If ICMP is configured, the ICMP “hello” request is sent to the subscriber and checked for a response, until the configured
maximum number of attempts is exceeded.
For IP subnet sessions, the peer (destination) IP address to be used for ICMP “hello” requests will be all the IP addresses
within the subnet. This means “hello” requests will be sent sequentially (not simultaneously) to all the possible hosts within
that subnet. If there is no response from any host in that subnet, the session will be disconnected.
Another option is to configure ICMP directed broadcast for keepalive requests. If the subscriber hosts recognize the IP subnet
broadcast address, the ISG can send the ICMP “hello” request to the subnet broadcast address. The subscribers need not be
on the same subnet as the ISG for this configuration to work. A directed broadcast keepalive request can work multiple hops
away as long as these conditions are satisfied:
The group of subscribers identified by the subnet must have the same subnet mask provisioned locally as the subnet provisioned
on the subnet subscriber session on the ISG. Otherwise, the subscriber hosts will not recognize the subnet broadcast address.
The router directly connected to the hosts must enable directed-broadcast forwarding, so that the IP subnet broadcast gets
translated into a Layer 2 broadcast.
When these two conditions are satisfied, you can optimize the ICMP keepalive configuration to minimize the number of ICMP
Because enabling directed broadcasts increases the risk of denial of service attacks, the use of subnet directed broadcasts
is not turned on by default.