H3C Technologies H3C S7500E Series Switches - Manual (Page 162)

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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:
z 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.
z 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 there is a tie 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, while 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 needs to be made on all routers in the 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 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.
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
still occur.
Follow these steps to configure a C-BSR:
To do… Use the command… Remarks
Enter system view system-view —
Enter public instance or VPN
instance PIM view
pim [ vpn-instance
vpn-instance-name ]
—
Configure an interface as a
C-BSR
c-bsr interface-type
interface-number [ hash-length
[ priority ] ]
Required
No C-BSRs are configured by
default.