1Version 3.0

Module 7Spanning Tree

Protocol

2Version 3.0

Redundancy

• Redundancy in a network is needed in case there is loss of connectivity in one segment.

• But redundancy in itself presents problems – loops.

• The Spanning-Tree Protocol is used in switched networks to create a loop free logical topology from a physical topology that has loops.

• Links, ports, and switches that are not part of the active loop free topology do not participate in the forwarding of data frames.

3Version 3.0

Redundancy

• Companies want 100% uptime, but 99.999% (5 nines) is the goal.

• Remember the goal is reliability without faults. Fault tolerance is achieved by redundancy.

• Example of having 1 car versus 2 cars – 1 is always available – redundancy

• So companies should: – eliminate single points of failure and– design alternate routes to a destination

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Reliability and 24x7 network demands have compelled LAN designers to construct multiple paths between user and resource

5Version 3.0

Redundant Switched Topologies

• Again, if one path fails, the other path or device can take over.

• This is good, but there is a downside that has to be accounted for:

– Broadcast storms– Multiple (or duplicate) frame copies– MAC address table instabilities

6Version 3.0

Redundant Paths and No Spanning Tree. . .

00-A1 BB-44

AA-11 CC-23

SATPort 1 Port 200-A1

SATPort 1 Port 200-A1

Port 1 Port 1

Port 2 Port 2

LAN Sw itch 1 LAN Sw itch 2

(1) 00-A1 sends frame to CC-23Sw itch 1 and Sw itch 2 learn about 00-A1

00-A1 BB-44

AA-11 CC-23

SATPort 1 Port 200-A1

SATPort 1 Port 200-A1

Port 1 Port 1

Port 2 Port 2

LAN Sw itch 1 LAN Sw itch 2

(2) LAN Sw itch 1 Floods packet out Port 2since CC-23 is not know n

00-A1 BB-44

AA-11 CC-23

SATPort 1 Port 200-A1

SATPort 1 Port 2 00-A1

Port 1 Port 1

Port 2 Port 2

LAN Sw itch 1 LAN Sw itch 2

(3) LAN Sw itch 2 learns (incorrectly) that(Source MAC ) 00-A1 is on Port 2

10Version 3.0

Or, A Broadcast Storm. . .

11Version 3.0

Broadcast Storms, like ARP requests

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch B

Host A

A

1

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Host B

12Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch B

Host A

A

1

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Because it is a Layer 2 broadcast frame, both switches, Switch A and Switch B, flood the frame out all ports, including their port A’s.

Host B

13Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch A

Host A

A

1

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Duplicateframe

Duplicateframe

Both switches receive the same broadcast, but on a different port. Doing what switches do, both switches flood the duplicate broadcast frame out their other ports.

Host B

14Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch B

A

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Duplicate Frame

Duplicate Frame

Here we go again, with the switches flooding the same broadcast again out its other ports. This results in duplicate frames, known as a broadcast storm!

Host A

Host B

15Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

A

Switch A

Switch B

A

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Layer 2 broadcasts not only take up network bandwidth, but must be processed by each host. This can severely impact a network, to the point of making it unusable.

Host A

Host B

16Version 3.0

Redundant Topology

• The traffic that switches flood out all ports can be caught in a loop, because in the Layer 2 header there is no TTL.

• (Remember that in Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

• You need switching (bridging) for reliability, but now the problem of loops – a switched network cannot have loops if it is to do what it is supposed to do.

• Solution? Allow physical loops, but create a loop-free topology

17Version 3.0

Spanning Tree Protocol

18Version 3.0

Broadcast Frame

Standby Link

• Switches forward broadcast frames• Prevents loops• Loops can cause broadcast storms and duplicate frames• Allows redundant links• Prunes topology to a minimal spanning tree• Resilient to topology changes and device failures• Main function of the Spanning Tree Protocol (STP) is to allow redundant

switched/bridged paths without suffering the effects of loops in the network

Spanning Tree Protocol

19Version 3.0

Root Bridge

RootBridge

Server

Server

= Backup Link

= Forwarding Path

The Spanning-Tree Protocol specifies an algorithm (Spanning-Tree Algorithm) that ultimately creates a logical loop-free topology

A

B

C

H

J

IE

G

FD

20Version 3.0

• The STA is used to calculate a loop-free logical topology.

• Spanning-tree frames called bridge protocol data units (BPDUs) are sent and received by all switches in the network at regular intervals and are used to determine the spanning tree topology.

• These BPDUs are used to determine the shortest path to the root bridge, and which ports will forward frames as part of the spanning tree – BPDUs sent out every 2 seconds

• A separate instance of STP runs within each configured VLAN.

