PIM v2 BSR
PIMv2 BSR (Bootstrap Router) 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.
Related Topics
Configuring Candidate BSRs, on page 61
Example: Configuring Candidate BSRs, on page 73
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:
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.
Catalyst 2960-XR Switch IP Multicast Routing Configuration Guide, Cisco IOS Release 15.0(2)EX1
42 OL-29426-01
Configuring PIM
PIM v2 BSR
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