Cabletron Systems NB30 Manual De Usuario

NB-30 Bridging

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Bridging Basics

Transparent bridges are most common in Ethernet networks. Individual 
Transparent bridges monitor packet trafÞc on attached network segments to learn 
where end stations reside in relation to each segment by mapping the Source 
Address of each received frame to the port (and segment) it was detected on. This 
information gets stored in the bridgeÕs Filtering Database.

When in the Forwarding state, the bridge compares a packetÕs destination address 
to the information in the Filtering Database to determine if the packet should be 
forwarded to another network segment or Þltered (i.e., not forwarded). A bridge 
Þlters a packet if it determines that the packetÕs destination address exists on the 
same side of the bridge as the source address.

If two or more bridges are connected to the same Ethernet LAN segmentÑplaced 
in parallelÑonly a single bridge must be allowed to forward data frames onto 
that segment. If two or more bridges were forwarding data frames onto the same 
Ethernet segment, the network would soon be ßooded. 

With a data loop in the topology, bridges would erroneously associate a single 
source address with multiple bridge ports, and keep proliferating data by 
forwarding packets in response to the ever-changing (but incorrect) information 
stored in their Filtering Database.

To avoid such data storms, Transparent bridges communicate with one another 
on the network by exchanging Bridge Protocol Data Units (BPDUs) to determine 
the network topology and collectively implement a Spanning Tree Algorithm 
(STA) that selects a controlling bridge for each LAN segment; this ensures that 
only a single data route exists between any two end stations and that topology 
information remains current.

With SPECTRUM Element Manager, you can view and manage each bridging 
interface supported by your NB-30. You manage your bridge by using the 
following windows:

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The Bridge Status window provides you with basic information about the 
current status of the NB-30Õs bridging interfaces, allows you to conÞgure each 
interface, and lets you enable or disable bridging across the bridge. The Bridge 
Status window also lets you access further windows to conÞgure bridging at 
the NB-30 (see 

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Bridge StatisticsÑincluding the Performance Graph, Bridge Summary 
Statistics 

and the Ethernet and Remote Port Statustics windowsÑgraphically 

display the trafÞc passing between your bridged networks, and let you 
compare and contrast trafÞc processed by each interface (see