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Configuring a BSR
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 in the backbone network. When configuring a router as a C-BSR,
be sure to specify a 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 PIM-SM domain and uses its interface IP
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 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 assumes
itself to be the BSR, and the winner retains its own BSR address and continues to assume itself to
be the BSR.
Configuring a legal range of BSR addresses enables filtering of bootstrap messages based on the
address range, therefore 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 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 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 this router as a C-BSR and make it win BSR election to control the right
of advertising RP information in the network. After a router is configured as a C-BSR, it automatically
floods the network with bootstrap messages.
Because a bootstrap message has a TTL 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.
These preventive measures can partially protect the security of BSRs in a network. However, if an attacker
controls a legal BSR, the problem still exists.
Because the BSR and the other devices exchange a large amount of information in the PIM-SM domain,
provide a relatively large bandwidth between the C-BSRs and the other devices.
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."
To configure a C-BSR:
