Cisco Systems BC-109 manual Enable the Automatic Spanning-Tree Function, source-bridge spanning

Enable the Automatic Spanning-Tree Function

Forwarding all-routes explorer packets is the default. However, in complicated source-route bridging topologies, using this default can generate an exponentially large number of explorers that are traversing the network. The number of explorer packets becomes quite large because duplicate explorer packets are sent across the network to every node on every path. Eventually each explorer packet will reach the destination device. The destination device will respond to each of these explorer packets. It is from these responses that the source device will collect the RIF and determine which route it will use to communicate with the destination device. Usually, the route contained in the first returned response will be used.

The number of explorer packets traversing the network can be reduced by sending spanning-tree explorer packets. Spanning-tree explorer packets are sent to specific nodes; that is, to only the nodes on the spanning tree, not to all nodes in the network. You must manually configure the spanning-tree topology over which the spanning-tree explorers are sent. You do this by configuring which interfaces on the routers will forward spanning-tree explorers and which interfaces will block them.

To enable forwarding of spanning-tree explorers on an outgoing interface, use the following command in interface configuration mode:

Command	Purpose
source-bridge spanning	Enable the forwarding of
	spanning-tree explorer packets on an
	interface.

Note While enabling the forwarding of spanning-tree explorer packets is not an absolute requirement, it is strongly recommended in complex topologies. Configuring an interface to block or forward spanning-tree explorers has no effect on how that interface handles all-routes explorer packets. All-routes explorers can always traverse the network.

To block forwarding of spanning tree explorers on an outgoing interface, use the following command in interface configuration mode:

Command	Purpose
no source-bridge spanning	Block spanning-tree explorer packets
	on an interface.

Enable the Automatic Spanning-Tree Function

The automatic spanning-tree function supports automatic resolution of spanning trees in SRB networks, which provides a single path for spanning explorer frames to traverse from a given node in the network to another. Spanning explorer frames have a single-route broadcast indicator set in the routing information field. Port identifiers consist of ring numbers and bridge numbers associated with the ports. The spanning-tree algorithm for SRB does not support Topology Change Notification bridge protocol data unit (BDPU).

Note Although the automatic spanning-tree function can be configured with source-route translational bridging (SR/TLB), the SRB domain and transparent bridging domain have separate spanning trees. Each Token Ring interface can belong to only one spanning tree. Only one bridge group can run the automatic spanning-tree function at a time.

Configuring Source-Route Bridging BC-115