Cisco MWR 2941 Installation Manual download pdf (Page 188)

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Command Reference, Cisco IOS Release 15.1(3)MR

OL-26895-01

Chapter Cisco MWR 2941 Router Command Reference, Release 15.1(3)MR

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, 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 will enable the router to register

traffic from the dense mode region with the rendezvous point (RP) in the sparse mode domain.

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 will always register 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.

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.

Sparse-Dense Mode

An alternative to choosing just dense mode or just sparse mode is to run PIM in a single region in sparse

mode for some groups and dense mode for other groups.

In sparse-dense mode, if the group is in dense mode, the interface will be treated as dense mode. If the

group is in sparse mode, the interface will be treated in sparse mode. The group is "sparse" if the router

knows about an RP for that group.

When an interface is treated in dense mode, it is populated in the outgoing interface list of the multicast

routing table when either of the following conditions is true:

• Members or DVMRP neighbors are on the interface.

• Any of the PIM neighbors on the interface have not pruned for the group.

When an interface is treated in sparse mode, it is populated in the outgoing interface list of the multicast

routing table when either of the following conditions is true:

• Members or DVMRP neighbors are on the interface.

• A PIM neighbor on the interface has received an explicit join message.

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