Multicast roles

Hi,

I am a bit confused about the following roles:

- Querier = The router that sends out queries, and receives reports from hosts. If multiple is elected on multiaccess networks, the highest IP address is elected.
- Forwarder = The router that actually sends out multicast traffic onto the subnet. Is elected based on the routing protocol (AD) and if tie, the metric, and the final tie breaker is highest IP address.
- DR = What does the DR do?

Thanks in advance!

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

kryolla

The DR sends the traffic from the server or client which is located on the same subnet to the RP. The same thing a DR does for OSPF

networker050184

The DR is the router closest to the router sending/receiving to/from the RP in the shared tree. The DR routes the multicast traffic to the RP.

kpjungle

So basically, a DR performs the same function to the RP as a forwarder does to the clients?

Example:

If the RP is located on a multiaccess network with two routers able to pass multicast traffic from the source to it, a DR is elected, not a forwarder, to pass the traffic to the DR?

EdTheLad

kryolla wrote:

The DR sends the traffic from the server or client which is located on the same subnet to the RP.

Not true, doesnt have to be on the same subnet as the RP.

networker050184 wrote:

The DR is the router closest to the router sending/receiving to/from the RP in the shared tree.

If the source or destination is connected to a multiaccess network with more than 2 routers then the above statement is useless.

The querier, this is the term used for the router closest to the receiver, this router is running IGMP and thus sends and receives IGMP queries.If the receiver was on a multiaccess network the router with the lowest IP address would become the querier.

The forwarder, what you are really looking at here is the PIM assert mechanism.This is used to stop duplicate flows being sent on a multiaccess network.If a PIM router receives an (S,G) on an interface that is already in its OIL for the specific (S,G) a pim assert message is sent out the interface, the router with the lowest combination of AD+METRIC wins, as you said final tie breaker is highest IP.So the winning router keeps forwarding while the others stop.

The DR,a PIM term, this is required on a multiaccess network,it is responsible for sending joins for other routers on the multiaccess network and for sending the PIM registration to the RP.Th PIM registration is unicast to the RP until an (S,G) branch between the DR and RP is created.The router with the hightest IP address is becomes the DR.

kpjungle

EdTheLad wrote:

kryolla wrote:

The DR sends the traffic from the server or client which is located on the same subnet to the RP.

Not true, doesnt have to be on the same subnet as the RP.

networker050184 wrote:

The DR is the router closest to the router sending/receiving to/from the RP in the shared tree.

If the source or destination is connected to a multiaccess network with more than 2 routers then the above statement is useless.

The querier, this is the term used for the router closest to the receiver, this router is running IGMP and thus sends and receives IGMP queries.If the receiver was on a multiaccess network the router with the lowest IP address would become the querier.

The forwarder, what you are really looking at here is the PIM assert mechanism.This is used to stop duplicate flows being sent on a multiaccess network.If a PIM router receives an (S,G) on an interface that is already in its OIL for the specific (S,G) a pim assert message is sent out the interface, the router with the lowest combination of AD+METRIC wins, as you said final tie breaker is highest IP.So the winning router keeps forwarding while the others stop.

The DR,a PIM term, this is required on a multiaccess network,it is responsible for sending joins for other routers on the multiaccess network and for sending the PIM registration to the RP.Th PIM registration is unicast to the RP until an (S,G) branch between the DR and RP is created.The router with the hightest IP address is becomes the DR.

Interesting, so a DR can exist, both between a source and the RP, but also between the RP towards the receiver? I assume that you always look upstream towards the RP or source, since that is the only way the joins exist?

EdTheLad

kpjungle wrote:

Interesting, so a DR can exist, both between a source and the RP, but also between the RP towards the receiver? I assume that you always look upstream towards the RP or source, since that is the only way the joins exist?

Yes, its all dependent on the topology, wherever you have a multiaccess network you will have a DR. You might have two multiaccess networks between the RP and reciever and hence you will have two DRs, the same logic follows between the source and the RP.

kpjungle

EdTheLad wrote:

kpjungle wrote:

Interesting, so a DR can exist, both between a source and the RP, but also between the RP towards the receiver? I assume that you always look upstream towards the RP or source, since that is the only way the joins exist?

Yes, its all dependent on the topology, wherever you have a multiaccess network you will have a DR. You might have two multiaccess networks between the RP and reciever and hence you will have two DRs, the same logic follows between the source and the RP.

Okay, thanks
Got it then i hope

kryolla

what I meant was the DR and the server has to be on the same subnet and the DR and client has to be on the same subnet. The DR then forwards the traffic to the RP where ever the RP is at.

kpjungle

kryolla wrote:

what I meant was the DR and the server has to be on the same subnet and the DR and client has to be on the same subnet. The DR then forwards the traffic to the RP where ever the RP is at.

If it is like OSPF, then i guess theres one for each multiaccess network all along the paths (except for the subnet with the receiver, in which case it is the forwarder and/or querier).

kryolla

Dont get confused between the DR and forwarder although they can be the same router depending on how close it is the source. If another router on the same subnet is closer to the source then it will be the forwarder but not the DR. If you have multiple groups then you can have multiple forwarders but 1 DR on the segment again depending on the route metric to get to the source. HTH

kpjungle

kryolla wrote:

Dont get confused between the DR and forwarder although they can be the same router depending on how close it is the source. If another router on the same subnet is closer to the source then it will be the forwarder but not the DR. If you have multiple groups then you can have multiple forwarders but 1 DR on the segment. HTH

Right... the defining difference for me is that a forwarder is "pointing" towards a receiver (forwarding onto the subnet the receiver is on) and is elected because of its distance/metric to the source, whereas a DR is not pointing towards a receiver, since it is only used by PIM and not IGMP, is that correct?

kryolla

yeah so the jist of it is the client will send a join message to the DR which is the querier so it will query all clients to see if anybody wants the multicast and if nobody replies it will stop sending. The DR will tell the RP that it wants to receive the multicast for this particular group. The DR upon receiving the s,g entry will ask its pim neighbors on the same segment if they have a shorter route to the source which in turn will be the forwarder. HTH

kpjungle

kryolla wrote:

yeah so the jist of it is the client will send a join message to the DR which is the querier so it will query all clients to see if anybody wants the multicast and if nobody replies it will stop sending. The DR will tell the RP that it wants to receive the multicast for this particular group. The DR upon receiving the s,g entry will ask its pim neighbors on the same segment if they have a shorter route to the source which in turn will be the forwarder. HTH

Okay, then i think we are in sync

To sum it up.. there are just as many DR's "elected" as there are multiaccess networks in the multicast network (at least on the tree(s)), there is one forwarder and one querier (not nessecarily the same) per receiving subnet.