The virtual-subnet command in the sub-if>dhcp context allows the system to snoop and record the default router address in DHCP ACK messages for a DHCPv4 ESM host. The system can answer ping or traceroute requests even if the default router address is not configured on the subscriber-interface.
This feature eliminates the need to configure every default-gw address on subscriber interface. Beside default router address, the system also calculates host’s subnet by using an assigned address and the subnet mask option in ACK. Both recorded default router address and the subnet can be displayed with the show service id virtual-subnet command.
Every ESM subscriber only has one set of default router address and subnet.