s Switch B will listen for BPDUs over the two links shown in the diagram. If no BPDUs are seen over the F0/0 link for a set amount of time, Switch B will no longer consider the F0/0 link to be a valid path to the root bridge. Because this was the primary path to the root bridge, and the root bridge can no longer be seen, Switch B will change F0/1 from blocking to forwarding to reestablish a path to the root bridge. At this point, there are no blocking ports on the two links connecting the switches, and a bridging loop exists. Unidirectional Links When a link is able to transmit in one direction but not another, the link is said to be unidirectional. While this can happen with copper Ethernet, the problem is most often seen when using fiber. A common issue when installing fiber plants is the cross-connection of individual fibers. Should a fiber pair be split, one fiber strand can end up on a different port or switch from the other strand in the pair. Figure 8-9 shows four switches. Switch A is supposed to be connected to Switch B by two pairs of fiber-one between the G0/1 ports on each switch, and another between the G0/2 ports on each switch. Somewhere in the cabling plant, the fiber pair for the G0/2 link has been split. Though the pair terminates correctly on Switch B, Switch A has only one strand from the pair. The other strand has been routed to Switch C and connected to port G0/3. Root bridge Switch A Forwarding Forwarding G0/1 Switch B Forwarding Forwarding Switch C Switch D Figure 8-9. Unidirectional link problem Fiber interfaces test for link integrity by determining whether there is a signal on the RX side of the pair. Switch B's port G0/2 has link integrity because its RX is active from Switch C, Switch A's port G0/2 has link integrity because its RX is active from Switch B, and Switch C's port G0/3 has link integrity because its RX is active from Switch D. Chapter 8: Spanning Tree Spanning tree is sending BPDUs out each interface on Switch A and Switch B because the links are active. Switch A is the ro