Specifications

CHAPTER 3
Spanning Tree
Chapter 3
Spanning Tree
Ethernet network design balances two separate imperatives. First, Ethernet has no capacity for detecting circular paths. If
such paths exist, traffic loops around and accumulates until new traffic is shut out. (This is called a broadcast storm.)
Second, having secondary paths is good preparation for inevitable link failure.
Spanning Tree is a protocol that prevents loop formation by detecting redundant links and disabling them until needed.
Designers can therefore build redundant links, and the protocol enables one to pass traffic and keep the other in reserve.
When the active link fails, the secondary link is enabled quickly.
Understanding the Spanning Tree Protocol
Switches either forward or filter Layer 2 frames. The way they make the forwarding/filtering decision can lead to loops in
a network with redundant links. Spanning Tree is a protocol that detects potential loops and breaks them.
A Layer 2 switch is functionally the same thing as a transparent bridge. Transparent bridges:
n Learn MAC (Media Access Control) addresses by looking at the source address of incoming frames. They build a
table mapping MAC address to port number.
n Forward broadcasts and multicasts out all ports except the one in which they came. (This is called flooding.)
n Forward unknown unicasts out all ports except the one in which they came. An unknown unicast is a message bound
for a unicast MAC address that is not in the switch’s table of addresses and ports.
n Do not make any changes to the frames as they forward them.
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue