Spanning Tree protocol- Chapter 5 STP.ppt 17/04/2017 Spanning Tree protocol- Chapter 5 CCNA Exploration Semester 3 Modified by Profs. Ward and Cappellino S Ward Abingdon and Witney College

Topics Redundancy in a converged network How Spanning Tree Protocol (STP) eliminates layer 2 loops The STP algorithm and its 3 steps Rapid spanning tree protocol

Semester 3 LAN Design Basic Switch Concepts Wireless STP- Ch 5 VLANs Inter-VLAN routing VTP

STP.ppt 17/04/2017 We want: Redundancy accomplished at the ____________________ ______________________________________________ What is redundancy? Multiple ________________ _________________ One link or device fails – another takes over. Redundancy allows flexibility but does have some challenges S Ward Abingdon and Witney College

Issues with Redundancy STP.ppt 17/04/2017 Issues with Redundancy Multiple ___________________ Create problems when all the links are active: ____________________ _____________________ See examples on following slides… S Ward Abingdon and Witney College

Broadcast storm 3. And so on with nothing to stop it STP.ppt 17/04/2017 Broadcast storm 3. And so on with nothing to stop it 2. Flood broadcast through non-source ports There’s a switching loop B 1. Send ARP request A C D S Ward Abingdon and Witney College

Duplicate Unicast Frame Transmissions STP.ppt Duplicate Unicast Frame Transmissions 17/04/2017 B A is on port 3 Don’t know B So flood A C Send frame to B Frame arrives A B And again D S Ward Abingdon and Witney College

STP.ppt 17/04/2017 Loops by mistake Even if there are no deliberate loops for redundancy, there can be loops set up by mistake. For example, by improper or no labeling of cables in the wiring closet ____________ S Ward Abingdon and Witney College

Etherchannel – the exception STP.ppt 17/04/2017 Etherchannel – the exception EtherChannel is a grouping of Ethernet ports on a switch that ____________________ ____________________________ Multiple connections do not make a loop where Etherchannel is used. The _________________________________ with the combined bandwidth. S Ward Abingdon and Witney College

Redundancy without loops STP.ppt 17/04/2017 Redundancy without loops There needs to be just ________________ _________________________, but _______ _________________ when they are needed. This must be done quickly and automatically. ______________________ does this. A blocked port does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops. S Ward Abingdon and Witney College

What is a spanning tree? A tree (extended star) topology Spanning all devices __________________________ Spanning tree is _____________ immediately __________________ _______________________

Not a spanning tree Not a tree - it has loops.

Not a spanning tree Not spanning. Device left out.

Spanning tree No loops. Includes all devices.

Spanning tree protocol Used by switches to __________________ _________________________________ ________ unwanted links by _____________ STP defined by ________________ _____________ defined by _____________ Switches __________________ – ________________________________

Spanning tree algorithm STP.ppt 17/04/2017 Spanning tree algorithm The switches use this algorithm to configure the ports _______ ____________________ Choose ___________ to be “____________” Choose a “_______” on ______ _____________ closest to the root bridge Choose a “__________” which are all _______________ that are still ___________________________________________________________________________ known as “non-designated” ports S Ward Abingdon and Witney College

Example of port designations Root bridge Designated port Designated port Root port Root port Non-designated port Designated port Root port Designated port

STP.ppt 17/04/2017 1. Choose the Root Bridge Root bridge serves as a __________________ ____________________________ ______________ in the broadcast domain __________________________________ Each switch has a bridge ID (___) containing priority value, extended system ID, followed by the MAC address of the switch- more to come on BID … A switch starts up. It sends out BPDU frames more to come on BPDU … containing the switch BID and the root ID every 2 seconds. At first each switch identifies itself as the root bridge. S Ward Abingdon and Witney College

Choose the root bridge (cont…) STP.ppt 17/04/2017 Choose the root bridge (cont…) ___________ Bridge Protocol Data Units (______) to _______________________ The switch with the __________________________ ________________________ As a switch receives a BPDU, it ___________________ ________________________________________ through which the BPDU was received. It passes on this information in its own BPDUs. Eventually all switches agree that the switch with the lowest BID is the root bridge. Administrator can set the priority to fix the selection S Ward Abingdon and Witney College

STP.ppt 17/04/2017 BPDU The _______________________________ ________________________________ The BPDU message is encapsulated in an Ethernet frame. The ______________ MAC address in the BPDU frame is a _________________ for the ___________________________ S Ward Abingdon and Witney College

BPDU Frame contains 12 fields: STP.ppt 17/04/2017 BPDU Frame contains 12 fields: 2 bytes Protocol ID Administrative uses 1 byte Version Message type Flags 8 bytes Root ID BID and path information- used to identify the root bridge and the cost 4 bytes Cost of path Bridge ID Port ID Message age Timer fields used to determine how frequently BPDU messages are sent Max age Hello time Forward delay S Ward Abingdon and Witney College

