Cisco XR 12000 Series Configuration Guide

Implementing Multicast Routing on Cisco IOS XR Software Cisco ASR 9000 Series Routers
Information About Implementing Multicast Routing
MCC-13
Multicast Configuration Guide
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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
has arrived on the RPF interface.
• If the packet arrives on the RPF interface, a router forwards the packet out the interfaces present in
the outgoing interface list of a multicast routing table entry.
• 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 performs the RPF check against the IP address of the source for the multicast packet.
• If a PIM router has no explicit source-tree state, this is considered a shared-tree state. The router
performs the RPF check on the address of the RP, which is known when members join the group.
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
To be able to offer Layer 3 multicast services to customers with multiple distributed sites, service
providers are looking for a secure and scalable mechanism to transmit customer multicast traffic across
the provider network. Multicast VPN (MVPN) provides such services over a shared service provider
backbone using native multicast technology similar to BGP/MPLS VPN.
MVPN emulates MPLS VPN technology in its adoption of the multicast domain (MD) concept, in which
provider edge (PE) routers establish virtual PIM neighbor connections with other PE routers connected
to the same customer VPN. These PE routers thereby form a secure, virtual multicast domain over the
provider network. Multicast traffic is then transmitted across the core network from one site to another
as if the traffic were going through a dedicated provider network.
Separate multicast routing and forwarding tables are maintained for each individual VPN routing and
forwarding (VRF) instance, with traffic being sent through VPN tunnels across the service provider
backbone. For information about MVPNv4 versus MVPNv6, see
MVPN IPv4 and IPv6 Connectivity,
page MCC-15.

Cisco XR 12000 Series Specifications

General

Brand	Cisco
Model	XR 12000 Series
Category	Network Router
Language	English