ip multicast-routing

[vivexmail]

hi,

As per my understanding every IGP like OSPF or EIGRP uses a multocast address for their hello packets.

So there is some process that underlies that allows multicast to pass through in cisco routers

My question , then why are we still typing in

# ip multicast-routing

in the config mode to enable multicasting ?

thanks

Viveknath

[turnera]

Certain multicast functions related to dynamic routing protocols, OSFP, EIGRP, etc are allowed by default. Link local multicasting is allowed for OSPF and EIGRP as part of the routers local communications.

However, if you wish to do IP multicasting you must enable that with the ip multicast-routing command and its associated configurations on the routers/switches you want to ip multicast to and from.

IP multicasting and link local multicasting are two different things.

[Peter Paluch]

Hello,

IP multicasting and link local multicasting are two different things. 

Nicely said! Sums up my own post nicely.

Best regards,

Peter

[Peter Paluch · Accepted solution]

Hello Viveknath,

Using multicast does not necessarily mean it is routed, i.e. forwarded from one router's interface to another. Using multicast has its meaning even on a single network. Multicast packets are encapsulated into frames in such a way that they are processed only by those stations that have subscribed to the particular multicast group. Other stations will ignore those frames as if they have never arrived. The fact that OSPF or EIGRP or even RIPv2 use multicasts does not mean that their messages are routed. On the contrary, all these multicasts are addressed within the 224.0.0.x range which has a link-local scope - such packets may never be routed off the segment they were sent on. At most, they can be received and processed by the stations on this segment.

So routers can subscribe to multicast groups for, say, OSPF, and send/receive multicasts because they do it as hosts - they do not actually route the OSPF multicast. That is why we do not need to configure the multicast routing in this sense.

However, if you need to perform multicast routing, i.e. take a multicast stream by one interface and possibly replicate it out several other interfaces, including the signalling necessary to join or leave a multicast distribution tree, you need to activate specific support in IOS for that (allocate and create the multicast routing table, start the PIM and IGMP support, etc.) - hence the ip multicast-routing command.

Best regards,

Peter

[Joseph W. Doherty]

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Posting

My question , then why are we still typing in
# ip multicast-routing
in the config mode to enable multicasting ?

Because it doesn't enable multicast packets, it enables multicast-routing.

Similar to a router using unicast packets (e.g. ping another IP host on a shared subnet) without IP routing being enabled.