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Chapter 9 - Multicast Support Commands

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Multicast over ATM
If the frame relay network in Figure 9-10 is replaced by an ATM network, then we can use multipoint virtual circuits (VC) to limit the replication f multicast packets. By default, PIM establishes a static multipoint VC that provides a connection to each PIM neighbor. If the hub receives a multicast packet that only one PIM neighbor needs, it is sent to all PIM neighbors.
Let’s say, for instance, we would like to modify this behavior so that the multicast packet is only forwarded to those neighbors that want to receive it. Assume the routers in the network are all running PIM Sparse-Mode and the Hub router is the RP. When router A sends a Join for a multicast group to the hub, the hub router sets up a multipoint VC for the group. If another spoke router joins the same group, the hub router just adds the spoke router to the multipoint VC. When traffic for the group is received by the hub, the router only needs to send one copy on the multipoint VC that was established for the group. Then the ATM switches between the hub and spoke routers are responsible for replicating and delivering the packets. This feature is configured using the interface command:
ip pim multipoint-signaling
This command can only be used on an ATM interface. To limit the maximum number of VCs that PIM can open for multicast traffic, use the interface command
ip pim vc-count number
number
Maximum number of VCs PIM can open. Default value is 200.
If the router needs to open another VC that causes the router to exceed the configured maximum VC count, then the VC with the least amount of activity is deleted. If there are multiple VCs with the same minimum amount of activity, then the VC that is connected to the fewest neighbors is deleted first. The activity level is measured in packets per second and by default all activity levels are considered when a VC needs to be deleted. To configure the activity level that determines whether VCs will be considered for deletion, use the interface command
ip pim minimum-vc-rate pps
pps
Set the minimum packets per second rate to the value given by pps.
If the number of VCs open already equals the maximum number allowed, then packets for new groups are sent over the static multicast VC.

 


 
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