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 (Virtual LAN).
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.
Some basic validity checks are performed (for example, a check to see if the VLAN has IP enabled)
A subnet broadcast route entry for the subnet is installed. For example, consider a system with the following configuration:
VLAN-A = 10.1.1.0/24, ports 1:1, 1:2, 1:3, 1:4
VLAN-B = 20.1.1.0/24, ports 1:5, 1:6, 1:7, 1:8
VLAN-C = 30.1.1.0/24, ports 1:9, 1:10, 1:11
If you enable IP directed broadcast forwarding on VLAN-A, you should install a route entry for 10.1.1.255 on this system.
A packet arriving on port 1:5 VLAN-B with destination IP (DIP) set to 10.1.1.255, the source IP (SIP) set to 20.1.1.3, the DMAC set to the router MAC, and the SMAC set to the originating system MAC, arrives at the installed route entry and is sent out on all the ports of VLAN-A, with DMAC set to be all FF and the SMAC set to the router's system MAC.
An IP packet arriving on port 1:1 VLAN-A with the DIP set to 10.1.1.255, the SIP set to 10.1.1.3, the DMAC set to all FF, and the SMAC set to the originator‘s MAC, causes Layer 2 flooding on all ports of VLAN-A.
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
IP subnet directed broadcast uses fast-path forwarding.
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