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Chapter 10

Configuring Multicast

-2 172.16.25.10 PIM/MBGP Reached RP/Core [198.32.163.0/24]

-3 ogig-den.oregon-gigapop.net (198.32.163.13) [AS 4600] PIM
Reached RP/Core [198.32.163.64/26]

-4 0car-0gw.oregon-gigapop.net (198.32.163.26) [AS 4600] PIM
[198.32.163.64/26]

RouterB#

As you can see, the outputs differ very little, but it is important to see how

the paths are established. From the first sample output, no group or destina-
tion was specified, so the router strictly used RPF to calculate the path from
the source to the router. In the other output, a group address was specified.
This caused the router to specifically use the existing forwarding tree for
group 224.2.243.55 to get back to the router.

These tools can be useful to determine connectivity as well as effectiveness

of placement of RPs and multicast sources. There are other show commands
that can aid you as well, but they are not related to the topic of this chapter.

Changing the IGMP Version

There are several settings that can be tweaked in the router to enhance or
change performance. The majority of them are beyond the scope of this
chapter. However, we will discuss one important feature, changing the
IGMP version.

It is important that you understand and know how to perform this change

because of the compatibility issues between IGMP versions, as discussed in
Chapter 9.

To put it simply, the IGMP version that runs on the hosts must also run

on the router. Cisco routers use IGMPv2 by default and do not auto-detect
the IGMP version the host is using. The command to change from IGMPv2
to IGMPv1, or vice versa, is ip igmp version (2 | 1). Because the IGMP
version needs to match only on the subnet, the command must be entered on
the interface that connects to the subnet that houses the IGMPv1 hosts. The
other interfaces on the router can remain on IGMPv2.

Enabling CGMP

CGMP must be used when hosts connect to a router via a Catalyst switch.
As we discussed in Chapter 9, Catalysts run CGMP so they can manage mul-
ticast membership reports from the router accordingly and so they can man-
age multicast ports on the switch. The router is the device that listens for the

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