3Com PathBuilder S200 Series Owner'S Manual Download

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Bridging

T0008-16F

Release 5.2M

Spanning Tree Protocol Entity (STPE)

Bridge Links

There are three types of bridge links within a given spanning tree network:

• The Root Bridge Link. The link representing the best path to the root bridge.

A root link is always on the spanning tree.

• The Designated Bridge Links. All the other bridge links on the spanning tree.

• The Standby Bridge Links. All other bridge links which are not on the

spanning tree.

All the bridge links of the root bridge are in the spanning tree and are designated
bridge links.

Forwarding and
Blocking States

After the spanning tree is determined, all root links and all designated links are
placed in a forwarding state and standby links are placed in a blocking state.
These states refer to the action that a link performs on data frames. For Source Route
Bridging, forwarding and blocking refer to Spanning Tree Explorer data frames
(Specifically Routed Frames and All Route Explorer frames are not subjected to
blocking/forwarding by this link state).

When a link is in blocking state, it still monitors and passes to its own bridge the
Hello message from the adjacent designated bridge link. Bridge links are not put into
a forwarding state immediately upon determining their link classification.
Forwarding Delay is used to allow the determination of the spanning tree network to
stabilize. This prevents the network from sending information frames into temporary
routing loops.

Topology Change
Notification

The Topology Change Notification Bridged Packet Data Unit (BPDU) is used by a
bridge that notices a topology change to send a notification in the direction of the
Root Bridge. This occurs only during Automatic spanning tree operation.

When the Root Bridge finally gets this notification, it sets the topology change
notification bit in the BPDU that it periodically generates. This informs all bridges
that there has been a change in topology and that they should expect that station
locations might have changed.

In a manual spanning tree, all the single paths are manually assigned. No Hello
frames are exchanged between bridges; rather, each bridge port in the network is
configured to either forward or block all Spanning Tree Explorer frames. If a link or
bridge goes down, then that path stays broken until the problem is fixed.

No topology change notification occurs in manual spanning tree operation. You have
to adjust the spanning tree to changes in your network either by reconfiguring the
spanning tree or wait until the lost path is restored.

A manual spanning tree forms fixed single route paths between LANs, and cannot
dynamically reestablish an alternate path for the broken one. However, the
PathBuilder S200 series switch LAN option can use its SVC rerouting capability to
overcome this problem. The PathBuilder S200 series switch senses the break, drops
the original SVC between the bridged nodes, and re-autocalls creating another SVC
connection using a different path across the WAN.

Since a manual spanning tree does not send Hello messages between bridges, it
minimizes network bandwidth overhead. An automatic spanning tree requires
sending and receiving BPDUs, which consumes processing cycles from the CPU of a
node and therefore increases CPU use. A manual spanning tree avoids this usage.