Backbone Fast Redundant Backbone Paths
In the network backbone, or Core layer, a different method is used to shorten STP convergence. BackboneFast works by having a switch actively determine if alternate paths exist to the root bridge in the event that the switch detects an indirect link failure. Indirect link failures occur when a link not directly connected to a switch fails. A switch detects an indirect link failure when it receives inferior BPDUs from its Designated Bridge on either its root port or a blocked port. (Inferior BPDUs are sent from a Designated Bridge that has lost its connection to the Root Bridge, making it announce itself as the new Root).
Normally, a switch must wait for the Max Age timer to expire before responding to the inferior BPDUs. However, BackboneFast begins to determine if other alternate paths to the Root Bridge exist according to the type of port that received the inferior BPDU. If the inferior BPDU arrives on a port in the Blocking state, the switch considers the root port and all other blocked ports to be alternate paths to the root bridge. If the inferior BPDU arrives on the root port itself, the switch considers all blocked ports to be alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no blocked ports, however, the switch assumes it has lost connectivity with the root bridge. In this case, the switch will assume that it has become the root bridge and BackboneFast will allow it to do so before the Max Age timer expires
Detecting alternate paths to the root bridge also involves an interactive process with other bridges. If the local switch has blocked ports, BackboneFast begins to use the Root Link Query (RLQ) protocol to see if there are upstream switches that have stable connections to the Root
Bridge. RLQ Requests are sent out. If a switch receives an RLQ Request and is either the Root Bridge or has lost connection to the Root, it sends an RLQ Reply. Otherwise, the RLQ Request is propagated on to other switches until an RLQ Reply can be generated. On the local switch, if an RLQ Reply is received on its current Root Port, the path to the Root Bridge is intact and stable. If it is received on a non-Root Port, an alternate Root Path must be chosen. The Max Age Timer is immediately expired so that a new Root Port can be found.
BackboneFast is simple to configure and operates by short-circuiting the Max Age Timer when needed. Although this function shortens the time a switch waits to detect a Root Path failure, ports still must go through full-length Forward Delay Timer intervals during the Listening and Learning states. Where PortFast and UplinkFast enabled immediate transitions, BackboneFast can only reduce the maximum convergence delay from 50 to 30 seconds.
To configure BackboneFast, use the following command:
Switch (enable) set spantree backbonefast {enable | disable}
When used, BackboneFast should be enabled on all switches in the network because BackboneFast requires the use of the RLQ request and reply mechanism to inform switches of Root Path stability. The RLQ protocol is only active when BackboneFast is enabled on a switch. By default, BackboneFast is disabled.
Continue reading here: NSAP Addresses
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