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STP – Spanning Tree Protocol in Depth

STP – Spanning Tree Protocol in Depth. CIS 187 Multilayer Switched Networks CCNP SWITCH version 7 Rick Graziani. Topics. Spanning Tree Protocol (STP) overview, its operations, and history Implement Rapid Spanning Tree Protocol (RSTP)

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STP – Spanning Tree Protocol in Depth

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  1. STP – Spanning Tree Protocolin Depth CIS 187 Multilayer Switched Networks CCNP SWITCH version 7 Rick Graziani

  2. Topics • Spanning Tree Protocol (STP) overview, its operations, and history • Implement Rapid Spanning Tree Protocol (RSTP) • Describe how and where to configure the following features: PortFast, UplinkFast, BackboneFast, BPDU Guard, BPDU Filter, Root Guard, Loop Guard, Unidirectional Link Detection, and FlexLinks • Configure Multiple Spanning Tree (MST) • Troubleshooting STP Rick Graziani graziani@cabrillo.edu

  3. Spanning Tree Protocol (STP) • “STP often accounts for more than 50 % of the configuration, troubleshooting, and maintenance headaches in real-world campus networks (especially if they are poorly designed).” • “Complex protocol that is generally poorly understood.” • Radia Perlman – Developer of STP Rick Graziani graziani@cabrillo.edu

  4. Spanning Tree Protocol (STP) • IEEE 802.1D • A loop-prevention protocol • Allows L2 devices to communicate with each other to discover physical loops in the network. • Algorithm that creates a loop-free logical topology. • STP creates a tree structure of loop-free leaves and branches that spans the entire Layer 2 network. Rick Graziani graziani@cabrillo.edu

  5. 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. • IP has a mechanism to prevent loops. IPv4 and IPv6 Rick Graziani graziani@cabrillo.edu

  6. 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. X Rick Graziani graziani@cabrillo.edu

  7. Spanning Tree Algorithm • STP executes an algorithm called Spanning Tree Algorithm (STA). • STA chooses a reference point, called a root bridge. • Then determines the available paths to that reference point. • If more than two paths exists, STA picks the best path and blocks the rest X Rick Graziani graziani@cabrillo.edu

  8. Two-key STP Concepts • STP calculations make extensive use of two key concepts in creating a loop-free topology: • Bridge ID • Path Cost Rick Graziani graziani@cabrillo.edu

  9. Bridge ID (BID) • Bridge ID (BID) is used to identify each bridge/switch. • The BID is used in determining the center of the network, in respect to STP, known as the root bridge. Bridge ID Without the Extended System ID Bridge ID with the Extended System ID Rick Graziani graziani@cabrillo.edu

  10. Bridge ID (BID) • Consists of two components: • A 2-byte Bridge Priority: Cisco switch defaults to 32,768 or 0x8000. • Usually expressed in decimalformat • A 6-byte MAC address • Usually expressed in hexadecimalformat. Rick Graziani graziani@cabrillo.edu

  11. Bridge ID (BID) • Each switch has a unique BID. • Original 802.1D standard, the BID = Priority Field +MAC address of the switch. • All VLANs were represented by a CST – one spanning tree for all vlans (later). • PVST requires that a separate instance of spanning tree run for each VLAN • BID field is required to carry VLAN ID (VID). • Extendedsystem ID to carry a VID. Rick Graziani graziani@cabrillo.edu

  12. What is the Priority of Access1? Priority = Priority (Default 32,768) + VLAN Access1#show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 19 Port 5(FastEthernet0/5) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0003.E461.46EC Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 VLAN0010 Spanning tree enabled protocol ieee Root ID Priority 32778 Address 0001.964E.7EBB Cost 19 Port 5(FastEthernet0/5) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32778 (priority 32768 sys-id-ext 10) Address 0003.E461.46EC Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 + = PVST coming later… + = Rick Graziani graziani@cabrillo.edu

  13. What is the BID of this switch? Core# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 4 Port 25(GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0001.C945.A573 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Rick Graziani graziani@cabrillo.edu

  14. Bridge ID (BID) • Used to elect a root bridge (coming) • 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) • Note: For simplicity, in our topologies we will use Bridge Priorities without the Extended System ID. (Same process, just done per VLAN.) Rick Graziani graziani@cabrillo.edu

  15. Path Cost – Original Spec (Linear) • Bridges use the concept of cost to evaluate how close they are to other bridges. • Used to create the loop-free topology . • Originally, 802.1D defined cost as 1 billion/bandwidth of the link in Mbps. • Cost of 10 Mbps link = 100 • Cost of 100 Mbps link = 10 • Cost of 1 Gbps link = 1 • Running out of room for faster switches including 10 Gbps Ethernet Rick Graziani graziani@cabrillo.edu

