Bridges

Question 1

Bridge

general characteristics

Answer 1

• simpler than using a router
• connects similar LANs
• identical protocols for physical and data link layers
• minimal processing
• reliable
• keep separately different kinds of traffic
• interconnect geographically separated LANs
• types of bridges: transparent and source routing

Question 2

Transparent Bridge

Answer 2

• reads all frames transmitted on one LAN and accepts those addresses to any station on the other LAN
• protocol relay
• no encapsulation
• exact bitwise copy of frame
• minimal buffering
• contains routing and addresses intelligence
• may connect more than two LANs
• bridging is transparent to stations

Question 3

Bridge Operations

Answer 3

Frame forwarding:
* maintains forwarding database for each port - list stations addresses reached through each port
* for a frame arriving on port X:
1. search forwarding database to see if MAC address is listed for any port except X;
2. if the address is not found, then forward to all ports except X;
3. if the address is listed for port Y, check port Y for blocking or forwarding state (blocking prevents port from receiving or transmitting);
4. if not blocked, transmit frame through port Y

Address Learning:
* can preload forwarding database
* can be learnt
* when frame arrives at port X, it has come from the LAN attached to port X
* use the source address to update forwarding database for port X to include that address
* timer on each entry in database
* each time frame arrives, source address checked against forwarding database

Loop resolution:
* use of spanning tree algorithm
* for any connected graph there is a spanning tree that maintains connectivity but contains no closed loops

Question 4

Spanning Tree

Answer 4

• develop routing table
• update in response to changes