bsm semantic fragmentation (IPv4)

Function

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.

Format

bsm semantic fragmentation

undo bsm semantic fragmentation

Parameters

None

Views

PIM view of public network instance or PIM view of VPN instance

Default Level

2: Configuration level

Usage Guidelines

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

IP multicast routing has been enabled using the multicast routing-enable command.

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. 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.

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 MDTs cannot be established on these devices.

Example

# Enable BSR message fragmentation in the public network instance PIM view.

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