Multicast over ATM
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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. |
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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: |
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ip pim multipoint-signaling |
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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 |
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ip pim vc-count number |
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number |
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Maximum number of VCs PIM can open. Default value is 200. |
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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 |
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ip pim minimum-vc-rate pps |
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pps |
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Set
the minimum packets per second rate to the value given by
pps. |
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If
the number of VCs open already equals the maximum number allowed, then packets
for new groups are sent over the static multicast VC. cp10
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