establish 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 most often occurs with 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 Common Spanning Tree Problems Root bridge Switch A Forwarding Forwarding G0/1 Switch B Forwarding Forwarding Switch C Switch D Figure 8-9. Unidirectional link problem 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. 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. Spanning tree is sending BPDUs out each interface on Switch A and Switch B because the links are active. Switch A is the root bridge. Switch B is only receiving BPDUs from the root bridge on one port: G0/1. Because Switch B is not receiving BPDUs from the root bridge on port G0/2, spanning tree does not block the port. Broadcasts received on Switch B on G0/1 will be retransmitted out G0/2. A loop is born. This problem can be difficult to uncover. A bridging loop causes mayhem in the net- work because th