multicast table look up

[sarahr202]

Hi everybody.

Suppose a router has following multicast routing table. Our router receives a packet with src ip 10.10.10.10 dst 227.7.7.7.

1)Which entry will router use to forward the packet?

*, 224.7.7.7), 04:00:33/00:02:41, RP 2.2.2.2, flags: S
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    Serial0/1, Forward/Sparse, 00:03:14/00:02:41

(10.10.10.10, 224.7.7.7), 00:13:53/00:03:22, flags: T
  Incoming interface: Serial0/0, RPF nbr 200.200.200.1
  Outgoing interface list:
    Serial0/1, Forward/Sparse, 00:03:14/00:02:44
========================================================

A router has following multicast routing table. Our router receives a packet with src ip 10.10.10.10 dst 227.7.7.7.

2)Which entry will router use to forward the packet? ( my guess since the 2nd entry (10.10.10.10, 224.7.7.7), is pruned, so it leaves only one valid entry

(*, 224.7.7.7), i am not sure ,just get into multicast)

*, 224.7.7.7), 04:19:21/00:03:26, RP 2.2.2.2, flags: S
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    Serial0/1, Forward/Sparse, 00:00:03/00:03:26

(10.10.10.10, 224.7.7.7), 00:07:20/00:01:39, flags: PT
  Incoming interface: Serial0/0, RPF nbr 200.200.200.1
  Outgoing interface list: Null

thanks and have a great weekend.

[Steve Fuller · Accepted solution]

Hi,

IP multicast traffic will by default, switch from the shared tree (*,G) to the source tree (S,G) upon arrival of the first packet. From the Internet Protocol IP Multicast Technology:

If the shared tree is not an optimal path between the source and the receiver the routers will dynamically create a source tree and stop traffic from flowing down the shared tree. This is the default behavior in IOS. Network Administrators can force traffic to stay on the shared tree by using a configuration option (ip pim spt-threshold infinity).

So if you want to delay the switching of traffic to the source tree you can set some value in Kbps, or infinity if you never want the traffic to switch to the source tree. Note that in later IOS releases you can only set 0 (always switch to source tree) or infinity (never switch to source tree).

Some reasons for doing this are discussed on Optimizing PIM Sparse Mode in a Large IP Multicast Deployment.

For the second case I can't say exactly as there's no details of the topology, but I don't believe the router will forward traffic on either tree. The extract you've provided is taken from the RP itself, and the interface in the Outgoing Interface List (OIL) is the path back towards the source. The RP can have state such as this when there's a turnaround router on the path between the source and the RP. The idea of the turnaround router is to prevent traffic coming unnecessarily to the RP. Take a look at the example from slide 66 of Module 6 of the IP Multicast Training Materials as this will explain in more detail with a step-by-step walk through of state creation etc.

While these presentations are a little old now, if you're new to multicast I would definitely recommend taking a look through them.

Regards

[sarahr202]

Hi Steve

Thanks for the great link.   I am not cleared about ":  DR" router.    for example:

My book says DR is the router connected that is directly to the source of  multicast traffic. 

And the above link says

A source sends traffic to a multicast group configured in PIM sparse mode (PIM-SM), the Designated Router (DR) leading toward the source must inform the rendezvous point (RP) about the presence of this source.

Please consider the following example.

1)What router will be considered DR?   R1 because is directly connected to Source or Both R1 and R2 as they both to lead to source.?

Source------------------------R1

                                          |

                                          |

                                         R2---------------------------Rp

source-----------R1

        |

        |----------------R2

2 )Above both R1 and R2 are attached to source, which one will be considered " DR"  ?

Thanks and have a nice weekend.

[Mohamed Sobair · Accepted solution]

Hello Sarah,

Happy Nice Weekend!

For the first Question:  Correct, The router Directly connected to the Source of the Multicast is considered the (DR).

The First diagram shows that (R1) is the DR, Here R1 is responsible of Sending Pim Register message and inform the RP about the presence of the Multicast Source.

All routers including R1 and R2 should be running Pim along with the RP for correct multicast routing and Pim operation, however, R1 is the DR router here.

For the Second Question:

For Such Scenario where multiple routers are connected to the Same Source, The DR will be choosen based on the following:

1- The Router with the least cost path towards the RP is considered the DR.

2- If there is a tie on the above, the Router with the Highest IP address is considered the DR.

Best Regards,

Mohamed

[sarahr202]

Hi Mohamed.

Would you please explain the following example ?

R2 is RP.

R4 s0/1------s0/1--R3 s0------------R2-----rest of network.

R4 has a client in group 224.7.7.7 while R3 has not any client in 224.7.7.7

R4 muticast routing table is shown below:

(*, 224.7.7.7), 03:50:44/stopped, RP 2.2.2.2, flags: SJC
  Incoming interface: Serial0/1, RPF nbr 150.150.150.3
  Outgoing interface list:
    FastEthernet0/0, Forward/Sparse, 03:50:44/00:02:20

(5.5.5.15, 224.7.7.7), 00:00:09/00:02:58, flags: JT
  Incoming interface: Serial0/1, RPF nbr 150.150.150.3
  Outgoing interface list:
    FastEthernet0/0, Forward/Sparse, 00:00:09/00:02:50

Which entry will be used to forward multicast packet src ip 5.5.5.5 dest ip 224.7.7.7? My book says the entry with " j" will be used. Just want to verify that.

R3  multicastrouting table is shown below:

(*, 224.7.7.7), 03:39:17/00:02:51, RP 2.2.2.2, flags: S
  Incoming interface: Serial0/0, RPF nbr 199.199.199.2
  Outgoing interface list:
    Serial0/1, Forward/Sparse, 03:39:17/00:02:51

(5.5.5.15, 224.7.7.7), 00:01:28/00:02:29, flags: T
  Incoming interface: Serial0/0, RPF nbr 199.199.199.2
  Outgoing interface list:
    Serial0/1, Forward/Sparse, 00:01:28/00:03:00

Here we dont have any " J" so which entry will R3 use to forward multicast traffic  src ip 5.5.5.5 ,dst ip 224.7.7.7 ?

I appreciate your help.

Thanks and have a great day.

[Mohamed Sobair · Accepted solution]

Hello Sarah,

1) Indeed, R4 will use the J entry which indicates a Join for the Source based Tree rooted at the source of the Multicast (5.5.5.15).

2) Because R3 has no interested Clients to Join the Multicast Group (224.7.7.7), there is no "J" Flag associated with the Multicast routing table, However, Because (R4) has interested clients for the Multicast Group 224.7.x, R3 forwards the Multicast routing table using both Trees. R4, would have sent a pim Join Message for the MG towards the RP previously before R3 begins to forward the Multicast Routing, R4 would also Choose which tree it should Join.

The Default behaviour on Cisco Equipment, is to Switch to the Source Based Tree As soon As the Source becomes known to the Router. If you dont wish to Switch to the Source Based Tree and want to always Join the Shared Tree which rooted at the RP, then You must issue the command (ip pim spt-threshhold infinity) at R4.

Best Regards,

Mohamed

[sarahr202]

thanks Mohamed.