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Configuring a BSR
A BIDIR-PIM 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 collects and advertises RP information in the
BIDIR-PIM domain.
Configuring a C-BSR
C-BSRs must 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 as
follows:
•
Initially, every C-BSR assumes itself to be the BSR of the BIDIR-PIM 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 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 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 must be made on all routers in the BIDIR-PIM domain. The following are 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 discard unwanted messages.
•
When an attacker controls a router on the network, the attacker can configure the router as a
C-BSR 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. Therefore, all routers can discard BSMs that are out of the legal address range.
The preventive measures can partially protect the security of BSRs in a network. If a legal BSR is
controlled by an attacker, the preceding problem still occurs.
When you configure a C-BSR, follow these guidelines:
•
Because a large amount of information needs to be exchanged between a BSR and the other
devices in the BIDIR-PIM domain, provide a relatively large bandwidth between the C-BSRs
and the other devices in the BIDIR-PIM domain.
•
For C-BSRs interconnected through a tunnel, static multicast routes need to be configured to
make sure the next hop to a C-BSR is a Tunnel interface. For more information about multicast
static routes, see "
Configuring multicast routing and forwarding
To configure a C-BSR:
Step Command
Remarks
1.
Enter system view.
system-view
N/A
2.
Enter public network PIM
view or VPN instance PIM
view.
pim
[
vpn-instance
vpn-instance-name
]
N/A
3.
Configure an interface as a
C-BSR.
c-bsr
interface-type
interface-number
[
hash-length
[
priority
] ]
No C-BSRs are configured by
default.
4.
Configure a legal BSR
address range.
bsr-policy
acl-number
Optional.
No restrictions on BSR address
