Configuring the Router-Alert Option

Context

Generally, a network device sends a message to the corresponding protocol stack for processing only when the destination address of the message is the address of a local interface. According to this rule, IGMP messages cannot be sent to the IGMP stack because their destination addresses are multicast addresses but not a local interface address. As a result, the network device cannot maintain group memberships. The Router-Alert option was introduced to solve this problem. If a message contains the Router-Alert option in the IP header, devices that receive the message send the message to the corresponding protocol stack without checking its destination IP address.

By default, the switch sends received IGMP messages to the IGMP stack for processing regardless of whether the messages contain the Router-Alert option in their IP headers. This ensures compatibility between the switch and other devices. Configuring the switch to discard IGMP packets without the Router-Alert option can improve device performance, reduce transmission cost, and enhance protocol security.

You can also configure the switch to send IGMP messages with or without the Router-Alert option. By default, the switch sends IGMP messages with the Router-Alert option. If the switch needs to communicate with a device that does not support the Router-Alert option, configure the switch to send IGMP messages without the Router-Alert option.

The Router-Alert option can be configured in the IGMP view or interface view.

Configuration in the IGMP view is valid globally, whereas configuration in the interface view is valid only for the specific interface.
If the Router-Alert option is configured in both the interface view and IGMP view, configuration in the interface view takes precedence over configuration in the IGMP view. If the Router-Alert option is not configured on an interface, the interface uses the configuration in the IGMP view.
If non-default configuration is made in the IGMP view, the default configuration in the interface view does not take effect.

Procedure

Configuring the Router-Alert option globally
1. Run system-view
  The system view is displayed.
2. Run igmp [ vpn-instance vpn-instance-name ]
  The IGMP view is displayed.
3. Run require-router-alert
  The switch is configured to check the Router-Alert option in received IGMP messages and drop IGMP messages without the Router-Alert option.
  
  By default, the switch does not check whether the received IGMP messages contain the Router-Alert option, and accepts all the received IGMP messages.
4. Run send-router-alert
  The switch is configured to send IGMP messages with the Router-Alert option.
  
  By default, IGMP messages sent by the switch contain the Router-Alert option.
Configuring the Router-Alert option on an interface
1. Run system-view
  The system view is displayed.
2. Run interface interface-type interface-number
  The interface view is displayed.
3. (Optional) On an Ethernet interface, run undo portswitch
  The interface is switched to Layer 3 mode.
  
  By default, an Ethernet interface works in Layer 2 mode.
  
  Only the S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S support switching between Layer 2 and Layer 3 modes.
4. Run igmp require-router-alert
  The switch is configured to check the Router-Alert option in IGMP messages received on the interface and drop IGMP messages without the Router-Alert option.
  
  By default, the switch does not check for the Router-Alert option in IGMP messages and processes all the IGMP messages received on an interface.
5. Run igmp send-router-alert
  The switch is configured to send IGMP messages with the Router-Alert option from this interface.
  
  By default, IGMP messages sent from an interface contain the Router-Alert option.