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Spanning Tree protocol- Chapter 5

Spanning Tree protocol- Chapter 5. CCNA Exploration Semester 3 Modified by Profs. Ward and Cappellino. 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. LAN Design.

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Spanning Tree protocol- Chapter 5

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  1. Spanning Tree protocol- Chapter 5 CCNA Exploration Semester 3 Modified by Profs. Ward and Cappellino

  2. 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

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

  4. 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

  5. Issues with Redundancy • Multiple ___________________ • Create problems when all the links are active: • ____________________ • _____________________ • ____________________ See examples on following slides…

  6. 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

  7. Duplicate Unicast Frame Transmissions 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

  8. 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 • ____________ ____________

  9. 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.

  10. 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.

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

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

  13. Not a spanning tree • Not spanning. Device left out.

  14. Spanning tree • No loops. Includes all devices.

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

  16. 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

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

  18. 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.

  19. 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

  20. BPDU • The _______________________________ ________________________________ • The BPDU message is encapsulated in an Ethernet frame. • The ______________ MAC address in the BPDU frame is a _________________ for the ___________________________

  21. BPDU Frame contains 12 fields:

  22. 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

  23. 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?

  24. 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?...

  25. 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

  26. 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)#int fa0/1 • SW1(config-if)#no spanning-tree cost • SW1(config-if)#end

  27. 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.

  28. 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 defaultF0/1 has port priority value of 128 and .1 asport ID giving: 128.1 F0/2 has 128.2 X F0/1 F0/2 F0/1 would be the root port…

  29. 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

  30. 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

  31. 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…

  32. Example…

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

  34. 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 – ___________________________

  35. 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

  36. 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

  37. 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

  38. Verify spanning tree Root bridge This switch

  39. 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

  40. STP developments

  41. PVST+ • ________________ can be implemented. • Separate instance of spanning tree runs for each VLAN

  42. 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.

  43. 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 _________

  44. 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 __________________________

  45. _______________ 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

  46. 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.

  47. 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 ________________________

  48. 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 ___________________

  49. 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

  50. 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

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