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166 Chapter 4: Bridges/Switches and LAN Design

When the network is up and no problems are occurring, the process works like this:

Step 1

The root sends a CBPDU, cost 0, out all its interfaces.

Step 2

The neighboring bridges send CBPDUs out their nonroot port
interfaces referring to the root, but with their cost added.

Step 3

Step 2 is repeated by each bridge in the network as it receives these
CBPDUs, as long as the CBPDU is received on a bridge's root
port.

Step 4

The root repeats Step 1 every Hello time.

Step 5

If a bridge does not get a CBPDU in Hello time, it continues as
normal, unless the larger MaxAge timer is passed.

Reacting to Changes in the Spanning Tree

The process used to react to changes in topology varies depending on the situation. This section
describes two instances, one briefly and the other in detail. Other variations than the two
instances covered here do occur. Regardless of the details, the process always begins when a
bridge does not receive a CBPDU on its root port in MaxAge time.

No CBPDUs Received on Any Ports

If the bridge whose MaxAge parameter expires is also not receiving any other CBPDUs on
ports that are not the root port, that bridge reacts by claiming to be the root bridge and begins
sending CBPDUs describing itself. This process reduces to the same logic as described earlier
in the section "Building an Initial Spanning Tree."

For instance, imagine that the root bridge failed in the network in Figure 4-17. Each bridge
would have MaxAge expire at about the same time. Each would claim to be the root; one would
be elected. A different Spanning Tree would result, but the process is the same as described
earlier.

CBPDUs Received on Some Ports

The process of recalculating the Spanning Tree occurs only if CBPDUs are no longer received
on the root port. However, a bridge can still be receiving CBPDUs on other ports. Consider the
familiar diagram shown in Figure 4-19. Bridge 5's E1 port has failed, preventing Bridge 4 from
receiving CBPDUs on its root port (E1) interface.

ch04.fm Page 166 Monday, March 20, 2000 5:02 PM