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Configuring IP Multicast Routing
Information About Cisco’s Implementation of IP Multicast Routing
Bootstrap Router
PIMv2 BSR is another method to distribute group-to-RP mapping information to all PIM routers and multilayer switches
in the network. It eliminates the need to manually configure RP information in every router and switch in the network.
However, instead of using IP multicast to distribute group-to-RP mapping information, BSR uses hop-by-hop flooding
of special BSR messages to distribute the mapping information.
The BSR is elected from a set of candidate routers and switches in the domain that have been configured to function as
BSRs. The election mechanism is similar to the root-bridge election mechanism used in bridged LANs. The BSR election
is based on the BSR priority of the device contained in the BSR messages that are sent hop-by-hop through the network.
Each BSR device examines the message and forwards out all interfaces only the message that has either a higher BSR
priority than its BSR priority or the same BSR priority, but with a higher BSR IP address. Using this method, the BSR is
elected.
The elected BSR sends BSR messages with a TTL of 1. Neighboring PIMv2 routers or multilayer switches receive the
BSR message and multicast it out all other interfaces (except the one on which it was received) with a TTL of 1. In this
way, BSR messages travel hop-by-hop throughout the PIM domain. Because BSR messages contain the IP address of
the current BSR, the flooding mechanism enables candidate RPs to automatically learn which device is the elected BSR.
Candidate RPs send candidate RP advertisements showing the group range for which they are responsible to the BSR,
which stores this information in its local candidate-RP cache. The BSR periodically advertises the contents of this cache
in BSR messages to all other PIM devices in the domain. These messages travel hop-by-hop through the network to all
routers and switches, which store the RP information in the BSR message in their local RP cache. The routers and
switches select the same RP for a given group because they all use a common RP hashing algorithm.
Multicast Forwarding and Reverse Path Check
With unicast routing, routers and multilayer switches forward traffic through the network along a single path from the
source to the destination host whose IP address appears in the destination address field of the IP packet. Each router
and switch along the way makes a unicast forwarding decision, using the destination IP address in the packet, by looking
up the destination address in the unicast routing table and forwarding the packet through the specified interface to the
next hop toward the destination.
With multicasting, the source is sending traffic to an arbitrary group of hosts represented by a multicast group address
in the destination address field of the IP packet. To decide whether to forward or drop an incoming multicast packet, the
router or multilayer switch uses a reverse path forwarding (RPF) check on the packet as follows and shown in
Figure 87
on page 681 :
1. The router or multilayer switch examines the source address of the arriving multicast packet to decide whether the
packet arrived on an interface that is on the reverse path back to the source.
2. If the packet arrives on the interface leading back to the source, the RPF check is successful and the packet is
forwarded to all interfaces in the outgoing interface list (which might not be all interfaces on the router).
3. If the RPF check fails, the packet is discarded.
Some multicast routing protocols maintain a separate multicast routing table and use it for the RPF check. However, PIM
uses the unicast routing table to perform the RPF check.
Figure 87 on page 681 shows port 2 receiving a multicast packet from source 151.10.3.21. Table 1 shows that the port
on the reverse path to the source is port 1, not port 2. Because the RPF check fails, the multilayer switch discards the
packet. Another multicast packet from source 151.10.3.21 is received on port 1, and the routing table shows this port is
on the reverse path to the source. Because the RPF check passes, the switch forwards the packet to all ports in the
outgoing port list.