BPDUs

Because each switch comes with a burned-in MAC address, if the switches use the
default priority, then the one with the lowest MAC address becomes the root
bridge. This means that this device will have a large number of packets passing
through it. If you have a 6509 and have spent lots of money on the fabric upgrades
to a 256GB backplane, the last thing you want is for an old switch in a closet to
become the root bridge. For this reason, it is strongly recommended that you
lower the number on the priority for core switches. Chapter 16 gives more information
on dealing with designs.
The following network analyzer output shows a BPDU broadcasted on a network. BPDUs
are sent out every two seconds by default. That might seem like a lot of overhead, but remember
that this is only a layer 2 frame, with no layer 3 information in the packet:
Flags: 0x80
802.3
Status: 0x00
Packet Length:64
Timestamp: 19:33:18.726314 02/28/2003
802.3 Header
Destination: 01:80:c2:00:00:00
Source: 00:b0:64:75:6b:c3
LLC Length: 38
802.2 Logical Link Control (LLC) Header
Dest. SAP: 0x42
802.1 Bridge Spanning Tree
Source SAP: 0x42
802.1 Bridge Spanning Tree
Command: 0x03 Unnumbered Information
802.1 - Bridge Spanning Tree
490
Chapter 15 
Layer 2 Switching and the Spanning Tree Protocol (STP)
Protocol Identifier: 0
Protocol Version ID: 0
Message Type: 0
Configuration Message
Flags: 000000
Root Priority/ID: 0x8000 / 00:b0:64:75:6b:c0
Cost Of Path To Root: 0x00000000 (0)
Bridge Priority/ID: 0x8000 / 00:b0:64:75:6b:c0
Port Priority/ID: 0x80 / 0x03
Message Age: 0/256 seconds (
exactly 0seconds
)
Maximum Age: 5120/256 seconds (exactly 20seconds)
Hello Time: 512/256 seconds (exactly 2seconds)
Forward Delay: 3840/256 seconds (exactly 15seconds)
Extra bytes (Padding):
........ 00 00 00 00 00 00 00 00
Frame Check Sequence: 0x2e006400
Notice the cost of path to root. It is 0 because this switch is actually the root bridge.
We’ll discuss path costs in more detail later in this chapter in the section, “Selecting the
Designated Port.”
The preceding network analyzer output also shows the BPDU timers, which are used to prevent
bridging loops, because the timers determine how long it will take the spanning tree to converge
after a failure.
BPDUs are susceptible to propagation delays, which can happen because of packet length,
switch processing, bandwidth, and switch utilization problems. These delays can lead to instability
in a network, because temporary loops might occur when BPDUs are not received in time
for remote switches in the network to include their information in the STP “plan.” The STP uses
timers to force ports to wait for the correct topology information.
As you can see in the output, the hello time is 2 seconds, the maximum age is 20 seconds, and
the forward delay is 15 seconds. These are the defaults.
When a switch first boots up, the only MAC address it knows is its own, so it advertises itself
as the root. As it collects BPDUs, it will acknowledge another device as the root, if necessary.
When a switch receives a BPDU advertising a device as root, with a better bridge ID than the
current root is using, the switch caches this information and waits. It will wait the duration of
the MaxAge timer before using the new root, allowing other switches in the network to also
receive the BPDU. This reduces the possibility of loops.