Understanding How Backbonefast Works - Cisco WS-C2948G-GE-TX Configuration Manual

Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard

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. This situation
allows 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 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
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