multicast pim bidir

Answered Question
Jan 25th, 2010

All,

When a non RP router received a mcast packet, where should the router forward the packet? Toward the RP or toward the mcast group OIL?

My question is, how will a non RP router pin point its own position? Whether it was in the path from the source to RP or from RP to the destination group?

Thanks

I have this problem too.
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Correct Answer by Giuseppe Larosa about 6 years 10 months ago

Hello Yuhi,

yes, the idea is to forward with the lightest overhead in terms of group states and encapsulation (no use of GRE towards RP even for the first packet)

Hope to help

Giuseppe

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Mohamed Sobair Mon, 01/25/2010 - 06:27

Hi,

All multicast pim enabled routers knows about the RP for some groups as defined i the configuration using one of the following methods:

1- Static RP

2- Auto-rp Annuouncment.

3- Boot strap BSR messages.

whenever a multicast packet is recieved on the interface, the router performs RPF check , and forward the multicast packet out its other ports. The RPF prevents multicast looping and shoould leads to the source (RP).

As for the RP itself, it recieves the following type of pim message from the multicast source send by the (designated forwarder):

1- register message

and would therfore send (register-stop) message as soon as registration is done.

Hope it helps,

Mohamed

Giuseppe Larosa Mon, 01/25/2010 - 13:21

Hello Yuhui,

http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_basic_cfg_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1054980

Packet Forwarding

A router creates (*, G) entries only for bidirectional groups. The olist of a (*, G) entry includes all the interfaces for which the router has been elected DF and that have received either an IGMP or PIM Join message. If a router is located on a sender-only branch, it will also create a (*, G) state, but the olist will not include any interfaces.

If a packet is received from the RPF interface toward the RP, the packet is forwarded downstream according to the olist of the (*, G) entry. Otherwise, only the router that is the DF for the receiving interface forwards the packet upstream toward the RP; all other routers must discard the packet.

Hope to help

Giuseppe

yuhuiyao Mon, 01/25/2010 - 14:23

Thanks Giuseppe.

Do you mean if there is a router in the path between the sender and the RP and it gets a mcast from the source, that router will have (*,G) state however, there will be no OIL. If so, how will that router route the mcast packet? Will the router just forward the mcast packet toward RP? If so, the router just does so without consulting mcast routing table?

Thanks

Jon Marshall Mon, 01/25/2010 - 15:08

yuhuiyao wrote:

Thanks Giuseppe.

Do you mean if there is a router in the path between the sender and the RP and it gets a mcast from the source, that router will have (*,G) state however, there will be no OIL. If so, how will that router route the mcast packet? Will the router just forward the mcast packet toward RP? If so, the router just does so without consulting mcast routing table?

Thanks

From the 12.4 multicast configuration guide -

Bidir-PIM also has unconditional forwarding of source traffic toward the RP upstream on the shared tree, but has no registering process for sources as in PIM-SM. These modifications allow forwarding of traffic in all routers based solely on the (*, G) multicast routing entries. This form of forwarding eliminates any source-specific state and allows scaling capability to an arbitrary number of sources.

Full link -

12.4 pim bidir

Jon

yuhuiyao Tue, 01/26/2010 - 06:02

Thanks all for your replies.

Is that safe to say if a mcast packet hit a router, and that router have a (*, G) without OIL, that router will forward the mcast packet toward RP?

Thanks.

Correct Answer
Giuseppe Larosa Tue, 01/26/2010 - 06:56

Hello Yuhi,

yes, the idea is to forward with the lightest overhead in terms of group states and encapsulation (no use of GRE towards RP even for the first packet)

Hope to help

Giuseppe

yuhuiyao Tue, 01/26/2010 - 07:03

Thanks Giuseppe for your reply.

In that case where the router is in the mid of the sender and RP, the router will have a (*, G) without OIL. The router will forward mcast packet toward RP wihtout consulting mrouting table. Am I right on that?

Thanks.

Jon Marshall Tue, 01/26/2010 - 09:27

yuhuiyao wrote:

Thanks Giuseppe for your reply.

In that case where the router is in the mid of the sender and RP, the router will have a (*, G) without OIL. The router will forward mcast packet toward RP wihtout consulting mrouting table. Am I right on that?

Thanks.

Whether the router forwards the packet depends on whether it has been elected the DF or not. It may be a good idea to read through that link i sent.

Jon

yuhuiyao Tue, 01/26/2010 - 09:49

My question is, will a DF in the mid of source and RP forward mcast traffic wihtout consulting mroute table? In the mroute table, (*, G) will have no OIL in that case.

Jon Marshall Tue, 01/26/2010 - 10:08

yuhuiyao wrote:

My question is, will a DF in the mid of source and RP forward mcast traffic wihtout consulting mroute table? In the mroute table, (*, G) will have no OIL in that case.

Sorry, wasn't entirely clear to me what you were asking.

Yes it will consult the mroute table because it still needs to see if it has a (*,G) entry and it doesn't know until it looks whether there is an OIL.

Jon

Giuseppe Larosa Tue, 01/26/2010 - 10:07

Hello Yuhui,

If a packet is received from the RPF interface toward the RP, the packet is forwarded downstream according to the olist of the (*, G) entry.

= receiving from core of distribution tree, interested users are downstream = there is a not empty OI list

Otherwise, only the router that is the DF for the receiving interface forwards the packet upstream toward the RP; all other routers must discard the packet.

= received from some segment in opposite direction from RP (more far then local node)  and that requires to be sent towards RP

Designated Forwarder election happens  per segment / IP subnet and it is the one allowed to forward traffic towards the RP because it is one with the best path towards the RP.

The different routers compare respective AD and metric of unicast routing path towards RP.

Only DF (for RP address) per interface propagates traffic to avoid unnecessary multicast traffic replication.

I would say in general a DF is chosen per segment and per : multicast source (rx from core = there is an OIlist)  or  RP address (tx to core)

Hope to help

Giuseppe

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