Bridge ID Used to ______________________ on a network STP.ppt 17/04/2017 Bridge ID Used to ______________________ on a network The bridge ID consists of: ____________________________ Lowest priority- ie. Lowest BID- becomes root bridge By default the priority is 32768 Value range is 1 - 65536 ___________________________________ Identifies the VLAN with which the BPDU is associated ________________________ MAC address used as ___________________________ MAC address with the lowest hexadecimal value has the lower BID Best to configure the desired root bridge and not count on MAC address S Ward Abingdon and Witney College

STP.ppt 17/04/2017 Configure priority Set a _____________________ if a specific switch is to become the root bridge Method 1: SW1#spanning-tree vlan 1 root primary Sets value to 24576 or 4096 less than lowest priority detected. SW1#spanning-tree vlan 1 root secondary Sets value to 28672. This switch should become the root bridge if the primary root bridge fails. Method 2: SW1#spanning-tree vlan 1 priority 24576 command gives more granular control over the bridge priority value by allowing the NA to configure the priority How would we verify bridge priority? S Ward Abingdon and Witney College

STP.ppt 17/04/2017 2. Select root ports After root bridge designation _________________________ ___________________________________________________________________________ in the broadcast domain. Every non-root bridge (Switch) selects a root port Only _________________________________ This is the _____________________________ to the root bridge Path information is determined by _______________________ _________ along the path from the destination to the root bridge. What is cost?... S Ward Abingdon and Witney College

Finding the cost of a link STP.ppt 17/04/2017 Finding the cost of a link Default port costs are defined by the speed at which the port operates Set by IEEE. Costs may change as newer, faster Ethernet is developed. Port cost is configurable by the NA Link speed Revised cost Previous cost 10 Gbps 2 1 1 Gbps 4 100 Mbps 19 10 10 Mbps 100 S Ward Abingdon and Witney College

Changing the cost of a link STP.ppt 17/04/2017 Changing the cost of a link SW1(config)#int fa0/1 SW1(config-if)#spanning-tree cost 25 SW1(config-if)#end To revert back to default value use “no”… SW1(config-if)#no spanning-tree cost S Ward Abingdon and Witney College

STP.ppt 17/04/2017 What is the best path? In the example, the path cost from switch S2 to the root bridge switch S1, over path 1 is 19, while the path cost over path 2 is 38. Because path 1 has a lower overall path cost to the root bridge, it is the preferred path. STP then configures the redundant path to be blocked, preventing a loop from occurring. S Ward Abingdon and Witney College

What if root ports have the same cost? STP.ppt 17/04/2017 What if root ports have the same cost? Switch uses the customizable port priority value. If those are the ____________? _____________________________ The port ID is ________________________________ port. This gives each port a unique number Example: by default F0/1 has port priority value of 128 and .1 as port ID giving: 128.1 F0/2 has 128.2 X F0/1 F0/2 F0/1 would be the root port… S Ward Abingdon and Witney College

Configure port priority STP.ppt 17/04/2017 Configure port priority SW2(config-if)#spanning-tree port-priority 112 Priority values range from 0 - 240, in increments of 16. The ____________________________ ________________________ Becomes ________________________ ____________________________________ _____________________________ by STA to prevent looping S Ward Abingdon and Witney College

3. Select designated/non-designated ports STP.ppt 17/04/2017 3. Select designated/non-designated ports ____________ ports - All _______________ __________________________________ For _____ bridges, ______________________ ports. For ___________bridges, a designated port is the _______________________________________________________________ as needed. Only ___________________________________ ______________ ports - All ports configured to be in a _______________ to prevent loops S Ward Abingdon and Witney College

Select designated/non-designated ports cont… STP.ppt 17/04/2017 Select designated/non-designated ports cont… If two switches are connected to the same LAN segment, which switch port will be the designated and which the non-designated port? Switches on the LAN segment in question exchange BPDU frames, which contain the switch BID. Lowest path cost to the root bridge will be the designated port. If _____________________? Switch with the ____________ has its port configured as a _______________________ Switch with the higher BID- non-designated port See example on next slide… S Ward Abingdon and Witney College

Example…

Summary of Port Roles STP makes: Root ports- forwarding Designated ports- forwarding Non-designated ports- shut down or blocked

Five Port states in traditional STP STP.ppt 17/04/2017 Five Port states in traditional STP Blocking – _______________ BPDU frames. Non-designated port which _________________ ___________________________________ Listening - ____________________ BPDU frames. Learning - _____________________ BPDU frames. _____________________ in preparation of participating in frame forwarding Forwarding – Fully active, _________________ Disabled – ___________________________ S Ward Abingdon and Witney College

