Network interconnection devices

Segments on a network can be interconnected using a:

repeater

which regenerates the signal of one segment onto one or more other segments.

bridge

which retransmits packets received on one segment onto another segment.

router

which receives its instructions for forwarding a packet from one segment to another from a network layer protocol.

Figure 4-7 OSI Representations of Network Devices

Repeaters

A repeater is a physical layer (OSI model) device that amplifies the signal of one segment onto one or more other segments. Repeaters are used to extend the maximum possible distance between end nodes on a segment. They are completely transparent to the sending and receiving nodes.

Bridges

A bridge is a data link layer device used to interconnect cable segments locally or over wide area network links. Instead of simply amplifying a signal as repeaters do, bridges retransmit packets received on one segment onto another segment. Bridges are considered data link layer devices because they examine the data link (or MAC header) portion of packets before retransmitting them onto other segments. There are two predominant types of bridge: source routing bridges and transparent bridges.

source routing bridges

Source routing allows Token-Ring segments to be interconnected by bridges, allowing administrators to segment network traffic. This requires that each workstation maintain a table of routes to the nodes with which it communicates. Furthermore, routing information must be included in the MAC header of each packet it sends. This information instructs bridges how to properly forward each packet to its destination. Source routing can be used instead of, or in conjunction with, NetWare routing. Because source routing is handled at the data link layer of the OSI model, it is beyond the scope of IPX/SPX.

transparent bridges

Transparent bridges interconnect two or more segments. They examine the MAC header source and destination fields of every packet transmitted on their connected segments. From the source address fields of packets, these bridges develop a table of the nodes that reside on (or are accessible through) each of their connected segments. With this table of information, a bridge can determine whether packets should be passed on to other segments.

Figure 4-8 Sample Transparent Bridge

After examining the packets transmitted on both segments, the bridge creates a table that tracks which nodes exist on each segment. With this table, the bridge can filter unnecessary traffic. For instance, if node 1 sends a packet to node 5, the bridge will not retransmit that packet on its port B. It will, however, retransmit packets sent from node 1 to node 7. Like repeaters, transparent bridges -- as their name implies -- are transparent to the sending and receiving nodes.

Routers

Routers interconnect different network segments. However, unlike bridges, routers, by definition, are network layer devices (see Figure 4-7). In other words, routers receive their instructions for forwarding a packet from one segment to another from a network layer protocol.

NetWare-compatible routers are available with NetWare or from third-party manufacturers. The routers that come packaged with NetWare have actually been misnamed ``bridges'' in the past. The NetWare routers include what has been called the internal bridge within NetWare file servers and the external bridge installed at workstations. Novell has officially renamed these two devices ``internal router'' and ``external router''.

NetWare-compatible routers can be configured to interconnect two or more segments. Each of these segments, however, must be assigned a unique network number to distinguish it from other segments on the network. A segment's network number must be configured into each of the routers connected to that segment. The network number serves as a common address for each node connected to a segment. For more information on assigning a segment's network number, see Appendix A, ``Configuration parameters'' in Configuring Network Connections.

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