Pim Bootstrap Router; Reverse-Path Forwarding; Multicast Vpn - Cisco XR12000 Series Configuration Manual

Implementing Multicast Routing on Cisco IOS XR Software

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 VPN

Multicast VPN (MVPN) provides the ability to dynamically provide multicast support over MPLS networks.
MVPN introduces an additional set of protocols and procedures that help enable a provider to support multicast
traffic in a VPN.
OL-30405-02
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.
Cisco IOS XR Multicast Configuration Guide for the Cisco XR12000 Series Router, Release 5.1.x
PIM Bootstrap Router
17