STP.ppt 17/04/2017 States and timers Amount of time that a port stays in the various port states depends on the ___________ During a topology change A port temporarily __________________________________________ for a specified period called the "forward delay interval.“ Possibly forward or remain in blocked state Up to ____________________ __________ again- full convergence S Ward Abingdon and Witney College

BPDU timers Timers are optimised for a 7-switch diameter network. STP.ppt 17/04/2017 BPDU timers Timers are optimised for a 7-switch diameter network. ______________________________________________________________ to travel from the ________________ on the broadcast domain The network has __________________ before switches forward user data. Typically timers and the diameter should not be adjusted though they are configurable S Ward Abingdon and Witney College

STP.ppt 17/04/2017 Cisco PortFast An access port leading to a workstation or server does not need to go through the STP modes because it will not be closed down. The switch port can be configured with _____________ ____________________________________________ If a switch is connected later and the port receives a BPDU, STP can put the port into a blocking state and then through the modes. Feature called ______________________ To configure portfast, enter interface config mode for the port where portfast is going to be used and then the command: spanning-tree portfast S Ward Abingdon and Witney College

Verify spanning tree Root bridge This switch

Topology change notification (TCN) STP.ppt 17/04/2017 Topology change notification (TCN) After the network converges, the root bridge sends out BPDUs, but the other switches do not normally send BPDUs back. If there is a ______________________________ ______________ called the topology change notification (________) ______________________ Each switch that receives the TCN sends an ____________________ and sends a TCN towards the root bridge until the root bridge receives it. The ____________________________ with the topology change (TC) bit set into the whole network S Ward Abingdon and Witney College

STP.ppt 17/04/2017 STP developments S Ward Abingdon and Witney College

PVST+ ________________ can be implemented. STP.ppt 17/04/2017 PVST+ ________________ can be implemented. Separate instance of spanning tree runs for each VLAN S Ward Abingdon and Witney College

PVST+ cont… The VLAN needs to be identified, so each BID has 3 fields: STP.ppt 17/04/2017 PVST+ cont… The VLAN needs to be identified, so each BID has 3 fields: Bridge Priority _____________________________________ MAC address. Original BID just had bridge priority and MAC address PVST+ is the default spanning-tree configuration for a Catalyst 2960 switch. S Ward Abingdon and Witney College

STP.ppt 17/04/2017 PVST+ cont… When the priority and extended system ID are prepended to the switch MAC address, each VLAN on the switch can be represented by a _________ S Ward Abingdon and Witney College

Rapid Spanning Tree Protocol STP.ppt 17/04/2017 Rapid Spanning Tree Protocol __________________________________ with it. RSTP (IEEE 802.1w) is an evolution of STP (IEEE 802.1D) Much _____________________ ____________________________ 2 in version field indicates it is RSTP Sends BPDUs with its current information every 2 seconds. Does not use timers in the same way as STP 3 missed BPDUs taken to mean loss of the link. (6 seconds) _____________________________ RSTP does not have a __________________________ S Ward Abingdon and Witney College

_______________ in RSTP STP.ppt 17/04/2017 _______________ in RSTP A port that will ______________________ __________________________ Immediately goes to forwarding state. Same idea as Cisco’s PortFast. An edge port becomes a normal spanning-tree port if it receives a BPDU Configuring an edge port uses the PortFast keyword as before. spanning-tree portfast S Ward Abingdon and Witney College

STP.ppt 17/04/2017 Link types ___________ are categorized into 2 link types, ____________________________ A _________________ between two switches is regarded as a _________________ A link ____________________ is regarded as a ____________________ Ports on a ________________ are able to _________________________ Designated ports Make the most use of the link type parameter. S Ward Abingdon and Witney College

STP.ppt 17/04/2017 Port states There are only _____________ in RSTP that correspond to the three possible operational states. The 802.1D ________________ states are ________ into a unique 802.1w ________________________ Operational STP RSTP Enabled Blocking Discarding Listening Learning Forwarding Disabled S Ward Abingdon and Witney College

STP.ppt 17/04/2017 RSTP Port Roles ____ - A ___________ that has been elected for the spanning-tree topology _______________ - A ____________________________________ Alternate - An _______ ___________________ This path is different than using the root port. Backup - A ___________________ to a segment where _____________________________________ _________ - Not strictly part of STP, a network administrator can ___________________ 48 S Ward Abingdon and Witney College

Design considerations STP.ppt 17/04/2017 Design considerations Root bridge should be a ______________ in the ______________ of the network. ________ the number of _______________ Use VTP ____________. Use ________________________ _________________ even if no blocked ports or physical redundancy exist S Ward Abingdon and Witney College

Troubleshooting Suggestions STP.ppt 17/04/2017 Troubleshooting Suggestions Before you troubleshoot a bridging loop, you need to know at least these items: Topology of the bridge network Location of the root bridge Location of the blocked ports and the redundant links How the network looks when it works correctly Use the “_________” command as needed S Ward Abingdon and Witney College