The bsm semantic fragmentation command enables BSR message fragmentation.
The undo bsm semantic fragmentation command disables BSR message fragmentation.
By default, BSR message fragmentation is not enabled.
Usage Scenario
A BSR message carries information about all C-RPs on the network. Therefore, if there is a large number of C-RPs on the network, the length of a BSR message exceeds the MTU of the outgoing interface. As a result, the BSR message cannot be processed and RP election fails. Consequently, multicast services cannot be transmitted normally. In this case, you can enable BSR message fragmentation to ensure that the devices on the network can learn consistent RP information and MDTs can be successfully established.
Prerequisites
IPv6 multicast routing has been enabled globally using the multicast ipv6 routing-enable command in the system view.
Configuration Impact
You can also configure IP fragmentation to solve the preceding problem. The difference between IP fragmentation and BSR message fragmentation is as follows:
If IP fragmentation is enabled, the protocol layer transmits the entire BSR message up to the IP layer regardless of the length of the BSR message. The BSR message is then fragmented at the IP layer. If one fragment is lost during transmission, the destination cannot parse the entire BSR message. As a result, the destination cannot learn RP information and MDTs cannot be established, which causes a multicast data forwarding failure.
If BSR message fragmentation is enabled, the protocol layer directly fragments a long BSR message. If one fragment is lost during transmission, only the information carried in this fragment is lost. As a result, only MDTs corresponding to the information carried in the lost fragments cannot be established. Since the other BSR message fragments can still reach the destination, the corresponding MDTs can be correctly established.
BSR message fragmentation is recommended because IP fragmentation causes all fragments to become unavailable when fragment information is lost.
Precautions
Enable BSR message fragmentation on all devices on the network. If BSR message fragmentation is not enabled on some devices, RP information on these devices is inconsistent with that on other devices and an MDT cannot be established correctly.