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Spanning Tree

Spanning Tree. V1.2. Objectives. Understand the origin of loop and the solution Understand the working principle of STP. Course Outline. Origin of loop and solution Working principle of STP STP configuration. Redundant Topology. Server/host X. Router Y. Segment 1. Segment 2.

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Spanning Tree

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  1. Spanning Tree V1.2

  2. Objectives • Understand the origin of loop and the solution • Understand the working principle of STP

  3. Course Outline • Origin of loop and solution • Working principle of STP • STP configuration

  4. Redundant Topology Server/host X Router Y Segment 1 Segment 2

  5. Broadcast Storms Server/host X Router Y Segment 1 Broadcast Switch A Switch B Segment 2 Host X sends a Broadcast frame

  6. Broadcast Storms Server/host X Router Y Segment 1 Broadcast Switch A Switch B Segment 2 Switch B forward Broadcast frame back to Switch A

  7. Broadcast Storms • Switches continue to propagate broadcast traffic over and over Server/host X Router Y Segment 1 Broadcast Switch A Switch B Segment 2

  8. Host X sends an unicast frame to router Y Router Y MAC address has not been learned by either switch yet Multiple Frame Copies Unicast Server/host X Router Y Segment 1 Switch A Switch B Segment 2

  9. Router Y will receive two copies of the same frame Server/host X Router Y Segment 1 Unicast Unicast Unicast Switch B Switch A Segment 2 Multiple Frame Copies

  10. Host X sends an unicast frame to Router Y Router Y MAC Address has not been learned by either Switch yet Switch A and B learn Host X MAC address on port 0 Server/host X Router Y Segment 1 Unicast Unicast Port 0 Port 0 Switch A Switch B Port 1 Port 1 Segment 2 MAC Database Instability

  11. Frame to Router Y is flooded Switch A and B incorrectly learn Host X MAC address on port 1 Server/host X Router Y Segment 1 Unicast Unicast Port 0 Port 0 Switch A Switch B Port 1 Port 1 Segment 2 MAC Database Instability

  12. Complex topology can cause multiple loops to occur Layer 2 has no mechanism to stop the loop Server/host Loop Broadcast Loop Loop Workstations Multiple Loop Problems

  13. Provides a loop free redundant network topology by placing certain ports in the blocking state Solution: Spanning-Tree Protocol x Block

  14. Course Outline • Origin of loop and solution • Working principle of STP • STP configuration

  15. 100baseT Designated port (F) Root port (F) Root bridge Non-root bridge SW X SW Y x Designated port (F) Non-designated port (B) 10baseT F: forwarding B: blocking Spanning-Tree Operations • One root bridge per network • One root port per non-root bridge • One designated port per segment

  16. BPDU = Bridge protocol data unit (default = sent every 2 seconds) Root bridge = Bridge with the lowest bridge ID Bridge ID = Bridge priority + bridge MAC address In the example, which switch has the lowest bridge ID? Spanning-Tree Protocol Root Bridge Selection Switch X Default priority 32768 (8000 hex) MAC 0c0011111111 Switch Y Default priority 32768 (8000 hex) MAC 0c0022222222 BPDU

  17. BPDU L/T DMA SMA LLC Header Payload

  18. Spanning-Tree Protocol Path Cost Link Speed Cost (reratify IEEE spec) Cost (previous IEEE spec) ---------------------------------------------------------------------------------------------------- 10 Gbps 2 1 1 Gbps 4 1 100 Mbps 19 10 10 Mbps 100 100

  19. Can you figure out: Which is the root bridge? Which are the designated, non-designated, and root ports? Which are forwarding or blocking ports? Spanning-Tree: Switch Z Mac 0c0011110000 Default priority 32768 Port 0 100baseT Port 0 Port 0 Switch X MAC 0c0011111111 Default priority 32768 Switch Y MAC 0c0022222222 Default priority 32768 Port 1 Port 1 100baseT

  20. Switch Z Mac 0c0011110000 Default priority 32768 Port 0 Designated port (F) 100baseT Root port (F) Root port (F) Port 0 Port 0 Switch X MAC 0c0011111111 Default priority 32768 Switch Y MAC 0c0022222222 Default priority 32768 Port 1 Port 1 Designated port (F) Non-designated port (BLK) 100baseT Spanning-Tree:

