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CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

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Chapter 5<br />

Spanning Tree Protocol<br />

- Most network engineers design LANs with redundant links between switches in order to provide<br />

higher availability as switch hardware and cable problems might occur.<br />

- Broadcast frames could loop forever in networks with redundant links – broadcast storm.<br />

Switching or bridging loops might caused by broadcast of ARP requests for locating an unknown<br />

or shutdown device, as switches were designed to forward unknown unicast frames.<br />

- Another problem is multiple frame copies, which could occur when a frame arrives from<br />

different segments at the same time, which could also lead to MAC address table thrashing.<br />

- Spanning Tree Protocol (STP) was introduced to solve these problems by placing switch ports<br />

in either forwarding or blocking state in forming a single active path called the spanning tree.<br />

The purpose of STP is to maintain a loop-free network topology in networks with redundant<br />

links. STP is enabled by default in <strong>Cisco</strong> Catalyst switches.<br />

- Switch ports in forwarding state can receive and forward frames.<br />

Switch ports in blocking state cannot receive and forward frames (but can still receive BPDUs).<br />

Note: If a blocking state port can receive frames, it will process and forward broadcast frames!<br />

PC1<br />

Fa0/1<br />

Fa0/1<br />

PC2<br />

SW1<br />

Fa0/2<br />

Blocking<br />

Fa0/2 PC1<br />

SW1<br />

Fa0/2<br />

Forwarding<br />

Fa0/2<br />

Fa0/1<br />

SW3<br />

Fa0/1<br />

SW3<br />

PC3<br />

Fa0/2<br />

SW2<br />

Figure 5-1: Network Setup for Spanning Tree Protocol<br />

- Figure 5-1A shows a network after the STP convergence – SW3’s Fa0/2 port is in blocking state.<br />

When PC1 sends a broadcast frame, SW1 will forward the broadcast frame to both SW2 and<br />

SW3. SW2 can forward the frame to SW3 as its Fa0/2 port is in forwarding state; whereas SW3<br />

cannot forward the frame to SW2 as its Fa0/2 port is in blocking state.<br />

- When the link between SW1 and SW3 fails (Figure 5-1B), SW3 will react as it no longer<br />

receiving any BPDU via its root port (Fa0/1). After the STP convergence, SW3 Fa0/2 will be<br />

changed to forwarding state, which allows SW3 to exchange frames with other switches.<br />

- The drawback of STP is it would cause some frames to traverse a longer and less-efficient path.<br />

Ex: If PC3 wants to send a frame to PC2, the frame will be traversed from SW3 SW1 SW2.<br />

39<br />

Fa0/1<br />

Fa0/1<br />

Figure 5-1A Figure 5-1B<br />

PC2<br />

PC3<br />

Fa0/2<br />

SW2<br />

Copyright © 2008 Yap Chin Hoong<br />

yapchinhoong@hotmail.com

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