bsm semantic fragmentation (IPv4)

Function

The bsm semantic fragmentation command enables BootStrap router (BSR) message fragmentation.

The undo bsm semantic fragmentation command disables BSR message fragmentation.

Format

bsm semantic fragmentation

undo bsm semantic fragmentation

Parameters

None

Views

PIM view

Default Level

2: Configuration level

Usage Guidelines

By default, BSR message fragmentation is not enabled.

A BSR message carries information about all Candidate-Rendezvous Points (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 Maximum Transmission Unit (MTU) of the outgoing interface. As a result, the BSR message cannot be processed and Rendezvous Point (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.

BSR message fragmentation is recommended because it can solve the problem caused by IP fragmentation in which all fragments become unavailable due to loss of fragment information.

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. During the transmission of BSR message fragments to the destination, if one fragment is lost, 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. During the transmission of BSR message fragments to the destination, if one fragment is lost, 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.

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 MDTs cannot be established on these devices.

Example

# Enable BSR message fragmentation.

<sysname> system-view
[sysname] multicast routing-enable
[sysname] pim
[sysname-pim] bsm semantic fragmentation