Cabletron Systems MicroMMAC 22ES, Reference Manual

Bridging and Routing

MicroMMAC Local Management Routing Addendum

 3

2.3

BRIDGING AND ROUTING

Bridging —

Bridging creates separate segments on a network. The 

bridge examines a portion of each network frame called the header. This 
header contains control information for the frame. The bridge compares 
the destination address of the frame to a table of source addresses. If the 
address indicates that the sending station and the destination station are 
on the same side of the bridge, the frame is discarded. If the addresses do 
not indicate that, then the bridge forwards the frame to its other 
interface(s).

During this process, the bridge formulates a table that allows it to identify 
which stations are connected to which LAN segment. The destination 
addresses of received frames are compared to this address table and 
decisions are made to discard or forward based on the outcome. 
Transparent bridging allows locally connected devices to send frames to 
all devices as if they are all on the same LAN. 

Bridging allows frames to be sent to all destinations regardless of the 
network protocols used. It also allows protocols that cannot be routed 
(such as NETBIOS) to be forwarded, and optimizes internetwork capacity 
by localizing traffic on LAN segments. A bridge extends the physical 
reach of networks beyond the limits of each LAN segment. Filters are 
used to increase network security in bridged networks. 

Routing —

Routing provides a way to transfer user data from source to 

destination over different LAN and WAN links using one or more 
network protocol formats. Routing relies on routing address tables to 
determine the best path for each packet. Routing tables can be seeded 
(i.e., addresses for remote destinations are placed in the table along with 
network address masks and a metric for path latency). Routing tables are 
also built dynamically (i.e., the location of remote stations, hosts and 
networks are updated from broadcast packet information). Routing helps 
to increase network capacity by localizing traffic on LAN segments and 
reducing the amount of broadcasts that would result from bridged traffic. 
It also provides security by isolating traffic on segmented LANs. Routing 
extends the reach of networks beyond the limits of each LAN segment.