3Com S7906E Configuration Manual page 1300

For the configuration of other timers in IPv6 PIM-SM, refer to
Configuring a BSR
An IPv6 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 IPv6 PIM-SM domain.
Configuring a C-BSR
C-BSRs should be configured on routers in the backbone network. When configuring a router as a
C-BSR, make sure to specify the IPv6 address of an IPv6 PIM-SM enabled interface on the router. The
BSR election process is summarized as follows:
Initially, every C-BSR assumes itself to be the BSR of this IPv6 PIM-SM domain, and uses its
interface IPv6 address as the BSR address to send bootstrap messages.
When a C-BSR receives the bootstrap message of another C-BSR, it first compares its own priority
with the other C-BSR's priority carried in the message. The C-BSR with a higher priority wins. If
there is a tie in the priority, the C-BSR with a higher IPv6 address wins. The loser uses the winner's
BSR address to replace its own BSR address and no longer assumes itself to be the BSR, while
the winner keeps its own BSR address and continues assuming itself to be the BSR.
Configuring a legal range of BSR addresses enables filtering of bootstrap messages based on the
address range, thus to prevent a maliciously configured host from masquerading as a BSR. The same
configuration needs to be made on all routers in the IPv6 PIM-SM domain. The following are typical
BSR spoofing cases and the corresponding preventive measures:
1) Some maliciously configured hosts can forge bootstrap messages 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 bootstrap
messages and discard unwanted messages.
2) When a router in the network is controlled by an attacker or when an illegal router is present in the
network, the attacker can configure this router as a C-BSR and make it win BSR election to control
the right of advertising RP information in the network. After being configured as a C-BSR, a router
automatically floods the network with bootstrap messages. As a bootstrap message has a hop limit
value of 1, the whole network will not be affected as long as the neighbor router discards these
bootstrap messages. Therefore, with a legal BSR address range configured on all routers in the
entire network, all these routers will discard bootstrap messages from out of the legal address
range.
The above-mentioned preventive measures can partially protect the security of BSRs in a network.
However, if a legal BSR is controlled by an attacker, the above-mentioned problem will also occur.
Follow these steps to complete basic BSR configuration:
To do...
Enter system view
Use the command...
system-view
1-21
Configuring IPv6 PIM Common
Remarks
—
Timers.