The second is to
improve the robustness of the spanning tree protocol, so the other bridges recognize the
problem and avoid creating loops.
The problem of protocol robustness is very amenable to a solution in the standard,
and in fact, the solution to this problem is one of the tasks in the Project Authorization
Request for IEEE Project 802.1aq Shortest Path Bridging. The solution is in three
parts:
1. The spanning tree protocols??™ state machines are modified slightly to transmit
BPDUs even from ports that are not designated ports. (This change can be made
in any bridge without disrupting its ability to interoperate with bridges that have
not been so updated.)
2. Each bridge port in configured as to whether the port is expected to connect to
another bridge or not.
3. If a bridge port is configured to expect another bridge, and if that other bridge is
not transmitting BPDUs, then the port is disabled.
Ethernet Bridging 403
Currently, in a bridge running spanning tree, if a bridge is not receiving BPDUs on
a given port, then it forwards traffic to that port. If a bridge goes ???brain dead,??? it stops
sending BPDUs, so all bridges forward traffic toward it, creating loops.
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