
The size of the cache used to store MSDP Source Active messages is 8K
Configuring MSDP
To configure MSDP on a BigIron RX, perform the following tasks:
•
Enable MSDP