 (LO FTE) in response to received TU Path AIS. It is sent upstream to peer LO PTE. LO Path RAI is indicated by setting bit 8 of LO POH V5 byte to "1." LO Path RAI is detected by peer LO PTE when bit 8 of received V5 byte is set to "1" or 10 consecutive multiframes. Its removal is detected when peer LO PTE receives 10 consecutive multiframes with bit 8 of V5 byte set to "0." Note: LO Path RAI is only available when generating and/or receiving "floating mode" TU payload structures. A.6.1 Response to Abnormal Conditions This section describes the response to the wide range of conditions that can be detected by the maintenance means built into the SDH frames, and the flow of alarm and indication signals. Appendix B: Spanning Tree Protocol B.1 Spanning Tree Protocol In Chapters 3 and 9, we have seen that the Ethernet technology, born for applications in local area network (LAN) and private networks, is one of the candidates for the develop- ment of a next generation, packet-based, metro network. In this appendix, we describe one of the main protocols of the Ethernet suite: the span- ning tree. We look at all the advantages of the spanning tree protocol, but also the scalabil- ity limits that its adversaries underline to discourage its use in the carrier network. The spanning tree protocol (STP) is a link-management protocol that is part of the Institute of Electrical and Electronics Engineers (IEEE) 802.1 standard for bridges and switches. All the material of this appendix is a synthesis of IEEE Ethernet standards, SO we will not repeat here the list of such standards, recommending the interested reader to use the list in the references of Chapter 3. The STP is used to determine the best path from source to destination for Ethernet frames in a switched Ethernet network. It creates a hierarchical tree spanning the entire network, including all bridges and switches. Searching such a tree, the STP determines all possible paths between two network users and, when needed, it activates only one of them. STP also provides pa