Cisco Catalyst 3560-X Series, Руководство по настройке программного обеспечения

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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
When a new receiver on a previously pruned branch of the tree joins a multicast group, the PIM DM
device detects the new receiver and immediately sends a graft message up the distribution tree toward
the source. When the upstream PIM DM device receives the graft message, it immediately puts the
interface on which the graft was received into the forwarding state so that the multicast traffic begins
flowing to the receiver.
PIM-SM
PIM-SM uses shared trees and shortest-path-trees (SPTs) to distribute multicast traffic to multicast
receivers in the network. In PIM-SM, a router or multilayer switch assumes that other routers or switches
do not forward multicast packets for a group, unless there is an explicit request for the traffic (join
message). When a host joins a multicast group using IGMP, its directly connected PIM-SM device sends
PIM join messages toward the root, also known as the RP. This join message travels router-by-router
toward the root, constructing a branch of the shared tree as it goes.
The RP keeps track of multicast receivers. It also registers sources through register messages received
from the source’s first-hop router (
designated router [DR]) to complete the shared tree path from the
source to the receiver. When using a shared tree, sources must send their traffic to the RP so that the
traffic reaches all receivers.
Prune messages are sent up the distribution tree to prune multicast group traffic. This action permits
branches of the shared tree or SPT that were created with explicit join messages to be torn down when
they are no longer needed.
When the number of PIM-enabled interfaces exceeds the hardware capacity and PIM-SM is enabled with
the SPT threshold is set to
infinity , the switch does not create (S,G) entries in the multicast routing table
for the some directly connected interfaces if they are not already in the table. The switch might not
correctly forward traffic from these interfaces.
PIM Stub Routing
The PIM stub routing feature, available in all software images, reduces resource usage by moving routed
traffic closer to the end user.
Note
The IP base image contains only PIM stub routing. The IP services image contains complete multicast
routing. On a switch running the IP base image, if you try to configure a VLAN interface with PIM
dense-mode, sparse-mode, or dense-sparse-mode, the configuration is not allowed.
In a network using PIM stub routing, the only allowable route for IP traffic to the user is through a switch
that is configured with PIM stub routing. PIM passive interfaces are connected to Layer 2 access
domains, such as VLANs, or to interfaces that are connected to other Layer 2 devices. Only directly
connected multicast (IGMP) receivers and sources are allowed in the Layer 2 access domains. The PIM
passive interfaces do not send or process any received PIM control packets.
When using PIM stub routing, you should configure the distribution and remote routers to use IP
multicast routing and configure only the switch as a PIM stub router. The switch does not route transit
traffic between distribution routers. You also need to configure a routed uplink port on the switch. The
switch uplink port cannot be used with SVIs. If you need PIM for an SVI uplink port, you should upgrade
to the IP services feature set.
You must also configure EIGRP stub routing when configuring PIM stub routing on the switch. For more
information, see the
“EIGRP Stub Routing” section on page 1-44 .