Next, end node K receives two copies of the frame originated by end node J.
While the reception of duplicate frames by a node is not generally fatal, at
best such duplication represents an inefficient use of available bandwidth.
Of graver consequence is the effect of duplicate frames on bridge 1 and
bridge 2. The frame flooded by bridge 1 onto interface 1 is ultimately read by
bridge 2 on interface 2. When bridge 2 reads this frame, it updates its
address table to indicate end node J is in the direction of the white LAN. In a
similar fashion, bridge 1 reads the frame flooded by bridge 2, and it updates
its address table to show end node J in the direction of the white LAN.
Consequently, the address tables of both bridges are corrupted and neither
bridge is now able to properly forward a frame to end node J.
This corruption is caused by the existence of alternate routes between hosts.
Such alternate routes are generally referred to as loops. The spanning tree
algorithm (fully described in IEEE 802.1 MAC Bridges) ensures the
existence of a loop-free topology in networks that contain parallel bridges.
The algorithm provides a single path (composed of bridges and intervening
LANs) between any two nodes in such an extended network. It also provides
a high degree of fault tolerance by allowing for the automatic reconfigura-
tion of the spanning tree topology in the face of bridge or data-path failure.
Five management-assigned values are required for derivation of the
spanning tree topology:
A multicast address specifying all bridges within the extended
network
A network-unique identifier for each bridge within the extended
network
A unique identifier for each bridge/LAN interface (called a port)
A priority specifying the relative priority of each port
A cost for each port
With these values assigned, bridges broadcast and process formatted frames
(called bridge protocol data units or BPDUs) to derive a single loop-free
topology throughout the extended network. BPDU frame exchange is
accomplished quickly, thus minimizing the time du which service is
unavailable between hosts.
In constructing a loop-free topology, the bridges within the extended
network first determine the root bridge, the bridge with the best (that is,
lowest) priority value. This bridge serves as the root of the loop-free
topology.
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Bridging Service
Transparent Bridging
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Summary of Contents for 600 Series
Page 1: ...Hewlett Packard Series 200 400 and 600 Routers HP Routing Services and Applications ...
Page 4: ......
Page 5: ...1 Product Notes ...
Page 6: ...Features of HP Routers Architecture and Technology Branch Office Routing Product Notes 1 2 ...
Page 38: ...Architecture and Technology Software Control Path Architecture 1 34 ...
Page 52: ...Branch Office Routing Future Directions 1 48 ...
Page 53: ...2 Routing Services Notes ...
Page 106: ...Bridging Service Traffic Prioritization 2 54 ...
Page 158: ...Novell IPX Routing Service NetBIOS Protocol Support 2 106 ...
Page 194: ...Data Compression for WAN Links Conclusion 2 142 ...
Page 195: ...3 Application Notes and Case Studies ...
Page 224: ...Improving Network Availability Application Recovery 3 30 ...
Page 234: ...ISDN Wide Area Network Design Dry Creek Joint Elem School District Performance 3 40 ...
Page 316: ......