Rp Paths And Spt Paths; Configuring Pim Sparse - Dell PowerConnect B-FCXs Configuration Manual

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PIM Sparse

RP paths and SPT paths

Figure 122
for the group. The source is attached to PIM Sparse Switch A and the recipient is attached to PIM
Sparse Switch C. PIM Sparse Switch B in is the RP for this multicast group. As a result, the default
path for packets from the source to the receiver is through the RP. However, the path through the
RP sometimes is not the shortest path. In this case, the shortest path between the source and the
receiver is over the direct link between Switch A and Switch C, which bypasses the RP (Switch B).
To optimize PIM traffic, the protocol contains a mechanism for calculating the Shortest Path Tree
(SPT) between a given source and receiver. PIM Sparse switches can use the SPT as an alternative
to using the RP for forwarding traffic from a source to a receiver. By default, Layer 3 Switches
forward the first packet they receive from a given source to a given receiver using the RP path, but
forward subsequent packets from that source to that receiver through the SPT. In
Switch A forwards the first packet from group 239.255.162.1 source to the destination by sending
the packet to Switch B, which is the RP. Switch B then sends the packet to Switch C. For the second
and all future packets that Switch A receives from the source for the receiver, Switch A forwards
them directly to Switch C using the SPT path.

Configuring PIM Sparse

To configure a Layer 3 Switch for PIM Sparse, perform the following tasks:
•
•
•
744
To enhance overall network performance, Layer 3 Switches use the RP to forward only the first
packet from a group source to the group receivers. After the first packet, the Layer 3 Switch
calculates the shortest path between the receiver and source (the Shortest Path Tree, or SPT)
and uses the SPT for subsequent packets from the source to the receiver. The Layer 3 Switch
calculates a separate SPT for each source-receiver pair.
NOTE
Dell recommends that you configure the same ports as candidate BSRs and RPs.
shows two paths for packets from the source for group 239.255.162.1 and a receiver
Configure the following global parameter:
•
Enable the PIM Sparse mode of multicast routing.
Configure the following interface parameters:
•
Configure an IP address on the interface
•
Enable PIM Sparse.
•
Identify the interface as a PIM Sparse border, if applicable.
NOTE
You cannot configure a routing interface as a PMBR interface for PIM Sparse.
Configure the following PIM Sparse global parameters:
•
Identify the Layer 3 Switch as a candidate PIM Sparse Bootstrap Router (BSR), if
applicable.
•
Identify the Layer 3 Switch as a candidate PIM Sparse Rendezvous Point (RP), if
applicable.
•
Specify the IP address of the RP (if you want to statically select the RP).
Figure
PowerConnect B-Series FCX Configuration Guide
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