The Router-Alert option ensures that packets can be sent to the routing protocol layer for processing.
Generally, packets are sent to and processed by the routing protocol only if the destination IP address is the IP address of an interface on the device. Therefore, Router-Alert, as a particular mechanism for marking protocol packets, is introduced. If a packet contains the Router-Alert option, it indicates that the packets must be sent to and processed by the routing protocol. In real applications, if the destination IP address of protocol packets is a multicast address or a particular IP address, the packets may not be sent to the routing protocol.
The destination IP address of IGMP packets is usually a multicast address, and the IGMP packets may not be sent to the routing protocol. In such a situation, the Router-Alert option can properly address the problem:
By default, Router-Alert is not checked, the multicast device sends the received IGMP messages to the routing protocol no matter whether these IGMP messages contain the Router-Alert option.
When Router-Alert is configured to be checked, only the IGMP messages containing the Router-Alert option can be sent to the routing protocol.