KB.15.15

Figure 69 Broadcast/multicast/unknown traffic flow access DT switches

IP routing and distributed trunking

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 Figure 70 (page 167) shows Layer 3 (IP unicast) forwarding in a DT topology. The

packet is sent as follows:

1. Switch A selects the link (using the trunk hash) to the DT pair. The packet is sent to the selected

link DT_SW_1.

2. 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.

3. When the packet arrives, DT_SW_2 performs a lookup and determines that the packet needs

to be sent to Switch B.

166 Port Trunking