Cabletron Systems NB30 manual About Transparent Bridging, Viewing and Managing Bridging Interfaces

NB-30 Bridging

About Transparent Bridging

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.

Viewing and Managing Bridging Interfaces

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:

¥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 Bridge Status, page 3-3).

¥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 Bridge Statistics, page 3-9).

3-2 Bridging Basics