Understanding How Ipv4 Bidirectional Pim Works; Default Ipv4 Multicast Layer 3 Switching Configuration - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Understanding How IPv4 Bidirectional PIM Works

access-list 100 permit ip any 224.0.1.0 0.0.0.255
access-list 100 deny ip any 224.0.0.0 15.255.255.255
The ACLs filter RPF failures and drop them in hardware so that they are not forwarded to the router.
Use the ACL-based method of filtering RPF failures only in sparse mode stub networks where there are
no downstream routers. For dense mode groups, RPF failure packets have to be seen on the router for
the PIM assert mechanism to function properly. Use CEF-based or NetFlow-based rate limiting to limit
the rate of RPF failures in dense mode networks and sparse mode transit networks.
For information on configuring ACL-based filtering of RPF failures, see the
Filtering of RPF Failures" section on page
Rate Limiting of RPF Failure Traffic
When you enable rate limiting of packets that fail the RPF check (non-RPF packets), most non-RPF
packets are dropped in hardware. According to the multicast protocol specification, the router needs to
receive the non-RPF packets for the PIM assert mechanism to function properly, so all non-RPF packets
cannot be dropped in hardware.
When a non-RPF packet is received, a NetFlow entry is created for each non-RPF flow.
When the first non-RPF packet arrives, the PFC3B bridges the packet to the PISA and to any bridged
ports and creates a NetFlow entry that contains source, group, and ingress interface information, after
which the NetFlow entry handles all packets for that source and group, sending packets only to bridged
ports and not to the PISA.
To support the PIM assert mechanism, the PFC3B periodically forwards a percentage of the non-RPF
flow packets to the PISA.
The first packets for directly connected sources in PIM sparse mode are also rate-limited and are processed
by the CPU.
Rate limiting of RPF failures is disabled by default.
Understanding How IPv4 Bidirectional PIM Works
The PFC3B supports hardware forwarding of IPv4 bidirectional PIM groups. To support IPv4
bidirectional PIM groups, the PFC3B implements a new mode called designated forwarder (DF) mode.
The designated forwarder is the router elected to forward packets to and from a segment for a IPv4
bidirectional PIM group. In DF mode, the supervisor engine accepts packets from the RPF and from the
DF interfaces.
When the supervisor engine is forwarding IPv4 bidirectional PIM groups, the RPF interface is always
included in the outgoing interface list of (*,G) entry, and the DF interfaces are included depending on
IGMP/PIM joins.
If the route to the RP becomes unavailable, the group is changed to dense mode. Should the RPF link to
the RP become unavailable, the IPv4 bidirectional PIM flow is removed from the hardware FIB.
For information on configuring IPv4 bidirectional PIM, see the
section on page

Default IPv4 Multicast Layer 3 Switching Configuration

Table 25-1
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
25-6
25-18.
shows the default IP multicast Layer 3 switching configuration.
Chapter 25 Configuring IPv4 Multicast Layer 3 Switching
25-13.
"Configuring IPv4 Bidirectional PIM"
"Configuring ACL-Based
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