Multicast Designated Router and PIM forwarder versus IGMP querier

[Steph1963]

Hi to Everybody,

Multicast DR Clarification

I have seen two different definition for a DR:

1) First Hop Designated Router that corresponds to the router that is directly connected to the multicast source (R5 in the attached diagram)

2) Router responsible for sending join/prune message to the RP.

Question 1:

Does these definitions indentify the same router

DR Election Algorithm

The DR is responsible for sending Join/Prune message to the RP. To elect the DR,each PIM router on the network examines the receive Hello messages and compare its IP address with those neighbors. The router with the highest IP address is the DR

In the following diagram:

LAN 1 DR: 10.0.0.2

LAN 2 DR: 10.0.0.6

LAN 3 DR: 192.68.3.254

Question 2:

Does the DR main tasks are the following:

Signal multicast receiver to the RP, tracking IGMP messages

Register active sources on the segment to the RP

If this is the case, it seems to me that the DR should always be upstream toward the RP in order to indicate the RP that a receiver downstream would like to join a multicast group.

How is this possible, if the DR elected does not provide an interface facing the upstream router like in the attached diagram.

Is there any concept of upstream/downstream interface when a join (*,G) message has to be sent. In the attached diagram, it does not make real sense to say that R4 can forward the join message to the RP. Seems to me that only R2 & R3 can do that job. How does the DR election algorithm can support this type of case

PIM Forwarder/versus IGMP querier

Are we talking here about two different roles and if this is the case, can somebody confirm if those definitions are correct:

PIM forwarder: Rourter that continues to forward multicast traffic in the case where two routers are receiving multicast from eah other (R2 & R3 in the attached diagram).  The PIM forwarder would be the lowest administrative distance or the lowest metric of the multicast source addres and the highest iP address in the case of a tie.

Querier: Router sending IGMP query message (lowest IP address on the LAN for IGMPv2)

In the case of IP Sparse Dense mode, does the PIM forwarder base on assert messages only applied for (S,G) when shortest path is calculated or is it also apply to identify the source for (*,G) when the router receive some information about the RP.

Thanks for your help

Stephane

[Giuseppe Larosa · Accepted solution]

Hello Stephane,

in each LAN segment where one or more multicast enabled routers are connected to you can see the three roles of PIM DR, IGMP querier, PIM forwarder.

The IGMP querier is the device responsible to send out IGMP queries at a regular time interval with the objective to keep updated the list of multicast groups with at least a receiver on the LAN segment/broadcast domain.

The PIM DR has to represent the LAN segment/broadcast domain in the PIM tree topology so it is responsible to contact the RP in PIM SM or sparse-dense mode.

For sources connected to the segment the PIM DR should use the register process because PIM tree is unidirectional and branches can only go out from RP.

The register process involves the use of GRE encapsulation to the RP and it is expensive/not efficient in the forwarding plane, so the threshold to switch to source based tree is really low (one packet) and the RP will join a distribution tree rooted at the PIM DR connected to the source.

if multiple multicast routers are present on the LAN segment with default settings one acts as the PIM DR and the one other one as the IGMP querier.

The PIM forwarder concept is specially important in PIM bidirectional.

for shared tree the PIM assert competiotion happens using a source the RP address.

for source based tree the PIM assert happens looking at the source.

to be noted the PIM assert process is the only case I know of that AD of a route is sent outside a router.

The PIM forwarder concept solves, specially for the source based trees, the possible not optimal location of PIM DR on the segment as you have noted. So looking at who has the better path to source ensures that RPF check is performed for the LAN segment as a while and a PIM forwarder is chosen for each source.

Another objective is to avoid to send in the segment multiple copies of the same stream.

>>>>About your questions: on router to router links the concept ot PIM DR is not meaningful; see it in action on leaf LAN segments where receivers or sources are connected to.

On router to router links the routers build a PIM neighborship and exchange PIM messages to express the interest in receiving some streams.

Hope to help

Giuseppe

[Steph1963]

Hi Giuseppe,

Thanks a lot for this great reply, I think that I have a much better understanding of the PIM Designated Router.

If I have understand correctly, PIM DR does not have any meaning when only routers are connected in a point-to-point (figrue 1) or in a point-to-multipoint topology. The DR is the router sending the join message to upstream router only in the case where there is more than one router on a LAN segment like in figure 2.

I know that the PIM dense-mode is using some periodic control mesages to make sure that the router downstream still requires the multicast traffic. As a reply to these periodic control, Prune Messages can be sent when the router does not have any multicast receiver. I think that the periodic control messages interval can be configured by the ip pim state-refresh origination-interval.

Does the PIM sparse mode is using a similar method to decide if it still has to flood the downstream interface after the shortest path has been establish? In other words, is there any requirement for the DR to send a join message to the upstream router to keep receiving the multicast traffic from the upstream router.

Thanks again for your precious help

Stephane

[Giuseppe Larosa · Accepted solution]

Hello Stephan,

thanks for your kind remarks.

>> Does the PIM sparse mode is using a similar method to decide if it still has to flood the downstream interface after the shortest path has been establish? In other words, is there any requirement for the DR to send a join message to the upstream router to keep receiving the multicast traffic from the upstream router.

PIM SM and SSM are based on explicit joins, but these joins have to be renewed over time, you can find out by yourself with a debug ip pim command or by using a packet capture.

There is an option in debug ip pim that allows to specify a single group in order to limit the output.

The difference is that PIM DM is flood an prune, PIM SM requires explicit joins ans it is driven by receivers.

PIM SM saves on the amount of states (= memory resources) as only sources with at least one live receiver have their SPT built.

Hope to help

Giuseppe

[pdiazq001]

Hi Chinar. how is everything

i have a doubt may be you can help solve it

………….……[Source]
|
(rp)—2——1—(r2)
| 2                       |1
|                          |
|                          |
|                          |
| 1                       |2
(r3)—2——1—(r4)
|
[Destination]

if have four routers, represented as ( ), and a source and destination [ ]. what is the steady flow for mmcast traffic if the ip addresses of the segments finish at the number indicated ( 1 or 2) for every side of the link
after commuting to the shortest path

suppose ospf is the IGP