Bridging, Switching, and Spanning Tree 145
For more information on Fast Ethernet and information on Gigabit Ethernet, try the following
Web pages:
·
wwwhost.ots.utexas.edu/ethernet/ethernet-home.html
·
www.ots.utexas.edu/ethernet/descript-100quickref.html
·
www.iol.unh.edu/training
·
www.cisco.com/warp/customer/cc/cisco/mkt/switch/fasteth/tech/feth_tc.htm
·
www.cisco.com/warp/customer/cc/cisco/mkt/switch/gig/tech/index.shtml
·
www.gigabit-ethernet.org
Bridging, Switching, and Spanning Tree
Transparent bridging and LAN switching are two topics you must understand to succeed on the
CCNA exam. The underlying logic between the two is very similar, so both are described in this
section. The Spanning-Tree Protocol, which prevents loops from occuring in a bridged/
switched network, is described after bridging and switching basics are completed. Finally, a
comparison of what happens when a single Ethernet is migrated to a pair of Ethernets--
separated by a bridge in one case, a switch in another case, and a router in the third case--serves
as a good review of the concepts behind all three types of campus forwarding devices.
The IOS also supports other types of bridging, namely source-route bridging (SRB), source-
route transparent bridging (SRT), and source-route translational bridging (SR/TLB). Cisco
expects CCNAs to be familiar with transparent bridging.
Transparent Bridging
Transparent bridging is called transparent because the endpoint devices do not need to know
that the bridge(s) exists. In other words, the computers attached to the LAN do not behave any
differently in the presence or absence of transparent bridges.
Transparent bridging is the process of forwarding frames, when appropriate. To accomplish
this, transparent bridges perform three key functions:
·
Learning MAC addresses by examining the source MAC addresses of each frame received
by the bridge
·
Deciding when to forward a frame and when to filter a frame, based on the destination
MAC address
·
Creating a loop-free environment with other bridges using the Spanning-Tree Protocol
ch04.fm Page 145 Monday, March 20, 2000 5:02 PM