  16. Path Cost – Revised Spec (Non-Linear) • IEEE modified the most to use a non-linear scale with the new values of: • 4 Mbps 250 (cost) • 10 Mbps 100 (cost) • 16 Mbps 62 (cost) • 45 Mbps 39 (cost) • 100 Mbps 19 (cost) • 155 Mbps 14 (cost) • 622 Mbps 6 (cost) • 1 Gbps 4 (cost) • 10 Gbps 2 (cost) • You can change the path cost by modifying the cost of a port. • Exercise caution when you do this! • BID and Path Cost are used to develop a loop-free topology . • Coming very soon! Rick Graziani graziani@cabrillo.edu

  17. Five-Step STP Decision Sequence • When creating a loop-free topology, STP always uses the same five-step decision sequence: Five-Step decision Sequence Step 1 - Lowest BID Step 2 - Lowest Path Cost to Root Bridge Step 3 - Lowest Sender BID Step 4 – Lowest Port Priority Step 5 - Lowest Port ID • Bridges use Configuration BPDUs during this five-step process. • We will assume all BPDUs are configuration BPDUs until otherwise noted. Rick Graziani graziani@cabrillo.edu

  18. FYI: BPDU key concepts BPDU key concepts: • Bridges save a copy of only the best BPDU seen on every port. • When making this evaluation, it considers all of the BPDUs received on the port, as well as the BPDU that would be sent on that port. • As every BPDU arrives, it is checked against this five-step sequence to see if it is more attractive (lower in value) than the existing BPDU saved for that port. • Only the lowest value BPDU is saved. • Bridges send configuration BPDUs until a more attractive BPDU is received. • Okay, lets see how this is used... Rick Graziani graziani@cabrillo.edu

  19. Elect one Root Bridge The STP algorithm uses three simple steps to converge on a loop-free topology: STP ConvergenceStep 1 Elect one Root BridgeStep 2 Elect Root PortsStep 3 Elect Designated Ports • When the network first starts, all bridges are announcing a chaotic mix of BPDUs. • All bridges immediately begin applying the five-step sequence decision process. • Switches need to elect a single Root Bridge. • Switch with the lowest BID wins! • Note: Many texts refer to the term “highest priority” which is the “lowest” BID value. • This is known as the “Root War.” Rick Graziani graziani@cabrillo.edu

  20. Elect one Root BridgeLowest BID wins! Who wins? Rick Graziani graziani@cabrillo.edu

  21. What is the BID of this switch? Who is the Root? Core# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 4 Port 25(GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0001.C945.A573 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 • Use this command to view the information on the other four switches. Rick Graziani graziani@cabrillo.edu

  22. What is the BID of this switch? Who is the Root? Distribution1# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 19 Port 3(FastEthernet0/3) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0005.5E0D.9315 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Rick Graziani graziani@cabrillo.edu

  23. What is the BID of this switch? Who is the Root? Distribution2# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 19 Port 3(FastEthernet0/3) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0060.47B0.5850 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Rick Graziani graziani@cabrillo.edu

  24. What is the BID of this switch? Who is the Root? Access1# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 19 Port 5(FastEthernet0/5) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0003.E461.46EC Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Rick Graziani graziani@cabrillo.edu

  25. What is the BID of this switch? Who is the Root? Access2# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0001.964E.7EBB Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Rick Graziani graziani@cabrillo.edu

  26. Elect one Root BridgeLowest BID wins! My BID is 32769.0001.C945.A573 Who wins? My BID is 32769.0005.5E0D.9315 My BID is 32769.0060.47B0.5850 My BID is 32769.0001.964E.7EBB I win! My BID is 32769.0003.E461.46EC Rick Graziani graziani@cabrillo.edu

  27. Bridge IDs 32769.0001.C945.A573 32769.0005.5E0D.9315 32769.0060.47B0.5850 32769.0001.964E.7EBB 32769.0003.E461.46EC Rick Graziani graziani@cabrillo.edu

  28. Elect one Root BridgeLowest BID wins! Its all done with BPDUs! Rick Graziani graziani@cabrillo.edu

  29. BPDUs BPDUs sent/relayed every two seconds. BPDU BPDU BPDU BPDU BPDU Rick Graziani graziani@cabrillo.edu

  30. Root Bridge Selection Criteria My BID is 32768.0001.C945.A573 I’m the root! Who wins? My BID is 32768.0060.47B0.5850 I’m the root! My BID is 32768.0005.5E0D.9315 I’m the root! My BID is 32768.0003.E461.46EC I’m the root! My BID is 32768.0001.964E.7EBB I’m the root! I win! • At the beginning, all bridges assume and declare themselves as the Root Bridge, by placing its own BID in the Root BID field of the BPDU. Rick Graziani graziani@cabrillo.edu

