Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB
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Appendix B Cisco MWR 2941 Router Command Reference
ip pim
Usage Guidelines Enabling PIM on an interface also enables Internet Group Management Protocol (IGMP) operation on
that interface. An interface can be configured to be in dense mode, passive mode, sparse mode, or
sparse-dense mode. The mode describes how the Cisco IOS software populates its multicast routing
table and how the software forwards multicast packets it receives from its directly connected LANs.
Dense mode interfaces are always added to the table when the multicast routing table is populated.
Sparse mode interfaces are added to the table only when periodic join messages are received from
downstream routers, or there is a directly connected member on the interface.
Dense Mode
Initially, a dense mode interface forwards multicast packets until the router determines that there are
group members or downstream routers, or until a prune message is received from a downstream router.
Then, the dense mode interface periodically forwards multicast packets out the interface until the same
conditions occur. Dense mode assumes that multicast group members are present. Dense mode routers
never send a join message. They do send prune messages as soon as they determine they have no
members or downstream PIM routers. A dense mode interface is subject to multicast flooding by default.
Dense Mode with Proxy Registering
For a router in a PIM sparse mode (PIM-SM) domain configured to operate in sparse mode or
sparse-dense mode, the ip pim dense-mode proxy-register command must be configured on the
interface leading toward the bordering dense mode region. This configuration enables the router to
register traffic from the dense mode region with the rendezvous point (RP) in the sparse mode domain.
Prior to Cisco IOS Release 12.0 S, an RP needed to be running on the border router leading toward a
dense mode region so that the RP could learn about traffic from sources in the dense mode region.
This command requires an extended access list or route map argument specifying which traffic the router
needs to register with the RP. This command applies only to sources reachable through a PIM router.
Cisco IOS software always registers traffic from remote sources if it arrives on a dense mode interface
and if the Reverse Path Forwarding (RPF) neighbor leading toward the source is a Distance Vector
Multicast Routing Protocol (DVMRP) but not a PIM router. This functionality has existed since Cisco
IOS Release 10.0 and cannot be modified (restricted) with an access list or route map.
Passive Mode
An interface configured with passive mode does not pass or forward PIM control plane traffic; it passes
or forwards only IGMP traffic. If passive mode is configured on an interface enabled for IP multicast,
the router does not send PIM messages on the interface nor does it accept PIM messages from other
routers on this interface. The router acts as the only PIM router on the network and works as the
designated router (DR) and the designated forwarder (DF) for all Bidirectional PIM group ranges.
The ip pim neighbor-filter command has no effect and is superseded by the ip pim passive command
when both commands are configured on the same interface.
Do not use the ip pim passive command on LANs that have more than one IP multicast router connected
to them, because all routers with this command become DR and DF, resulting in duplicate traffic
(PIM-SM, PIM-DM, PIM-SSM) or looping traffic (Bidir-PIM). To limit PIM messages to and from valid
routers on LANs with more than one router, use the ip pim neighbor-filter command
Sparse Mode
A sparse mode interface is used for multicast forwarding only if a join message is received from a
downstream router or if group members are directly connected to the interface. Sparse mode assumes
that no other multicast group members are present. When sparse mode routers want to join the shared
path, they periodically send join messages toward the RP. When sparse mode routers want to join the
source path, they periodically send join messages toward the source; they also send periodic prune
messages toward the RP to prune the shared path.