More on SSM

As you’ve noticed I’ve been studying SSM and what better way to learn than to blog about it. I recently got a Safari subscription which has been great so far. I’ve been reading some in the book Interdomain Multicast Routing: Practical Juniper Networks and Cisco Systems Solutions which has been great so far.

We are still using the same topology and now we will look a bit more in detail what is happening.

R1 will be the source, sending traffic from its loopback. R3 will be the client running IGMPv3 on its upstream interface to R2. As explained in previous post I am doing this to simulate an end host otherwise I would configure it on R3 downstream interface and then it would sen a PIM Join upstream.

To run SSM we need IGMPv3 or use some form of mapping as described in previous post. It is important to note though that IGMPv3 is not specific for SSM. With SSM a (S,G) pair is described as a channel. Instead of join/leave it is now called subscribe and unsubscribe.

So the first thing that happens is that the client (computer or STB) sends IGMPv3 membership report to the destination IP 224.0.0.22. This is the IP used for IGMPv3. This is how the packet looks in Wireshark.

The destination IP is 224.0.0.22 which corresponds to the multicast MAC 01-00-5E-00-00-16. 16 in hex is 22 in decimal.

We clearly see that it is version 3 and the type is Membership Report 0x22. Number of group records show how many groups are being joined.

Then the actual group record is shown (225.0.0.1) and the type is Allow New Sources. The number of sources is 1. And then we see the channel (S,G) that is joined.

Then R2 sends a PIM Join towards the source.

We can see that it is a (S,G) join. The SPT is built.

R2 will send general IGMPv3 queries to see if there are still any receivers connected to the LAN segment.

The query is sent to all multicast hosts (224.0.0.1) and if still receiving the multicast the host will reply with a report.

The type is Membership Query (0x11). The Max Response Time is 10 seconds which is the time that the host has to reply within.

We can see in this report that the record type is Mode is include (1) compared to Allow New Sources when the first report was sent.

Now R3 unsubscribes to the channel and the IGMP report is used once again.

The type is now Block Old Sources (6).

After this has been sent the IGMP querier (router) has to make sure that there are no other subscribers to the channel so it sends out a channel specific query.

If noone responds to this the router will send a PIM Prune upstream as can be seen here.

Finally. How can we see which router is the IGMP querier? Use the show ip igmp interface command.

R2#show ip igmp interface fa0/0
FastEthernet0/0 is up, line protocol is up
  Internet address is 23.23.23.2/24
  IGMP is enabled on interface
  Current IGMP host version is 3
  Current IGMP router version is 3
  IGMP query interval is 60 seconds
  IGMP querier timeout is 120 seconds
  IGMP max query response time is 10 seconds
  Last member query count is 2
  Last member query response interval is 1000 ms
  Inbound IGMP access group is not set
  IGMP activity: 2 joins, 1 leaves
  Multicast routing is enabled on interface
  Multicast TTL threshold is 0
  Multicast designated router (DR) is 23.23.23.2 (this system)
  IGMP querying router is 23.23.23.2 (this system)
  Multicast groups joined by this system (number of users):
      224.0.1.40(1)

We can see some interesting things here. We can see which router is the designated router and IGMP querier. By default the IGMP querier is the router with the lowest IP and the DR is the one with highest IP. DR can be affected by chancing the DR priority. We can also see which timers are used for query interval and max response time among other timers.

So by now you should have a good grasp of SSM. It does not have a lot of moving parts which is nice.