Pim Bootstrap Router; Reverse-Path Forwarding; Multicast Non-Stop Routing - Cisco ASR 9000 Series Configuration Manual

PIM Bootstrap Router

PIM Bootstrap Router
The PIM bootstrap router (BSR) provides a fault-tolerant, automated RP discovery and distribution mechanism
that simplifies the Auto-RP process. This feature is enabled by default allowing routers to dynamically learn
the group-to-RP mappings.
PIM uses the BSR to discover and announce RP-set information for each group prefix to all the routers in a
PIM domain. This is the same function accomplished by Auto-RP, but the BSR is part of the PIM Version 2
specification. The BSR mechanism interoperates with Auto-RP on Cisco routers.
To avoid a single point of failure, you can configure several candidate BSRs in a PIM domain. A BSR is
elected among the candidate BSRs automatically. Candidates use bootstrap messages to discover which BSR
has the highest priority. The candidate with the highest priority sends an announcement to all PIM routers in
the PIM domain that it is the BSR.
Routers that are configured as candidate RPs unicast to the BSR the group range for which they are responsible.
The BSR includes this information in its bootstrap messages and disseminates it to all PIM routers in the
domain. Based on this information, all routers are able to map multicast groups to specific RPs. As long as a
router is receiving the bootstrap message, it has a current RP map.

Reverse-Path Forwarding

Reverse-path forwarding (RPF) is an algorithm used for forwarding multicast datagrams. It functions as
follows:
• If a router receives a datagram on an interface it uses to send unicast packets to the source, the packet
• If the packet arrives on the RPF interface, a router forwards the packet out the interfaces present in the
• If the packet does not arrive on the RPF interface, the packet is silently discarded to prevent loops.
PIM uses both source trees and RP-rooted shared trees to forward datagrams; the RPF check is performed
differently for each, as follows:
• If a PIM router has an (S,G) entry present in the multicast routing table (a source-tree state), the router
• If a PIM router has no explicit source-tree state, this is considered a shared-tree state. The router performs
Sparse-mode PIM uses the RPF lookup function to determine where it needs to send joins and prunes. (S,G)
joins (which are source-tree states) are sent toward the source. (*,G) joins (which are shared-tree states) are
sent toward the RP.

Multicast Non-Stop Routing

Multicast Non-Stop Routing (NSR) enables the router to synchronize the multicast routing tables on both the
active and standby RSPs so that during an HA scenario like an RSP failover there is no loss of multicast data.
Multicast NSR is enabled through the multicast processes being hot standby. Multicast NSR supports both
Cisco ASR 9000 Series Aggregation Services Router Multicast Configuration Guide, Release 6.0.x
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has arrived on the RPF interface.
outgoing interface list of a multicast routing table entry.
performs the RPF check against the IP address of the source for the multicast packet.
the RPF check on the address of the RP, which is known when members join the group.
Implementing Layer-3 Multicast Routing on Cisco IOS XR Software