multicasting with ospf and eigrp

[vishalpatil86]

hi,

In, ospf and eigrp, routing updates are sent trhu multicasting.

DO we have to enable multicasting on router interface running ospf or eigrp?

[Peter Paluch]

Hello Vishal,

DO we have to enable multicasting on router interface running ospf or eigrp?

No, certainly not. OSPF, EIGRP and RIPv2 use link-local multicasts which means no multicast routing support is necessary, and even if it was activated, the link-local multicasts are not routed (so the multicast routing itself is actually irrelevant to this specific issue). Simply activating the OSPF, EIGRP or RIPv2 will work without manually modifying any multicast settings.

Best regards,

Peter

[vishalpatil86]

it means router to receive multicast has to run one of the routing protocols??

if yes, then in which case we use PIM DENSE command?

[cadet alain]

Hi,

First answer is yes.

PIM is used only to forward multicast feeds through routers when you a multicast source sending to multicast clients.

As Peter explained above routing protocol multicast address are link-local scope(224.0.0.0 to 224.0.0.255) and so they have a TTL of 1 and can't be forwarded by routers.

Regards.

Alain.

[Jon Marshall]

Vishal

Just to clarify -

it means router to receive multicast has to run one of the routing protocols??

No, it just means that PIM uses whatever routes are in the routing table. So you can run an routing protocol such as EIGRP/OSPF but you can also simply have static routes in your routing and PIM would use these.

Jon

[cadet alain]

Hi jon,

If the OP was asking about routing protocol multicast addresses like OSPF,EIGRP or RIPv2 then I think the answer is yes imho.

Regards.

Alain.

[Jon Marshall]

Alain

To me the question seemed pretty straightforward ie.

does a routing protocol have to be used for PIM to work to which the answer is no you don't need to run a routing protocol.

No problem though, questions can and often are interpreted differently by different people

Jon

[Jon Marshall]

Alain

actually the question wasn't about PIM, or indeed routing multicast traffic (i have now had my first cup of coffee ) so it think i read it wrongly.

Vishal

it means router to receive multicast has to run one of the routing protocols??

No it doesn't. Routers can receive multicast through a variety of traffic eg. routing protocols such as EIGRP/OSPF, HSRP packets etc. All of these are link local as Peter has said and will not be routed.

For a router to route multicast traffic it has to run multicast routing and with Cisco this is generally PIM. PIM uses the routing table to determine where to send packets.

Jon

[Peter Paluch]

Dear friends,

My understanding of Vishal's question was:

If routing protocols like OSPF, EIGRP or RIPv2 use multicast communication between routers (i.e. the routing information exchanged in these protocols is encapsulated into multicast IP packets), is it necessary in some way to configure the routers to support multicasts?

Naturally, the answer is no, as the pure capability of an interface to send and receive these kinds of multicasts is initialized by the routing protocol itself, and no further multicast-related protocols like IGMP or PIM are necessary in order for plain EIGRP, OSPF or RIPv2 to work.

Best regards,

Peter

[vishalpatil86]

peter got my question correctly...!!!!