Multicast Forwarding Information Base Overview

• Simplifies multicast operation through the separation of the control and forwarding planes.
• Protects mission critical multicast applications by enabling new services such as multicast high availability (HA).
• Eliminates the need for the route cache maintenance associated with demand caching schemes such as multicast fast switching.

The following tables are used for general multicast routing and forwarding:

• IGMP--Contains local IGMP memberships on the router.
• Multicast Route (Mroute)--Contains (*, G) and (S, G) multicast states on the router (including PIM mode, incoming interfaces, and outgoing interfaces).
• Multicast Source Discovery Protocol (MSDP)--Contains all Source-Active (SA) messages.
• Multicast Routing Information Base (MRIB)--Contains (*, G), (S, G), and (*, G/m) MRIB entries.
• MFIB--Contains (*, G), (S, G), and (*, G/m) MFIB entries.

Multicast tables can be further defined by the following contexts:

• Global--Non-VRF context.
• VRF--Layer-3 VPN context.
• IPv4--IPv4 address family context.
• IPv6--IPv6 address family context.

• (*, G)--Shared tree entries used by PIM sparse mode (PIM-SM) and bidirectional PIM (bidir-PIM).
• (S, G)--Source tree entries used by PIM-SM and Source Specific Multicast (PIM-SSM).
• (*, G/mask)--Shared tree entries used by the bidir-PIM and the MFIB.

Note	For more information about (*, G/mask) entries, see the Introduction of New Multicast Forwarding Entries section.

The following sections describe the components that make up the MFIB architecture:

The figure illustrates the components that make up the MFIB architecture.

Figure 1. IPv4 MFIB Architecture

Note	When you enter the show ip mrib client command on a multicast router in an IP multicast network, PIM, the mroute table, and IGMP will appear as one client to the MRIB. For more information, see the Cisco IOS IP Multicast Command Reference.

The figure illustrates the IPv4 MFIB distributed architecture.

Figure 2. IPv4 MFIB Distributed Architecture

• MFIB
  
• Distributed MFIB
  
• MRIB
  
• Multicast Control Plane
  

The core forwarding engine used by the MFIB is shared by both interrupt-level (fast switching) and process-level forwarding (process switching) as shown in the figure. Multicast packets received with a forwarding entry in the MFIB will be fast-switched by the MFIB, and multicast packets received without a forwarding entry that require the creation of a new forwarding entry will be process-switched by the MFIB.

Figure 3. Multicast Forwarding Engine

The table lists the significant MFIB and MRIB entry and interface flags used by IPv4 multicast.

The MFIB architecture introduces (*, G/mask) entries to describe a group range present in a router’s local group-to-RP mapping cache (static, Auto-RP, Bootstrap Router [BSR]).

(*, G/mask) entries are used by the MFIB to:

• Create (S, G) entries if they are not already present in the MFIB table (for PIM-SM)
• Create (*, G) entries along source-only branches (for bidir-PIM)
• Forward multicast traffic along shared-tree branches (for bidir-PIM)

Note	(*, G/mask) entries are present until the group-to-RP mapping cache either times out or is cleared.

The MFIB architecture introduces PIM tunnel interfaces. PIM tunnel interfaces are used by the MFIB for the PIM-SM registration process. Two types of PIM tunnel interfaces are used by the MFIB:

• A PIM encapsulation tunnel (PIM Encap Tunnel)
• A PIM decapsulation tunnel (PIM Decap Tunnel)

The PIM Encap Tunnel interface is dynamically created whenever a group-to-RP mapping is learned (via Auto-RP, BSR, or static RP configuration). The PIM Encap Tunnel interface is used to encapsulate multicast packets sent by first-hop designated routers (DRs) that have directly connected sources.

Similar to the PIM Encap Tunnel, the PIM Decap Tunnel interface is dynamically created--with the exception that it is created on the RP only whenever a group-to-rp mapping is learned. The PIM Decap Tunnel interface is used by the RP to decapsulate PIM registers.

Note	PIM tunnels will not appear in the running configuration. To display information about PIM Tunnel interfaces, use the show ip pim tunnel command.

The following syslog message will appear when a PIM tunnel interface is created:

* %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel<interface_number>, changed state to up

In the MFIB forwarding model, the MFIB maintains multicast state packet and byte counts and packet rates. The MFIB calculates these statistics as it forwards traffic. There is no periodic polling of these statistics by the control plane, nor does the MFIB periodically upload these statistics to the control plane. The MFIB has an API to these statistics allowing the control plane to query multicast counters when requested from the command-line interface (CLI) for the show ip mroute countcommand and for MIB statistics objects.