Network Virtualization using Extreme Fabric Connect

Network Virtualization Using Extreme Fabric Connect
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Fabric Connect PIM Gateway is able to interoperate with both PIM-SM and PIM-SSM.
We’ll start by looking at the most common approach, dealing with PIM-SM, and then revisit PIM-SSM in a
later section below.
PIM-SM is a very complex protocol. At the heart of PIM-SM is the Rendezvous Point (RP), which is an IP
router designated to consolidate knowledge of available Multicast Sources and is the focus of all PIM Joins
when a receiver expresses a wish to join a stream. When a new Multicast stream is received by a PIM
Router, it immediately PIM Registers (encapsulates) that stream towards the RP. Likewise, if a new
Multicast Receiver expresses a wish to receive a given Multicast stream, the PIM router where the IGMP
entry is added will trigger a chain of PIM *.G Joins, hop-by-hop by every PIM Router along the reverse
forwarding path towards the RP.
Forwarding of the multicast stream requires two conditions: the stream sender exists and a multicast
receiver has joined the stream. When both these conditions are met, the multicast stream is initially
forwarded via the RP. To this effect, if the RP is no longer receiving the stream, it also triggers a chain of
PIM S.G Joins, hop-by-hop by every PIM Router along the reverse forwarding path, this time towards PIM
Router where the sender is located. Forwarding IP multicast via the RP is not efficient as the path is unlikely
to be optimal (except if the RP is close to the sender) and software forwarding is typically used on the RP. It
is however necessary to get the stream going to start with, because the last hop PIM router (where the
receiver is located) has no idea what the source IP of the requested stream is (the receiver is not aware of
the source IP and only asked for the Class D Group address). However, once the stream gets going, the last
hop PIM router will be able to infer the source IP of the sender, and at this point (also depending on vendor
implementations of PIM), the last hop PIM router will trigger yet again a chain of PIM S.G Joins, hop-by-hop
by every PIM Router along the reverse forwarding path, this time directly towards PIM Router where the
sender is located. Not surprisingly, PIM-SM suffers from a very high latency when first establishing multicast
streams.
The key to interact with PIM-SM is to have direct access to the RP function where all the knowledge of
multicast sources in the PIM cloud is concentrated. The IETF has defined the Multicast Source Discovery
Protocol (MSDP) precisely for this purpose. This is yet another protocol which needs to be piled on top of
the protocol stack but is the only way that separate PIM clouds can be made to interoperate.
The Extreme Fabric Connect PIM Gateway therefore has no other choice but to match the same complexity
and implement both PIM signalling and MSDP.
Note
Fabric Connect PIM Gateway requires MSDP in order to interface with a PIM-SM network.
Ensure that PIM-SM RP router operating in PIM cloud is able to support MSDP.
Figure 47 PIM Gateway to Legacy IP Multicast Routed Networks