Cabletron Systems 9H42X-XX Bridging

5-1

Bridging

Bridge management overview; the Bridge Status window; bridge Performance Graphs; changing

bridge Spanning Tree parameters; conﬁguring the Filtering Database; setting duplex modes; using the

port Source Addressing window

When conﬁgured to operate as a traditional switch, your 9H42x-xx Fast Ethernet

SmartSwitch modules have the ability to act as 802.1d Transparent Bridges to

direct trafﬁc between the network segments connected to their front panel and the

interface to the INB backplane.

Bridges are used in local area networks to connect two or more network segments

and to control the ﬂow of packets between the segments. Ideally, bridges forward

packets to another network segment only when necessary.

Bridges are also used to increase the fault tolerance in a local area network by

creating redundant bridge paths between network segments. In the event of a

bridge or bridge segment failure, an alternate bridge path will be available to

network trafﬁc, without signiﬁcant interruption to its ﬂow.

The method a bridge uses to forward packets, choose a bridge path, and ensure

that a sending station’s messages take only one bridge path depends on the

bridge’s type: Transparent (generally used in Ethernet or FDDI environments) or

Source Routing (generally used in Token Ring environments).

Transparent bridging relies on a “Filtering Database” to make forwarding

decisions. The source addresses of data coming into each bridge interface are read

and stored in a Filtering Database that associates each address with the interface it

was detected on. When a packet is received by the bridge, it can then compare the

destination address of the packet to the addresses in the Filtering Database, and

determine which bridge interface to send the packet to.

In Source Route bridging, the source node sends “explorer” packets to a

destination node that pass through a bridged network. Each bridge that sees the

explorer packet will append Routing Information (in the form of LAN segment

numbers) to it. When the destination node receives these explorer packets, it will