Cisco WS-C2980G-A Software Guide - Page 117

Chapter 7 Configuring Spanning Tree Understanding How MST Works Message Age and Hop Count IST and MST instances do not use the Message Age and Maximum Age timer settings in the BPDU. IST and MST use a separate hop count mechanism that is very similar to the IP TTL mechanism. You can configure each MST bridge with a maximum hop count. The root bridge of the instance sends a BPDU (or M-record) with the remaining hop count that is equal to the maximum hop count. When a bridge receives a BPDU (or M-record), it decrements the received remaining hop count by one. The bridge discards the BPDU (M-record) and ages out the information held for the port if the count reaches zero after decrementing. The nonroot bridges propagate the decremented count as the remaining hop count in the BPDUs (M-records) they generate. The Message Age and Maximum Age timer settings in the RST portion of the BPDU remain the same throughout the region, and the same values are propagated by the region's designated ports at the boundary. MST-to-PVST+ Interoperability These guidelines apply in a topology where you configure MST switches (all in the same region) to interact with PVST+ switches that have VLANs 1-100 set up to span throughout the network: • Configure the root for all VLANs inside the MST region. The ports that belong to the MST switch at the boundary simulate PVST+ and send PVST+ BPDUs for all the VLANs. This example shows the ports simulating PVST: Console> (enable) show spantree mst 3 Spanning tree mode MST Instance 3 VLANs Mapped: 31-40 Designated Root Designated Root Priority Designated Root Cost Designated Root Port 00-10-7b-bb-2f-00 8195 (root priority:8192, sys ID ext:3) 0 Remaining Hops 20 1/0 Bridge ID MAC ADDR Bridge ID Priority 00-10-7b-bb-2f-00 8195 (bridge priority:8192, sys ID ext:3) Port State Role Cost Prio Type 6/1 forwarding BDRY 10000 30 P2P, Boundary(PVST) 6/2 blocking BDRY 20000 32 P2P, Boundary(PVST) If you enable loop guard on the PVST+ switches, the ports might change to a loop-inconsistent state when the MST switches change their configuration. To correct the loop-inconsistent state, you must disable and reenable loop guard on that PVST+ switch. • Do not locate the root for some or all of the VLANs inside the PVST+ side of the MST switch, because when the MST switch at the boundary receives PVST+ BPDUs for all or some of the VLANs on its designated ports, root guard sets the port to the blocking state. Do not designate switches with a slower CPU running PVST+ as a switch running MST. 78-15486-01 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 7-21