Cisco WS-C2980G-A Software Guide - Page 160

Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Understanding How BackboneFast Works As soon as the switch transitions the alternate port to the forwarding state, the switch begins transmitting dummy multicast frames on that port, one for each entry in the local Enhanced Address Recognition Logic (EARL) table (except those entries that are associated with the failed root port). By default, approximately 15 dummy multicast frames are transmitted per 100 ms. Each dummy multicast frame uses the station address in the EARL table entry as its source MAC address and a dummy multicast address (01-00-0C-CD-CD-CD) as the destination MAC address. Switches receiving these dummy multicast frames immediately update their EARL table entries for each source MAC address to use the new port, allowing the switches to begin using the new path almost immediately. If connectivity on the original root port is restored, the switch waits for a period equal to twice the forward delay time plus 5 seconds before transitioning the port to the forwarding state to allow the neighbor port enough time to transition through the listening and learning states to the forwarding state. Understanding How BackboneFast Works BackboneFast provides fast convergence in the network backbone after a spanning tree topology change occurs. A switch detects an indirect link failure (the failure of a link to which the switch is not directly connected) when the switch receives inferior BPDUs from its designated bridge on its root port or blocked ports. These inferior BPDUs indicate that the designated bridge has lost its connection to the root bridge. An inferior BPDU identifies a single switch as both the root bridge and the designated bridge. Under normal spanning tree rules, the switch ignores inferior BPDUs for the configured maximum aging time (specified by the set spantree maxage command). The switch tries to determine if it has an alternate path to the root bridge. If the inferior BPDU arrives on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root bridge. If the inferior BPDU arrives on the root port, all blocked ports become alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity to the root bridge, causes the maximum aging time on the root to expire, and becomes the root switch according to normal spanning tree rules. If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new kind of protocol data unit (PDU) called the Root Link Query PDU out all alternate paths to the root bridge. If the switch determines that it still has an alternate path to the root, it causes the maximum aging time on the ports on which it received the inferior BPDU to expire. If all the alternate paths to the root bridge indicate that the switch has lost connectivity to the root bridge, the switch causes the maximum aging times on the ports on which it received an inferior BPDU to expire. If one or more alternate paths can still connect to the root bridge, the switch makes all ports on which it received an inferior BPDU its designated ports and moves them out of the blocking state (if they were in the blocking state), through the listening and learning states, and into the forwarding state. Figure 8-3 shows an example of a BackboneFast network topology. Switch A, the root switch, connects directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that connects directly to Switch B over link L3 is in the blocking state. Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 8-4 78-15486-01