JUNOS Enterprise Switching

LAN Interconnection

Or: Repeaters, Bridges, and Routers, Oh My!

This section focuses on terminology and technology surrounding modern LAN interconnection. Given the qualifier, you can safely presume there will be no discussion of Source Route Bridging (SRB), translational bridging, or multiprotocol routing. When you’re bridging in a purely Ethernet environment there is no need to translate, and SRB is but a part of the dark Token Ring past that surfaces only to haunt humanity in the occasional bad dream.

Figure 1-11 shows the relationship of LAN interconnect devices to the OSI model.

Once again the OSI model shows its remaining utility. One simple figure makes it clear that repeaters operate at Layer 1, bridges at Layer 2, and routers at Layer 3. This means that repeaters spit bits, bridges frames, and routers packets. You cannot get to a packet without first dealing with the frame in which it was encapsulated. Thus, Figure 1-11 shows that routers process the Link and Physical layers. However, unlike a repeater or bridge, a router terminates both in its bid to obtain access to the inner Network layer packets, which are reframed and sent on the egress link.

Some key characteristics of each device are described in the following subsections.

Figure 1-11. LAN interconnect and the OSI

Repeaters

As noted, a repeater is a Physical layer device that regenerates the signal to compensate for transmission ...