  21. Port state D: data B: BPDU

  22. Disabled （1） （2） （1，2） Listening （3） （5） （4） （1） （1，2） （4） Blocking Learning （2） （4） （5） Forwarding （1，2） 4） elected as RP or DP 1）port enabled 2）port disabled 5）Forward Delay 3）elected as RP or DP Port States Transfer

  23. Blocking (20 sec) Listening (15 sec) Learning (15 sec) Forwarding Spanning-Tree Port States • Spanning-tree transitions each port through several different state:

  24. 100baseT Designated port Root port (F) Port 0 Port 0 x MAXAGE x Switch X MAC 0c0011111111 Default priority 32768 Switch Y MAC 0c0022222222 Default priority 32768 Root Bridge BPDU Port 1 Port 1 x Designated port Non-designated port (BLK) 10baseT Spanning-Tree Recalculation x Root bridge

  25. Spanning Tree Practice MAC=0000.8c00.1201 Switch A Default priority=32768 MAC=0000.8c00.8955 Switch B MAC=0000.8c00.1202 Switch C MAC=0000.8c00.2101 Switch D MAC=0000.8c00.9870 Switch E

  26. Course Outline • Origin of loop and solution • Working principle of STP • STP configuration

  27. SSTP Configuration • The default mode of ZXR10 T160G/T64G is MSTP. Whichever mode configured can be compatible and interconnected with other two modes. zte# configure zte(config)#spanning-tree enable zte(config)#spanning-tree mode sstp

  28. RSTP (Rapid Spanning Tree Protocol) provides higher convergence speed than STP (i.e. SSTP mode). When the network topology is changing, the status of old redundant switch port can be transferred (From Discard to Forward) quickly in the case of point-to-point connection. RSTP

  29. RSTP Port Type

  30. RSTP Port State

  31. Alternate And Backup Ports D D R R A D B

  32. Speed Up • Edge Port • Handshake

  33. 10,20,30 10,20 20,30 10,30 10,30 30 20 10 MSTP

  34. The concept of instance and VLAN mirroring are added in MSTP (Multiple Spanning Tree Protocol); SSTP mode and RSTP mode can both be considered to be instances of MSTP mode, namely, the case that only one instance 0 exists. MSTP mode also provides fast convergence and load balance in VLAN environment. In SSTP and RSTP modes, there is no such kind of concept as VLAN, there is only one status for each port, namely, the forwarding statuses of ports in different VLANs are consistent. While in MSTP mode, there exist multiple spanning tree instances, the forwarding statuses of ports are different in different VLANs. Multiple independent subtree instances can be formed inside MST region to achieve load balance. By default, MSTP is enabled MSTP

  35. VLAN 10 topology VLAN 20 topology VLAN  Instance Default Instance 0 VLAN 30 topology Per-VLAN STP And Instance

  36. Interface Loop Protection ZXR10(config)#interface fei_1/1 ZXR10(config-if)#switchport mode trunk ZXR10(config-if)#switchport trunk vlan 1-2 ZXR10(config-if)#exit ZXR10(config)#loop-detect interface fei_1/1 enable ZXR10(config)#loop-detect interface fei_1/1 vlan 1-2 enable Switch A Fei_1/1 ZXR10(config)#show loop-detect interface-detail fei_1/1 isUp isMonitor isLoop isProtected enable enable no disable npNum portNum reopenTime counter 1 24 600 158 loopVlan vlanRange 0 1 2 Switch B

  37. zesr add vlan 4094 xgei_4/1 xgei_4/2 TRANSIT standard LINK DOWN D LINK DOWN C E Transit Transit Transit B F Transit To F Transit Master primary secondary A zesr add vlan 4094 xgei_4/1 xgei_4/2 MASTER standard zesr set healthtime 100 failtime 300 Theory speed: 50ms ZTE Ethernet Switch Ring

  38. Loop is easily formed whenever there is a redundant design LAN switch runs STP to resolve loop problem STP uses BPDU to exchange information and make decision Ports that enable STP will transfer to different states according to current topology changing Summarization

  39. How are unicast and broadcast packets processed by LAN switch? How many STP port states there are? When and how will they transfer? How is root bridge elected? And how about root port? How to speed up the convergence? What is the role of BPDU? Questions

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