IP Broadcast Handling Overview

To understand how IP broadcast handling functions in the ExtremeXOS software, consider the following two examples.

For the first example, a system sends an IP packet (such as the IP packet generated by the ping command) to an IP subnet directed broadcast address which is directly connected to that system. In this case, the IP packet goes out as a Layer 2 broadcast with the destination media access control (DMAC) addresses all set to FF, while the source media access control (SMAC) is set to the system MAC. This packet is sent out of all the ports of the VLAN.

In the second example, a system sends a packet (such as the IP packet generated by the ping command) to an IP subnet directed broadcast address which is remotely connected through a gateway. In this case, the IP packet goes out as a Layer 2 unicast packet with the DMAC equal to the gateway's MAC address, while the SMAC is set to the system MAC. At the gateway router, the existing IP packet forwarding mechanism is sufficient to send the packet out of the correct interface if the router is not the final hop router.

When the packet reaches the final hop router, which is directly connected to the target IP subnet, IP directed broadcast forwarding needs to be turned on.

The IP broadcast handling feature is applicable only at the final hop router directly attached to the target subnet. At the final hop router, when IP subnet directed broadcast forwarding is enabled on an IP VLAN via the command line, the following happens:

When IP subnet directed broadcast is disabled on an IP VLAN, it is disabled on all VLAN ports and all IP subnet directed broadcast entries are deleted.

Note

Note

IP subnet directed broadcast uses fast-path forwarding.