Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 479
53-1002269-02
PIM Sparse
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• PMBR – A PIM switch that has some interfaces within the PIM domain and other interface
outside the PIM domain. PBMRs connect the PIM domain to the Internet.
NOTE
You cannot configure a routing interface as a PMBR interface for PIM Sparse in the current
software release.
• BSR – The Bootstrap Router (BSR) distributes RP information to the other PIM Sparse switches
within the domain. Each PIM Sparse domain has one active BSR. For redundancy, you can
configure ports on multiple switches as candidate BSRs. The PIM Sparse protocol uses an
election process to select one of the candidate BSRs as the BSR for the domain. The BSR with
the highest BSR priority (a user-configurable parameter) is elected. If the priorities result in a
tie, then the candidate BSR interface with the highest IP address is elected. In the example in
Figure 87, PIM Sparse switch B is the BSR. Port 2 is configured as a candidate BSR.
• RP – The RP is the meeting point for PIM Sparse sources and receivers. A PIM Sparse domain
can have multiple RPs, but each PIM Sparse multicast group address can have only one active
RP. PIM Sparse switches learn the addresses of RPs and the groups for which they are
responsible from messages that the BSR sends to each of the PIM Sparse switches. In the
example in Figure 87, PIM Sparse Switch B is the RP. Port 2 is configured as a candidate
Rendezvous Point (RP).
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.
RP paths and SPT paths
Figure 87 shows two paths for packets from the source for group 239.255.162.1 and a receiver 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 Figure 87, 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: