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Configuring MSDP Mesh Groups

MSDP mesh groups are groups of peers configured in a full-mesh topology that limits the flooding of source-active messages to neighboring peers. Every mesh group member must have a peer connection with every other mesh group member. When a source-active message is received from a mesh group member, the source-active message is always accepted but is not flooded to other members of the same mesh group. However, the source-active message is flooded to non-mesh group peers or members of other mesh groups. By default, standard flooding rules apply if mesh-group is not specified.

To configure an MSDP mesh group, define a peer group, and include the mode mesh-group statement:




group group-name { 

local-address address; 

mode mesh-group; 

peer address;
}

You can include this statement at the following hierarchy levels:

[edit protocols msdp]
[edit logical-systems logical-system-name protocols msdp]
[edit logical-systems logical-system-name routing-instances routing-instance-name protocols msdp]
[edit routing-instances routing-instance-name protocols msdp]

Caution: When configuring MSDP mesh groups, you must configure all members the same. If you do not configure a full mesh, excessive flooding of source-active messages can occur.

A common application for MSDP mesh groups is peer-reverse-path-forwarding (peer-RPF) check bypass. For example, if there are two MSDP peers inside an autonomous system (AS), and only one of them has an external MSDP session to another AS, the internal MSDP peer often rejects incoming source-active messages relayed by the peer with the external link. Rejection occurs because the external MSDP peer must be reachable by the internal MSDP peer via the next hop toward the source in another AS, and this next-hop condition is not certain. To prevent rejections, configure an MSDP mesh group on the internal MSDP peer so it always accepts source-active messages.

Table 14 explains how flooding is handled by peers in this configuration. Figure 39 illustrates source-active message flooding between different mesh groups and peers within the same mesh group.

Table 14: Source-Active Message Flooding Explanation

Source-Active Message Received From	Source-Active Message Flooded To	Source-Active Message NOT Flooded To
Peer 21	Peer 11, Peer 12, Peer 13, Peer 31, Peer 32	Peer 22
Peer 11	Peer 21, Peer 22, Peer 31, Peer 32	Peer 12, Peer 13
Peer 31	Peer 21, Peer 22, Peer 11, Peer 12, Peer 13, Peer 32

Figure 39: Source-Active Message Flooding

Image g017098.gif

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