Enterasys Networks BRIM-E6 manual Ethernet Bridging

ETHERNET BRIDGING

As with repeaters, IEEE recommends a maximum number of bridges that can be in a signal path. With repeaters we are able to have a signal path of 4 repeaters, 5 segments. With bridges we can have a signal path of 7 bridges, 8 networks. The bridge count includes both local and remote bridges.

Spanning Tree Algorithm

Since bridges play a very important role in the transfer of data from one network to another, a helpful feature would be the ability to set up a redundant bridge that would commence operation automatically if the primary bridge failed. In the IEEE draft standard 802.1d, IEEE chose to build some fault tolerance into the bridge specification. The 802.1d specification defines bridge operation, redundancy and a process called Spanning Tree Algorithm (STA). STA manages the primary and backup bridges and also guards against data loops and duplicate data.

When a bridge is powered up, it goes through a series of self tests to check its internal operation. During this time the bridge is in a standby, or blocking condition and will not forward traffic. Also during this standby period, the bridge sends out special bridge management packets called Configuration Bridge Protocol Data Units (BPDU). A BPDU is 1 byte in length and serves to identify the BPDU type as either a configuration or topology change BPDU. Bridges use the BPDU packets as a way of communicating with each other.

Spanning Tree Operation

Upon power up, Bridge 1 and 2 enter a standby, or blocking condition. Bridge 2 transmits a Configuration BPDU from its root port claiming it is the root. The BPDU is seen by Bridge 1 which inspects the BPDU for address and priority along with other pertinent information. Assuming equal priorities, Bridge 1 will transmit a BPDU to inform Bridge 2 of Bridge 1's address and priority. Upon seeing the response from Bridge 1, Bridge 2 determines that Bridge 1 has the higher priority and is therefore the primary (or Root) bridge. Bridge 2 will remain in a standby or blocking condition and will continue to monitor the network, listening for Bridge 1 Configuration BPDUs. Once Bridge 1 is sure there are no data loops, Bridge 1 comes on-line and normal network operations will resume. This process is called spanning.

BRIM-E6 USER’S GUIDE Page 5