1 basic but difficult multicasting question

att-sgcops · ‎11-18-2005

I setup a testing lab for multicast as you can see from the attachment. I define R1 as static RP, and using PIMSM as routing protocol. please see part of the configuration from R2:

ip pim rp-address 192.168.100.11

ip pim spt-threshold infinity.

And "ip igmp join-group 224.1.2.3" configured on Receiver1. When I did "ping 224.1.2.3" from R2, and "show ip mroute", I can see the following:

(*, 224.1.2.3), 00:02:07/00:03:24, RP 192.168.100.11, flags: SF

Incoming interface: Serial1, RPF nbr 172.16.1.1

Outgoing interface list:

Serial0.1, Forward/Sparse, 00:02:07/00:03:24

(172.16.1.2, 224.1.2.3), 00:00:08/00:03:21, flags: T

Incoming interface: Serial0.1, RPF nbr 172.16.3.2

Outgoing interface list:

Serial1, Forward/Sparse, 00:00:08/00:03:21

(172.16.2.1, 224.1.2.3), 00:00:35/00:03:00, flags: FT

Incoming interface: Ethernet0, RPF nbr 0.0.0.0, Registering

Outgoing interface list:

Serial0.1, Forward/Sparse, 00:00:35/00:03:23

Serial1, Forward/Sparse, 00:00:35/00:03:20

(172.16.3.1, 224.1.2.3), 00:00:35/00:02:24, flags: T

Incoming interface: Serial0.1, RPF nbr 0.0.0.0

Outgoing interface list:

Serial1, Forward/Sparse, 00:00:37/00:03:17

(172.16.9.1, 224.1.2.3), 00:02:11/00:01:48, flags:

Incoming interface: Serial0.1, RPF nbr 172.16.3.2

Outgoing interface list:

Serial1, Forward/Sparse, 00:00:12/00:03:17

(*, 224.0.1.40), 00:02:48/00:02:59, RP 192.168.100.11, flags: SCL

Incoming interface: Serial1, RPF nbr 172.16.1.1

Outgoing interface list:

Serial0.1, Forward/Sparse, 00:02:30/00:02:59

And someone explain the output of the "show ip mrouote"? I can underastand the first entry and the last one.

BTW, I am preparing for CCIE, is it necessary to understand the multicast routing fully?

thanks for your reply

Georg Pauwen · ‎11-18-2005

Hello,

basically, the mrouting table shows the IP addresses of the source router and the IP address of the multicast group. The 'incoming interface' is the interface through which traffic for the multicast group is received. The RPF is the IP address of the upstream router to the multicast address.

You might want to have a look at the link below, which describes all the fields in the output (scroll down to Table 42):

show ip mroute

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a0080086f3e.html#wp1056899

As with regard to your question about CCIE and multicast: it is definitely important that you fully understand multicast. Your best source is the documentation CD, which gives you in-depth details about the various multicast aspects:

Configuring IP Multicast Routing

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/himc_c/ch05/index.htm

HTH,

GP

jsayer · ‎11-18-2005

wasn't able to download your attachment, but the output shows four separate sources sending to the group 224.1.2.3. Since you're configured for Sparse-Mode only, you'd only see these (s,g) entries if you were in the path between the sources and the RP. The timers tell you how long the multicast state has been known for each source - 172.16.2.1 and 172.16.3.1 are identical. I don't recall how ping responses are handled with multicast, but this could be other receivers on the group sending ping replies. this would make them each a source, requiring them to send register packets, etc. to the RP.

hope this helps,

john