Routing Multicast Traffic with MOSPF

Routing Multicast Traffic with MOSPF

Problem

You want to distribute your multicast routing tables with MOSPF.

Solution

Unfortunately, Cisco does not support MOSPF. As mentioned in the Introduction to this chapter, MOSPF is a set of multicast extensions to OSPF that uses LSA Type 6. By default, when a Cisco router receives a Type 6 LSA packet it will generate a %OSPF- 4-BADLSATYPE error message. To avoid this error message, you can configure your routers to ignore Type 6 LSA packets:

Router#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#router ospf 65530
Router(config-router)#ospf ignore lsa mospf
Router(config-router)#end
Router#

Discussion

MOSPF has not enjoyed a particularly wide acceptance for several reasons, mostly related to the fact that it uses a dense-mode multicast-forwarding scheme, and because it is protocol dependant. It turns out to be most useful in networks that meet several key requirements:

  • They should use a relatively small number of multicast applications.

  • These applications should have few servers and many group members, with the group members scattered throughout the network.

  • The network must use OSPF as its unicast protocol.

  • The applications should deliver a flow of multicast traffic that is neither heavy enough to cause congestion problems on the slowest links in the network, nor so light that relationships time out in the routers.

Few router vendors have implemented MOSPF. Increasingly, the multicast routing protocol of choice appears to be PIM-SM.

However, it is important to note that the normal default behavior for PIM is to use the standard IP unicast routing table. If you happen to be using OSPF for your standard IP routing tables, PIM will use the same OSPF routing tables to construct its Reverse Path Forwarding (RPF) trees back to the multicast source or Rendezvous Point (RP) by default. So you can use OSPF with PIM, without needing any additional configuration.

The only thing that you miss by not being able to use MOSPF is the ability to dynamically distribute a multicast routing table that reflects a different topology than the unicast routing table.