Table of Contents

Keywords
Examples
Related Commands

Bridging Global

This section is used to modify parameters that affect the way bridging and the IEEE Spanning Tree algorithm operate. These parameters are global to the device and are not associated with a particular interface.

Keywords

After entering the configure command for the section, enter one or more of the following keywords.

Mode = {IEEE | Learning | Off}	The Mode keyword specifies whether bridging will be enabled and how it will be configured for the system as a whole. To disable bridging, set the mode to Off. The bridge supports two operating modes: IEEE and Learning. The IEEE mode configures the bridge to support the IEEE 802.1D Spanning Tree algorithm. The Spanning Tree algorithm is used by bridges to detect loops (i.e., two or more pathways to the same destination) and "prune" them into a tree-like, loop-free topology by establishing a root bridge and then calculating the best path from each bridge to the root bridge. Traffic is then forwarded only along this path. If the network to which the bridge is attaching contains loops, Spanning Tree must be enabled to prevent packet duplication. The Learning mode configures the bridge for operation with the Spanning Tree algorithm disabled. The bridge listens to all network traffic and builds an Ethernet address cache of the devices on each interface. When a bridge receives a packet on one interface which is destined for an address on another interface, it looks up the destination in its address cache. If it has an entry, it forwards the packet directly to the appropriate interface. If it doesn't have an entry, it forwards the packet to all interfaces except the one from which it was received. If there is a loop in the network topology, a bridge that doesn't employ the Spanning Tree algorithm will endlessly forward the same packet back and forth on its interfaces because it cannot detect the loop formed by the second pathway. Learning mode should only be used on networks without active loops. Note   Because the parameters in this section are global to the device, it isn't possible to turn on IEEE (Spanning Tree) or Learning for individual interfaces. When the mode is IEEE, the root bridge dictates the parameters for the whole network.
AgingTime = Number	The AgingTime keyword sets the time that entries can remain in the bridge's Ethernet address cache. Each time the bridge receives traffic for an address, the aging timer is reset for that address. If no traffic comes through for the address and the aging time expires, the entry is purged. The default value is 300 seconds. Values range from 10 to 100,000 seconds.
HashTableSize = Number	The HashTableSize keyword sets the maximum number of address entries in the bridge's Ethernet address cache. The bridge only allocates as many entries as it needs, up to the limit specified in this parameter. The default value is 1024. Values range from 256 to 16,384.
BridgePriority = Number	The BridgePriority keyword is used by the Spanning Tree algorithm to calculate the root bridge. The bridge priority is combined with the bridge's Ethernet address to create an 8-byte bridge ID. The Spanning Tree algorithm uses the bridge ID to determine the root bridge for a network. The numerically lowest bridge ID on a network will be the root bridge for that network. There will only be one root bridge on a network. The IEEE recommended default value is 32,768; values range from 0 to 65,535.
MaxAge = Number	The MaxAge keyword is used to determine when a Spanning Tree configuration packet is considered stale and its information is discarded. The default value recommended by the IEEE specification is 20 seconds; values range from 6 to 40 seconds.
HelloTime = Number	The HelloTime keyword sets the interval between Spanning Tree configuration packets sent by the bridge. The default value recommended by the IEEE specification is 2 seconds; values range from 1 to 10.
ForwardDelay = Number	The ForwardDelay keyword sets the time that a bridge will spend determining whether or not to include an interface in the network's Spanning Tree. If included, the interface will spend this same amount of time listening to network traffic and building its address cache before it begins forwarding packets. It is also used as the aging time during periods of topology change on the network. The recommended default value is 15 seconds; values range from 4 to 30 seconds.

Examples

The following example shows a bridge configuration for a network with an unstable topology. By setting the Spanning Tree parameters to the minimum values, the topology changes will be detected quicker at the expense of more Spanning Tree protocol traffic on the network.

[ Bridging Global ]
Mode            = IEEE
AgingTime       = 300
HashTableSize   = 1024
MaxAge          = 6
HelloTime       = 1
ForwardDelay    = 4
 

To set this as the root bridge, set the bridge priority to a lower value.

[ Bridging Global ]
BridgePriority  = 1000

Related Commands

Command	Description
configure Bridging	Sets bridging parameters for an interface
set bridge	Sets bridge configuration parameters
show bridge	Shows bridge configuration, status and statistics

Posted: Wed Sep 27 10:29:08 PDT 2000
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