Spanning Tree Algorithm

21Version 3.0

Spanning TreeFor every switched network:

•One root bridge

•One root port per non root bridge

•One designated port per segment

•Unused, non-designated ports

22Version 3.0

Step 1: Electing a Root Bridge

• Bridge Priority

• Bridge ID

• Root Bridge

Step 2: Electing Root Ports

• Path Cost or Port Cost

• Root Path Cost

• Root Port

Step 3: Electing Designated Ports

• Path Cost or Port Cost

• Root Path Cost

3 Steps to Spanning Tree

23Version 3.0

Step 1: Electing a Root Bridge

• The first step is for switches to select a Root Bridge.

• The root bridge is the bridge from which all other paths are decided.

• Only one switch can be the root bridge.

Election of a root bridge is decided by:

1. Lowest Bridge Priority

2. Lowest Bridge ID (tie-breaker)

24Version 3.0

Bridge Priority

• This is a numerical value.

• The switch with the with the lowest bridge priority is the root bridge.

• The switches use BPDU’s to accomplish this.

• All switches consider themselves as the root bridge until they find out otherwise.

• All Cisco Catalyst switches have the default Bridge priority of 32768.

25Version 3.0

A B

A B

A B

1

1

A

B

C

10BaseT Ports (12)

10BaseT Ports (24)

10BaseT Ports (24)

100BaseT Ports

100BaseT Ports

100BaseT Ports

Bridge Priorities

26Version 3.0

Catalyst 1900 - Spanning Tree Configuration - Option 1 ----------------------- Information ------------------------------------ [V] VLANs assigned to option 1-1005 ----------------------- Settings --------------------------------------- [B] Bridge priority 32768 (8000 hex) [M] Max age when operating as root 20 second(s) [H] Hello time when operating as root 2 second(s) [F] Forward delay when operating as root 15 second(s)

Switch A: Bridge Priority

27Version 3.0

In case of a tie, the Bridge ID is used…

Bridge ID

• The Bridge ID is the MAC address assigned to the individual switch.

• The lower Bridge ID (MAC address) is the tiebreaker.

• Because MAC addresses are unique, this ensures that only one bridge will have the lowest value.

• NOTE: There are other tie breakers, if these values are not unique, but we will not cover those situations.

28Version 3.0

Catalyst 1900 Management Console Copyright (c) Cisco Systems, Inc. 1993-1998 All rights reserved. Enterprise Edition Software Ethernet Address: 00-B0-64-26-6D-00 PCA Number: 73-3122-04 PCA Serial Number: FAB03503222 Model Number: WS-C1912-EN System Serial Number: FAB0351U08M Power Supply S/N: PHI033301VQ PCB Serial Number: FAB03503222,73-3122-04

29Version 3.0

A B

A B

A B

1

1

A

B

C

10BaseT Ports (12)

10BaseT Ports (24)

10BaseT Ports (24)

100BaseT Ports

100BaseT Ports

Priority: 32768 ID: 00-B0-64-26-6D-00

Priority: 32768 ID: 00-B0-64-58-CB-80

Priority: 32768 ID: 00-B0-64-58-DC-00

Bridge Priorities and Bridge Ids

Which one is the lowest?

30Version 3.0

A B

A B

1

1

A

B

C

10BaseT Ports (12)

10BaseT Ports (24)

10BaseT Ports (24)

100BaseT Ports

Priority: 32768 ID: 00-B0-64-26-6D-00

Priority: 32768 ID: 00-B0-64-58-CB-80

Priority: 32768 ID: 00-B0-64-58-DC-00

Lowest: A becomes the root bridge

A B

31Version 3.0

States initially set, later modified by STP

Server ports can be configured to immediately enter STP forward mode

Understanding STP States

• Blocking• Listening• Learning• Forwarding• Disabled

32Version 3.0

Blocking - No frames forwarded, BPDUs received

Listening - No frames forwarded, listening for frames

Learning - No frames forwarded, but learning MAC addresses

Forwarding – Receiving BPDUs, Forwarding data traffic, receiving data traffic, learns MAC addresses

Disabled - No frames forwarded, no BPDUs heard

Understanding STP States

50 seconds from blocking to forwarding

33Version 3.0

Rapid Spanning Tree Protocol

• IEEE 802.1w

• Will eventually replace 802.1d

• Port states and roles will be clarified

• A set of link types will be defined that will allow going to a forwarding stage quicker

• All switches will generate their own BPDUs instead of relying on the root bridge.

• Link types would be:– Point to point – Edge-type– Shared

Can go to forward state immediately

34Version 3.0

Module 7Spanning Tree

Protocol