H3C Technologies H3C S12500-X Series Switches - Manual (Page 87)

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Configuring a BSR
You must configure a BSR if C-RPs are configured to dynamically select the RP. In a network with a static
RP, this configuration task is unnecessary.
A PIM-SM domain can have only one BSR, but must have at least one C-BSR. Any router can be
configured as a C-BSR. Elected from C-BSRs, the BSR is responsible for collecting and advertising RP
information in the PIM-SM domain.
Configuring a C-BSR
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 PIM-SM 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, and the winner retains its own
BSR address and continues to regard itself as the BSR.
In a PIM-SM domain, the BSR collects C-RP information from the received advertisement messages from
the C-RPs, encapsulates the C-RP information in the RP-set information, and distributes the RP-set
information to all routers in the PIM-SM domain. 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, thereby
preventing a maliciously configured host from masquerading as a BSR. The same configuration must be
made on all routers in the PIM-SM 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
attacks often occur on border routers. Because a BSR is inside the network whereas hosts are
outside the network, you can protect a BSR against attacks from external hosts by enabling the
border routers to perform neighbor checks and RPF checks on BSMs and to discard unwanted
messages.
• When an attacker controls a router in the network or when an illegal router is present in the network,
the attacker can configure the router as a C-BSR and make it win the BSR election to advertise RP
information in the network. After a router is configured as a C-BSR, it automatically floods the
network with BSMs. Because a BSM has a TTL value of 1, the whole network will not be affected as
long as the neighbor router discards these BSMs. Therefore, with a legal BSR address range
configured on all routers in the network, all these routers can discard BSMs from 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.
When you configure a C-BSR, reserve a relatively large bandwidth between the C-BSR and the other
devices in the PIM-SM domain.
When C-BSRs connect to other PIM routers through tunnels, static multicast routes must be configured to
make sure the next hop to a C-BSR is a tunnel interface. Otherwise, RPF check is affected. For more
information about static multicast routes, see " Configuring multicast routing and forwarding ."
To configure a C-BSR: