In switch-to-switch distributed trunking, the peer DT switches behave like independent Layer 3 devices with their own IP addresses in each active VLAN. If a DT switch receives a packet destined for the peer DT switch, it switches the packet through the ISC link. Interfaces on a VLAN using DT typically use a single default gateway pointing to only one of the DT switches in a DT pair.
The example in the following figure shows Layer 3 (IP unicast) forwarding in a DT topology. The packet is sent as follows:
Switch A selects the link (using the trunk hash) to the DT pair. The packet is sent to the selected link DT_SW_1.
When DT_SW_1 receives the packet, it determines, based on the MAC address, that the packet must be sent over the ISC link to DT_SW_2.
When the packet arrives, DT_SW_2 performs a lookup and determines that the packet needs to be sent to Switch B.
Layer 3 forwarding (IP unicast) in DT topology
Another example in the next figure shows Layer 3 (IP unicast) forwarding in a DT topology. The packet is sent as follows:
Host 2 sends a packet to Switch C.
Switch C performs a lookup in the routing table and determines that the default gateway IP address is 10.0.0.1.
Layer 2 lookup determines that the outgoing interface is the DT port.
Hashing determines that the trunk member chosen is DT_SW_2 and the packet is sent there.
DT_SW_2 determines that the packet needs to be sent over the ISC link to DT_SW_1 based on the MAC address.
DT_SW_1 performs a lookup and determines that the packet goes to Switch A.
The packet is only forwarded if the outgoing interface is not a DT port, or if the outgoing DT port does not have an active interface on the peer switch.
