HP MSR2003 Configuration Manual page 116

For C-BSRs interconnected through a GRE tunnel, configure static multicast routes to make sure the next
hop to a C-BSR is a tunnel interface. For more information about static multicast routes, see "Configuring
multicast routing and forwarding."
C-BSRs should be configured on routers on the backbone network. The BSR election process is
summarized as follows:
1. Initially, each C-BSR regards itself as the BSR of the BIDIR-PIM domain and sends BSMs to other
routers in the domain.
2. When a C-BSR receives the BSM from another C-BSR, it compares its own priority with the priority
carried in the message. The C-BSR with a higher priority wins the BSR election. If a tie exists in the
priority, the C-BSR with a higher IP address wins. The loser uses the winner's BSR address to
replace its own BSR address and no longer regards itself as the BSR. The winner retains its own
BSR address and continues to regard itself as the BSR.
In a BIDIR-PIM domain, the BSR does the following:
Collects C-RP information from the received advertisement messages from the C-RPs.
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Encapsulates the C-RP information in the RP-set information.
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Distributes the RP-set information to all routers in the BIDIR-PIM domain.
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All routers use the same hash algorithm to get an RP for a specific multicast group.
Configuring a legal BSR address range enables filtering of BSMs based on the address range, which
prevents a maliciously configured host from masquerading as a BSR. The same configuration must be
made on all routers in the BIDIR-PIM domain. The following describes the typical BSR spoofing cases and
the corresponding preventive measures:
Some maliciously configured hosts can forge BSMs to fool routers and change RP mappings. Such
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attacks often occur on border routers. A BSR is inside the network and receiver hosts are outside the
network. To protect a BSR against external attacks, you can enable the border routers to do the
following
Perform neighbor checks and RPF checks on BSMs.
Discard unwanted messages.
When an attacker controls a router on the network, the attacker can configure the router as a C-BSR
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to win the BSR election. Through this router, the attacker controls the advertising of RP information.
For security purposes, you can configure a legal BSR address range on all routers on the network.
All routers will discard BSMs that are out of the legal address range.
These preventive measures can partially protect the BSR in a network. However, if an attacker controls a
legal BSR, the problem still exists.
To configure a C-BSR:
Step
1. Enter system view.
2. Enter PIM view.
3. Configure a C-BSR.
Command
system-view
pim [ vpn-instance
vpn-instance-name ]
c-bsr ip-address [ scope
group-address { mask-length
| mask } ] [ hash-length
hash-length | priority
priority ] *
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Remarks
N/A
N/A
By default, no C-BSR is configured.