Troubleshooting Transparent Bridging Environments - Internetworking Troubleshooting - page 553

Frame Format

Transparent bridges exchange configuration messages and topology change messages. Configuration
messages are sent between bridges to establish a network topology. Topology change messages are sent
after a topology change has been detected to indicate that the STA should be rerun.
The IEEE 802.1d configuration message format is shown in Figure 20-4.

Figure20-4 The Transparent Bridge Configuration

Protocol
identifier

Version Messa

type

ge Flags Root

ID

Root
path
cost

Bridge Port

ID

Messa
age

ge Maxim

age

um Hello

time

Forward
delay

2 bytes 1 byte 1 byte 1 byte 8 bytes 4 bytes 8 bytes 2 bytes 2 bytes 2 bytes 2 bytes 2 bytes

Message Format

The fields of the transparent bridge configuration message are as follows:
• Protocol identifier —Contains the value 0.
• Version —Contains the value 0.
• Message type —Contains the value 0.
• Flag —A 1-byte field, of which only the first 2 bits are used. The topology change (TC) bit signals
a topology change. The topology change acknowledgment (TCA) bit is set to acknowledge receipt
of a configuration message with the TC bit set.
• Root ID —Identifies the root bridge by listing its 2-byte priority followed by its 6-byte ID.
• Root path cost —Contains the cost of the path from the bridge sending the configuration message
to the root bridge.
• Bridge ID —Identifies the priority and ID of the bridge sending the message.
• Port ID —Identifies the port from which the configuration message was sent. This field allows loops
created by multiply attached bridges to be detected and dealt with.
• Message age —Specifies the amount of time since the root sent the configuration message on which
the current configuration message is based.
• Maximum age —Indicates when the current configuration message should be deleted.
• Hello time —Provides the time period between root bridge configuration messages.
• Forward delay —Provides the length of time that bridges should wait before transitioning to a new
state after a topology change. If a bridge transitions too soon, not all network links may be ready to
change their state, and loops can result.
Topological change messages consist of only 4 bytes. They include a Protocol Identifier field, which
contains the value 0; a Version field, which contains the value 0; and a Message Type field, which
contains the value 128.

Different IOS Bridging Techniques

Cisco routers have three different ways of implementing bridging: