Case Study: MSDP Mesh Groups
In the preceding case study, routers Rodan and Megalon are RPs in the same AS. Large multicast domains can frequently have many RPs to share the workload or to localize multicast trees. Although MSPD has been presented so far as a tool for sharing inter-AS source information, it also proves useful when there are multiple RPs in a single domain, and sources always register to certain RPs but members throughout the domain must find any source.
Every RP in the domain commonly has an MSDP peering session to every other RP in the domain, for redundancy and robustness. Figure 7-16 shows an example. The four RPs in the illustration are in the same AS, and each is peered to the other three. The four routers may or may not be directly connected and are probably physically remote from each other.
Example 7-16 shows the configurations of the four routers in Figure 7-16.
Example 7-16 Configuring MSDP on the Four Routers in Figure 7-16
Frankenstein
ip pim rp-address 10.100.1.1
ip msdp peer 10.100.1.3 connect-source Loopback0
ip msdp description 10.100.1.3 to Mummy
ip msdp peer 10.100.1.2 connect-source Loopback0
ip msdp description 10.100.1.2 to Wolfman
ip msdp peer 10.100.1.4 connect-source Loopback0
ip msdp description 10.100.1.4 to Dracula
_______________________________________________________________________
Wolfman
ip pim rp-address 10.100.1.2
ip msdp peer 10.100.1.1 connect-source Loopback0
ip msdp description 10.100.1.1 to Frankenstein
ip msdp peer 10.100.1.3 connect-source Loopback0
ip msdp description 10.100.1.3 to Mummy
ip msdp peer 10.100.1.4 connect-source Loopback0
ip msdp description 10.100.1.4 to Dracula
_______________________________________________________________________
Mummy
ip pim rp-address 10.100.1.3
ip msdp peer 10.100.1.1 connect-source Loopback0
ip msdp description 10.100.1.1 to Frankenstein
ip msdp peer 10.100.1.2 connect-source Loopback0
ip msdp description 10.100.1.2 to Wolfman
ip msdp peer 10.100.1.4 connect-source Loopback0
ip msdp description 10.100.1.4 to Dracula
_______________________________________________________________________
Dracula
ip pim rp-address 10.100.1.4
ip msdp peer 10.100.1.1 connect-source Loopback0
ip msdp description 10.100.1.1 to Frankenstein
ip msdp peer 10.100.1.2 connect-source Loopback0
ip msdp description 10.100.1.2 to Wolfman
ip msdp peer 10.100.1.3 connect-source Loopback0
ip msdp description 10.100.1.3 to Mummy
The problem with the configuration as it stands is that an SA message generated by one router is flooded by all the other routers, causing large numbers of peer RPF flooding failures and resulting MSDP notification messages. If every RP has an MSDP connection to every other RP, however, no flooding is necessary. Every RP receives a copy of every SA directly from the originator. To remedy the flooding problem, an MSDP mesh group is built.
An MSDP mesh group is a set of fully meshed MSDP peers such as the ones shown in Figure 7-16, but no transiting of SA messages takes place. That is, when an RP receives an SA from a peer, it does not forward the message to any other peer.
Mesh groups are configured with the ip msdp mesh-group command. The group is given an arbitrary name (so that one RP can belong to more than one mesh group, if necessary), and the members of the mesh group are specified. The configurations in Example 7-17 add the RPs in Figure 7-16 to a mesh group named Boogeymen.
Example 7-17 Adding the RPs in Figure 7-16 to Mesh Group Boogeymen
Frankenstein
ip pim rp-address 10.100.1.1
ip msdp peer 10.100.1.3 connect-source Loopback0
ip msdp description 10.100.1.3 to Mummy
ip msdp peer 10.100.1.2 connect-source Loopback0
ip msdp description 10.100.1.2 to Wolfman
ip msdp peer 10.100.1.4 connect-source Loopback0
ip msdp description 10.100.1.4 to Dracula
ip msdp mesh-group Boogeymen 10.100.1.3
ip msdp mesh-group Boogeymen 10.100.1.2
ip msdp mesh-group Boogeymen 10.100.1.4
_______________________________________________________________________
Wolfman
ip pim rp-address 10.100.1.2
ip msdp peer 10.100.1.1 connect-source Loopback0
ip msdp description 10.100.1.1 to Frankenstein
ip msdp peer 10.100.1.3 connect-source Loopback0
ip msdp description 10.100.1.3 to Mummy
ip msdp peer 10.100.1.4 connect-source Loopback0
ip msdp description 10.100.1.4 to Dracula
ip msdp mesh-group Boogeymen 10.100.1.1
ip msdp mesh-group Boogeymen 10.100.1.3
ip msdp mesh-group Boogeymen 10.100.1.4
_______________________________________________________________________
Mummy
ip pim rp-address 10.100.1.3
ip msdp peer 10.100.1.1 connect-source Loopback0
ip msdp description 10.100.1.1 to Frankenstein
ip msdp peer 10.100.1.2 connect-source Loopback0
ip msdp description 10.100.1.2 to Wolfman
ip msdp peer 10.100.1.4 connect-source Loopback0
ip msdp description 10.100.1.4 to Dracula
ip msdp mesh-group Boogeymen 10.100.1.1
ip msdp mesh-group Boogeymen 10.100.1.2
ip msdp mesh-group Boogeymen 10.100.1.4
_______________________________________________________________________
Dracula
ip pim rp-address 10.100.1.4
ip msdp peer 10.100.1.1 connect-source Loopback0
ip msdp description 10.100.1.1 to Frankenstein
ip msdp peer 10.100.1.2 connect-source Loopback0
ip msdp description 10.100.1.2 to Wolfman
ip msdp peer 10.100.1.3 connect-source Loopback0
ip msdp description 10.100.1.3 to Mummy
ip msdp mesh-group Boogeymen 10.100.1.1
ip msdp mesh-group Boogeymen 10.100.1.2
ip msdp mesh-group Boogeymen 10.100.1.3
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