MSDP

MSDP is a mechanism that allows RPs to share information about active sources. When RPs in remote domains learn about the active sources, they can pass on that information to the local receivers and multicast data can be forwarded between the domains. MSDP allows each domain to maintain an independent RP that does not rely on other domains but enables RPs to forward traffic between domains. PIM-SM is used to forward the traffic between the multicast domains.

Using PIM-SM, multicast sources and receivers register with their local RP by the closest multicast router. The RP maintains information about the sources and receivers for a specific group. RPs in other domains do not have any knowledge about sources located in other domains.

MSDP is required to provide inter-domain multicast services using Any Source Multicast (ASM). Anycast RP for MSDP enables fast convergence when an MSDP/PIM PR router fails by allowing receivers and sources to rendezvous at the closest RP.

When an RP in a PIM-SM domain first learns of a new sender, for example, by PIM register messages, it constructs a source-active (SA) message and sends it to its MSDP peers. The SA message contains the following fields:

• source address of the data source

• group address the data source sends to

• IP address of the RP

Each MSDP peer receives and forwards the message away from the RP address in a peer-RPF flooding fashion. The peer-RPF flooding applies to forwarding SA messages. The Multicast Routing Information Base (MRIB) is examined to determine which peer toward the originating RP of the SA message is selected. Such a peer is called an RPF peer.

If the MSDP peer receives the SA message from a non-RPF peer toward the originating RP, it drops the message. Otherwise, it forwards the message to all its MSDP peers (except the one from which it received the SA message).

When an MSDP peer that is also an RP for its own domain receives a new SA message, it determines if any group members within the domain are interested in any group described by an (S,G) entry within the SA message. That is, the RP checks for a (*,G) entry with a non-empty outgoing interface list. This implies that some system in the domain is interested in the group. In this case, the RP triggers an (S,G) join event toward the data source as if a join/prune message addressed to the RP was received. This sets up a branch of the source-tree to this domain. Subsequent data packets arrive at the RP by this tree branch and are forwarded down the shared-tree inside the domain. If leaf routers choose to join the source-tree, they have the option to do so according to existing PIM-SM conventions. If an RP in a domain receives a PIM join message for a new group G, the RP must trigger an (S,G) join event for each active (S,G) for that group in its SA cache.

This procedure is called flood-and-join because if any RP is not interested in the group, the SA message can be ignored; otherwise, they join a distribution tree.

MSDP peer groups are typically created when multiple peers have a set of common operational parameters. Group parameters not specifically configured are inherited from the global level.

MSDP mesh groups are used to reduce SA flooding primarily in intra-domain configurations. When a number of speakers in an MSDP domain are fully meshed, they can be configured as a mesh group. The originator of the SA message forwards the message to all members of the mesh group. Because of this, forwarding the SA between non-originating members of the mesh group is not necessary.

MSDP routing policies allow for filtering of inbound or outbound, or both, SA messages. Policies can be configured at different levels:

• global level

  Global level applies to all peers.

• group level

  Group level applies to all peers in the peer group.

• neighbor level

  Neighbor level applies only to a specified peer.

The most specific level is used. If multiple policy names are specified, the policies are evaluated in the order they are specified. The first policy that matches is applied. If no policy is applied, SA messages are passed.

Match conditions include the following:

• neighbor - matches on a neighbor address is the source address in the IP header of the SA message

• route filter - matches on a multicast group address embedded in the SA message

• source address filter - matches on a multicast source address embedded in the SA message

Perform the following basic MSDP configuration tasks:

• enable MSDP (required)

• configure peer

• configure local address

Use commands in the following context to configure and enable basic MSDP.

configure router msdp

The following example shows a basic MSDP configuration with default values and the peer and peer local addresses specified.

[ex:/configure router "Base" msdp]
A:admin@node-2# info
    ... 
    peer 10.20.1.1 {
        local-address 10.20.1.6
    }
    ...

A:node-2>config>router>msdp# info
----------------------------------------------
            ...
            peer 10.20.1.1
                local-address 10.20.1.6
            exit
            ...
----------------------------------------------

MSDP is enabled by default. Use the following commands to disable MSDP:

• MD-CLI

  configure router msdp admin-state disable

• classic CLI

  configure router msdp shutdown

The following example shows an MSDP configuration that is shutdown.

[ex:/configure router "Base" msdp]
A:admin@node-2# info
    admin-state disable
    group "test" {
        active-source-limit 50000
        export-policy ["LDP-export"]
        import-policy ["LDP-import"]
        local-address 10.10.10.103
        mode mesh-group
        receive-message-rate {
            rate 100
            time 300
            threshold 5000
        }
        peer 10.10.10.104 {
        }
    }
    peer 10.20.1.1 {
        local-address 10.20.1.6
    }

A:node-2config>router# info
----------------------------------------------
...
#------------------------------------------
echo "MSDP Configuration"
#--------------------------------------------------
        msdp
            shutdown
            peer 10.20.1.1
                local-address 10.20.1.6
            exit
            group "test"
                active-source-limit 50000
                receive-msdp-msg-rate 100 interval 300 threshold 5000
                export "LDP-export"
                import "LDP-import"
                local-address 10.10.10.103
                mode mesh-group
                peer 10.10.10.104
                exit
            exit
        exit