ccna 3 v3.0 module 7 spanning-tree protocol

1© 2003, Cisco Systems, Inc. All rights reserved.

CCNA 3 v3.0 Module 7 Spanning-Tree Protocol

Cisco Networking Academy


Spanning-Tree Objectives

• Redundant Topologies

• Spanning-Tree Protocol


Redundancy

Redundant networking topologies are designed to ensure that networks continue to function in the presence of single points of failure.


Redundant Topologies

• A goal of redundant topologies is to eliminate network outages caused by a single point of failure.

• All networks need redundancy for enhanced reliability.

• However, transparent bridging begins to have problems when redundant paths are added to the Layer 2 network.


Simple Redundant Switched Topology


Media Access Control Database Instability

In a redundant switched network, it is possible for switches to learn the wrong information. A switch can learn that a MAC address is on a port when it is not.

Unknown Unicast


Broadcast Storm

1/1 2/1

2/21/2

The process of continually propagating a broadcast is known as a broadcast storm which will eventually bring the network down when the switches’ processor utilization reaches 100%.


Using Bridging Loops for Redundancy


Spanning-Tree Protocol


L2 Loops

• Broadcasts and Layer 2 loops can be a dangerous combination.

• Ethernet frames have no TTL field.

• After an Ethernet frame starts to loop, it will probably continue until someone shuts off one of the switches or breaks a link.

• Ethernet switches associate the source MAC in the Layer 2 header with the a port number


Spanning-Tree Protocol (STP)

• STP is a loop-prevention protocol.

• allows L2 devices to communicate with each other to discover physical loops in the network.

• specifies an algorithm that L2 devices can use to create a loop-free logical topology.

• creates a tree structure of loop-free leaves and branches that spans the entire Layer 2 network.


STP Prevents Loops

• The purpose of STP is to avoid and eliminate loops in the network by negotiating a loop-free path through a root bridge.

• STP determines where the are loops and blocks links that are redundant.

–Ensures that there will be only one active path to every destination.


Spanning-Tree Algorithm

• STP executes an algorithm called STA

(Spanning-Tree Algorithm).

• STA chooses a reference point, called a root bridge, and then determines the available paths to that reference point.

–If more than two paths exists, STA picks the best path and blocks the rest.

–How does STP learn the network topology?


Spanning-Tree BPDUs

• All switches using STP must exchange Bridge Protocol Data Units with other switches.

• Sent every 2 seconds on every active port.


BPDUs

The exchange of BPDU messages results in the following:

–The election of a root switch

–The election of a designated switch for every switched segment

–The removal of loops in the switched network by placing redundant switch ports in a backup state


BPDUs

Bridge protocol data unit (BPDU)


Bridge ID


Bridge ID

• Consists of two components:

1. A 2-byte priority: Cisco switch defaults to 32,768 or 0x8000.

2. A 6-byte MAC address

• Used to elect a root bridge.

• Lowest Bridge ID is the root.

• If all devices have the same priority, the bridge with the lowest MAC address becomes the root bridge -- Yikes!!!


Spanning-Tree Operation 802.1d

• One root bridge per network.

• One root port per nonroot bridge.

• One designated port per segment.

• Nondesignated ports are unused.


Root Port

• Root Port: The port receiving the best BPDU on a bridge is the root port.

– This is the port that is the closest to the root bridge in terms of path cost

– The root bridge sends BPDUs that are more useful than the ones that any other bridge can send.

– The root bridge is the only bridge in the network that does not have a root port.


Designated Port

• A port is designated if it can send the best BPDU on the segment to which it is connected.

• On a given segment, there can be only one path toward the root bridge otherwise redundant paths would create a bridging loop.

– All bridges connected to a given segment listen to each other's BPDUs and agree on the bridge sending the best BPDU as the designated bridge for the segment.


Spanning-Tree Example


Spanning-Tree Link Costs

Shortest path is based on cumulative link costs. Link costs are based on the speed of the link.


Spanning-Tree Port States

These values have been calculated on an assumption that there will be a maximum of seven switches (diameter of seven) in any branch of the spanning tree from the root bridge.


Spanning-Tree Port States

• The Max Age allows topology change information to propagate from one side of the network to the other.

• In the listening state, switches determine if there are any other paths to the root bridge and path cost is compared.

• In the learning state user data is not forwarded, but MAC addresses are learned from any traffic that is seen.

• In the forwarding state user data is forwarded and MAC addresses continue to be learned.


Spanning-Tree Recalculation

A switched internetwork has converged when all the switch and bridge ports are in either the forwarding or blocked state.

Convergence on a new spanning-tree topology using the IEEE 802.1D standard can take up to 50 seconds.

Root port

Root port


Spanning-Tree Recalculation

• During the time that the Layer 2 network is converging, MAC addresses that can no longer be reached still exist in the CAM table.

• STP topology change process forces the switch to purge MAC addresses in the CAM table faster.

• When a root bridge sends out a topology change BPDU the topology change is set to a period of time equal to the sum of the max age and fwd delay timers (50 seconds).

• A switch receiving the topology change BPDU uses the fwd delay timer (15 seconds) to age out entries in its MAC address table – the default time is five minutes.


Rapid Spanning-Tree Protocol (RST)

• The IEEE 802.1w LAN standard and protocol introduce the following:

1. Clarification of new port states and roles

2. Definition of a set of link types that can go to forwarding state rapidly

3. Allowing switches, in a converged network, to generate their own BPDUs rather than relaying root bridge BPDUs


Rapid Spanning-Tree

• According to the 802.1w standard:

1. The “blocked” state of a port has been renamed as the “discarding” state.

2. A role of a discarding port is an “alternate port”.

3. The discarding port can become the “designated port” in the event of the failure of the designated port for the segment.

4. Link types have been defined as point-to-point, edge-type, and shared.


Alternate and Backup Ports

• These two port roles correspond to the blocking state of 802.1d (discarding MACs).

–A blocked port is defined as not being the designated or root port.

–A blocked port receives a more useful BPDU than the one it would send out on its segment.

• An alternate port is a port blocked by receiving more useful BPDUs from another bridge.

• A backup port is a port blocked by receiving more useful BPDUs from the same bridge it is on.


Alternate and Backup Ports


New BPDU Format

• BPDU are sent every hello-time, and not simply relayed anymore.

• With 802.1d, a non-root bridge would only generate BPDUs when it received one on its root port.

– Actually, every bridge only relays the root bridge’s BPDU

– With 802.1w, every bridge generates it’s own BPDU even if it does not receive one from the root bridge.

• The Rapid Spanning-Tree Protocol, IEEE 802.1w, will eventually replace the Spanning-Tree Protocol, IEEE 802.1D.


Rapid Spanning-Tree Port Designations

The Rapid Spanning Tree Protocol, IEEE 802.1w, will eventually replace the Spanning Tree Protocol, IEEE 802.1D.

ccna 3 v3.0 module 7 spanning-tree protocol

Documents