KB.15.15

Forwarding traffic with distributed trunking and spanning tree

Refer to Figure 67 (page 164) for the following discussion about forwarding traffic when spanning

tree is enabled. In this example, it is assumed that traffic is sent from a host off switch B to a server,

and from the server back to the host. STP can block any one of the upstream links; in this example,

STP has blocked all the links except the I1 link connected to DT1.

NOTE: STP is automatically disabled on the DT ports.

Figure 67 Distributed trunking with STP forwarding unicast, broadcast, and multicast traffic

Forwarding unicast traffic

Refer to Figure 68 (page 165) for the following discussion about forwarding traffic with

switch-to-switch distributed trunking. Traffic from Host X or Y that is destined for Host F is always

forwarded by Switch A over one of its standard 802.1AX trunk links to either Switch B or Switch

C. When either Switch B or Switch C receives incoming traffic from Switch A, the traffic is directly

forwarded to Switch F without traversing the ISC link.

Traffic from Host Y to Host D may go over the ISC if Switch A sends it to Switch C instead of

sending it to Switch B.

164 Port Trunking