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Spanning Tree Protocol (STP)
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which one has the lowest (best) ID. It's like this: If two switches--I'll name
them A and B--both use the default priority of 32,768, then the MAC address
will be used instead. If switch A's MAC address is 0000.0c00.1111.1111 and
switch B's MAC address is 0000.0c00.2222.2222, then switch A would
become the root bridge. Just remember that the lower value is the better one
when it comes to electing a root bridge.
BPDUs are sent every 2 seconds, by default, out all active ports on a
bridge/switch, and the bridge with the lowest (best) bridge ID is elected the
root bridge. You can change the bridge's ID so that it will become a root
bridge automatically. Being able to do that is important in a large switched
network--it ensures that the best paths are chosen.
Changing STP parameters is beyond the scope of this book, but it's covered
in CCNP: Switching Study Guide (Sybex, 2000).
Selecting the Designated Port
If more than one link is connected to the root port, then port cost becomes
the factor used to determine which port will be the root port. So, to deter-
mine the port or ports that will be used to communicate with the root bridge,
you must first figure out the path's cost. The STP cost is an accumulated total
path cost based on the available bandwidth of each of the links. Table 2.1
shows the typical costs associated with various Ethernet networks.
The IEEE 802.1D specification has recently been revised to handle the
new higher-speed links. The Cisco 1900 switches use the original IEEE
802.1D specifications.
T A B L E 2 . 1
Typical Costs of Different Ethernet Networks
Speed
New IEEE Cost
Original IEEE Cost
10Gbps
2
1
1Gbps
4
1
100Mbps
19
10
10Mbps
100
100
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