PIM-DM multicast hello messages

AgJames86 · ‎12-06-2008

Hello,

I would like to clarify a question about Protocol Independent Multicast which operates in a Dense mode.

As stated by RFC 3973, PIM is a protocol which is used between the _routers_ in order to ensure multicast packets routing and forwarding in the network.

The same RFC states that hello messages are used in order to detect other PIM routers. When I captured the traffic between the host and a router I saw a number of hello messages that are destined as they should be to the address (224.0.0.13 - all PIMv2 routers) and which are going from the router interface which is a default gateway for the hosts in the LAN.

So, my question is failry simple - why router sends hello messages from that interface?

Is it like, that the router will send hello messages over all active ethernet interfaces even if there are no other routers connected to that particular interface?

I was just thinking, that underlying routing protocol such as OSPF will discover the neigbors and thus it is possible to know that there are no more active routers for that interface and thus no need of hello messages.

Just wondering, guys...

Jon Marshall · ‎12-06-2008

"Is it like, that the router will send hello messages over all active ethernet interfaces even if there are no other routers connected to that particular interface?"

Yes a router will send hello's every 30 seconds by default on every interface that has PIM enabled on it.

"I was just thinking, that underlying routing protocol such as OSPF will discover the neigbors and thus it is possible to know that there are no more active routers for that interface and thus no need of hello messages."

Good point but it does not necessarily follow that because the router is not running OSPF or nay other dynamic routing protocol it is not running PIM on it's interface.

Jon

Giuseppe Larosa · ‎12-07-2008

Hello Igor,

the router will send out PIM hello messages in order to discover PIM neighbors exactly as OSPF would do.

The fact that there isn't any other PIM enabled router on the link doesn't stop the local router to send them.

On the other hand as soon as a new PIM enabled router is connected to the subnet they discover each other and become PIM neighbors.

This is needed to decide what router has to forward a mutlicast stream on the LAN (IP PIM Assert message is based on best routing path to the source)

>> I was just thinking, that underlying routing protocol such as OSPF will discover the neigbors and thus it is possible to know that there are no more active routers for that interface and thus no need of hello messages.

This is contrary to my own experience with OSPF (and theory): you can passive an interface, but if you don't do it, you get an OSPF hello every 10 seconds and you can see them with a packet capture on your PC regardless to the number of neighbors in the subnet.

Again, the reason is to be able to detect a new neighbor as soon as it is connected/activated on the link.

there is

sh ip ospf nei

and also

sh ip pim nei

you can see what interfaces haven't neighbors by comparing the output of:

sh ip pim interface

sh ip pim neigh

the same happens for ospf

sh ip ospf int br

sh ip ospf neigh

Hope to help

Giuseppe