Exploration_LAN_Switching_Chapter5A.pdf

7/28/2019 Exploration_LAN_Switching_Chapter5A.pdf

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2006 Cisco Systems, Inc. All rights reserved. Cisco Publ icITE I Chapter 6 1

Implement Spanning

Tree Protocols

LAN Switching and Wireless Chapter 5


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2006 Cisco Systems, Inc. All rights reserved. Cisco Publ icITE 1 Chapter 6 2

Objectives

Explain the role of redundancy in a converged network

Summarize how STP works to eliminate Layer 2 loops in aconverged network

Explain how the STP algorithm uses three steps to

converge on a loop-free topology

Implement rapid per VLAN spanning tree (rapid PVST+) ina LAN to prevent loops between redundant switches.


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Redundancy in a Switched Network

The role of redundancy in a hierarchical network


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Redundancy can disable a hierarchical network


7/42 2006 Cisco Systems, Inc. All rights reserved. Cisco Publ icITE 1 Chapter 6 7


Redundancy can disable a hierarchical network




Issues with Redundancy




The Spanning Tree Protocol (STP)



STP eliminates L2 Loops in a Network

The STP algorithm




The STP algorithm

Root bridge: elected and used as a final destination; after the rootbridge is determined, the STA calculates the shortest path to theroot bridge

Ports:

Root ports: ports closest to the root bridge Designated ports: all non-root ports

permitted to forward traffic

Non-designated ports: portsconfigured to be in blocking state




The STP algorithm

Root bridge: the one with the lowest Bridge ID




The BPDU

All switches in the broadcast domain participate in the election

After boot, switches send out BPDU frames every 2 seconds

The BPDU information helps on the identification of the root bridge




The Bridge ID (BID):

Between 0 and 65536

Increments of 4096




The cost




Electing the Root Bridge


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Forcing the Root Bridge


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Port roles support the operation of STP: Root port:

Exists on non-root bridges (only 1 port per bridge)

Best path to the root bridge

Designated port:

Exists on root and non-root bridges

On root bridges: all ports

On non-root: forwards towards R.B.

ONLY 1 per segment

Non-designated port:

Switch port that is blocked

Is not forwarding data frames


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Port priority: Helps determine the root port on a switch

When there are 2 same lowest-cost-to-root-bridge ports, the lowestport priority value determines the root port

Port priority values range from 0 to 240 (in steps of 16)

If both port priority values are the same, use the lowest port ID



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Port Role Decisions



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Port Role Decisions



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Port Role Decisions



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Port Role Decisions



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Port Role Decisions



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Port Role Decisions



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Port Role Decisions



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Verifying port roles and port priority


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STP port states: Blocking: does not participate in frame forwarding; receives BPDUs

Listening: preparing to participate; sends/receives BPDUs

Learning: begins to populate MAC address table

Forwarding: forwards frames; sends/receives BPDUs

Disabled: the port is administratively disabled (down)



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BPDU timers: define amount of time a port stays in thevarios port states



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Adjusting BPDU timers: Timers optimized for the seven-switch diameter

To modify timers, adjust the Spanning-tree diameter on the rootbridge


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