Configuring PIM-DM plus IGMP 155
vlan 100
igmp-snooping enable
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Precautions ■Only one C-BSR can be configured on a Layer 3 switch. Configuration of a
C-BSR on another interface overwrites the previous configuration.
■It is recommended that C-BSRs and C-RPs be configured on Layer 3 switches in
the backbone network.
■If you do not specify a group range for a C-RP, the C-RP will serve all multicast
groups when it becomes the RP in the domain; otherwise it will serve the
specified group range.
■You can configure a basic ACL to filter related multicast IP addresses, thus to
control the multicast group range that a static RP serves.
■If you configure a static RP, you must perform the same configuration on all the
routers in the PIM-SM domain.
■If the configured static RP address is the address of an interface in the up state
on the local device, the local device will serve as a static RP.
■When the elected RP works properly, the static RP does not take effect.
■It is not necessary to enable PIM on the interface that serves as a static RP.
■Configuring a legal BSR address range can prevent the legal BSR from being
replaced maliciously. With a legal BSR address range configured on all Layer 3
switches in the entire network, all these switches will discard bootstrap
messages from out of the legal address range, thus to safeguard BSR in the
network.
■To guard against C-RP spoofing, you can configure a legal C-RP address range
and the range of multicast groups to be advertised by each C-RP.
Configuring PIM-DM plus IGMP
PIM-DM is a type of dense mode multicast protocol. It uses the “push mode” for
multicast forwarding, and is suitable for small-sized networks with densely
distributed multicast group members.
The basic implementation of PIM-DM is as follows:
■PIM-DM assumes that at least one multicast group member exists on each
subnet of the network, and therefore multicast data is flooded to all nodes on
the network. Then, branches without multicast receivers are pruned from the
forwarding tree, leaving only those branches that contain receivers. This “flood
and prune” process takes place periodically, that is, pruned branches resume
multicast forwarding periodically.
■When a new receiver on a previously pruned branch joins a multicast group, to
reduce the join latency, PIM-DM uses a graft mechanism to resume data
forwarding to that branch.
In PIM-DM, the multicast forwarding path is a source tree, with the multicast
source as its “root” and multicast group members as its “leaves”. Because the
source tree is the shortest path from the multicast source to the receivers, it is also
called shortest path tree (SPT).