User guide

A-14 Cisco 2500 Series Access Server User Guide
LANs and WANs
In addition to data link layer addressing, network protocols (Layer 3) were developed to
allow more precise delivery of packets (or routing), within a larger internetwork. This
strategy is similar to the post office’s method of delivering mail. Everyone has a name (like
a MAC address), but that is not enough information to route letters efficiently. Zip codes
(like Layer 3 addresses) help mail reach its exact destination more efficiently by routing
them through the national mail infrastructure.
Bridged LANs
You can use data link layer bridges to extend the physical distance limitations of LANs.
(See Figure A-3.) It is important to note that the network layer logical addressing is not
affected by bridges, and network layer devices (such as access servers and routers) treat a
bridged Ethernet or bridged Token Ring as one data link for addressing purposes. The two
common kinds of bridges are transparent and source-route bridges.
Figure A-3 Two LANs Connected with a Bridge to Form Network 2
Translational bridging between two dissimilar LANs such as a Token Ring LAN and an
Ethernet LAN can cause problems for some Layer 3 protocols like AppleTalk and IPX.
Translational bridging should only be implemented after careful design consideration.
WAN Technologies
WANs connect networks together across longer distances, such as between cities, or across
continents. Figure A-4 illustrates a typical WAN architecture.
H3570
Layer 3
Layer 2
Layer 3
LAN A LAN B
Network 2
Network 1 Network 3
Router Bridge Router
Layer 1
Layer 2
Layer 1
Layer 2
Layer 1
Layer 2
Layer 1
Layer 2
Layer 1 Layer 1