  31. Elect one Root BridgeLowest BID wins! Rick Graziani graziani@cabrillo.edu

  32. Once all of the switches see that Access2 has the lowest BID, they are all in agreement that Access2 is the Root Bridge. Root Bridge Rick Graziani graziani@cabrillo.edu

  33. Elect Root Ports I will select one Root Port that is closest, best path to the root bridge. STP ConvergenceStep 1 Elect one Root BridgeStep 2 Elect Root PortsStep 3 Elect Designated Ports • Now that the Root War has been won, switches move on to selecting Root Ports. • A bridge’s Root Port is the port closest to the Root Bridge. • Bridges use the cost to determine closeness. • Every non-Root Bridge will select one Root Port! • Specifically, bridges track the Root Path Cost, the cumulative cost of all links to the Root Bridge. Rick Graziani graziani@cabrillo.edu

  34. Determining (Electing) the Root Port Rick Graziani graziani@cabrillo.edu

  35. Root Bridge, Access2 sends out BPDUs, containing a Root Path Cost of 0. • Access1, Distribution1, and Distribution2 receives these BPDUs and adds the Path Cost of the FastEthernet interface to the Root Path Cost contained in the BPDU. • Access1, Distribution1, and Distribution2 add Root Path Cost 0 PLUS its Path (port) cost of 19 = 19. • This value is used internally and used in BPDUs to other switches. Path Cost BPDU Cost=0+19=19 BPDU Cost=0+19=19 19 19 Root Bridge 0 0 19 0 BPDU Cost=0 BPDU Cost=0+19=19 Rick Graziani graziani@cabrillo.edu

  36. Root Path Cost • Cumulative cost to the Root Bridge. • This is the value transmitted in the BPDU. • Calculated by adding the receiving port’s Path Cost to the valued contained in the BPDU. Difference b/t Path Cost and Root Path Cost Path Cost: • The value assigned to each port. • Added to BPDUs received on that port to calculate Root Path Cost. Path Cost BPDU Cost=0+19=19 BPDU Cost=0+19=19 19 19 Root Bridge 0 0 19 0 BPDU Cost=0 BPDU Cost=0+19=19 Rick Graziani graziani@cabrillo.edu

  37. What are the Path Costs for Root Bridge Access2? Path Cost Access2# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0001.964E.7EBB Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------------- Fa0/1 Desg FWD 19 128.1 P2p Fa0/3 Desg FWD 19 128.3 P2p Fa0/5 Desg FWD 19 128.5 P2p Rick Graziani graziani@cabrillo.edu

  38. What are the Path Costs for Distribution1? Path Cost Distribution1# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 19 Port 3(FastEthernet0/3) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0005.5E0D.9315 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ---------------------- Gi0/1 Desg FWD 4 128.25 P2p Gi0/2 Altn BLK 4 128.26 P2p Fa0/3 Root FWD 19 128.3 P2p Fa0/5 Desg FWD 19 128.5 P2p Rick Graziani graziani@cabrillo.edu

  39. What are the Path Costs for Access1? Path Cost Access1# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 19 Port 5(FastEthernet0/5) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0003.E461.46EC Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ---------------------- Fa0/5 Root FWD 19 128.5 P2p Gi1/1 Desg FWD 4 128.25 P2p Gi1/2 Desg FWD 4 128.26 P2p Rick Graziani graziani@cabrillo.edu

  40. What are the Path Costs for Distribution2? Path Cost Distribution2# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 19 Port 3(FastEthernet0/3) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0060.47B0.5850 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ---------------------- Fa0/3 Root FWD 19 128.3 P2p Fa0/5 Altn BLK 19 128.5 P2p Gi0/1 Altn BLK 4 128.25 P2p Gi0/2 Desg FWD 4 128.26 P2p Rick Graziani graziani@cabrillo.edu

  41. show spanning-tree detail Distribution1# show spanning-tree detail VLAN0001 is executing the ieee compatible Spanning Tree Protocol Bridge Identifier has priority of 32768, sysid 1, 0005.5E0D.9315 Configured hello time 2, max age 20, forward delay 15 Current root has priority 32769 Root port is 3 (FastEthernet0/3), cost of root path is 19 Topology change flag not set, detected flag not set Number of topology changes 0 last change occurred 00:00:00 ago from FastEthernet0/1 Times: hold 1, topology change 35, notification 2 hello 2, max age 20, forward delay 15 Timers: hello 0, topology change 0, notification 0, aging 300 Path Cost Use this command to view the Root Path Cost of an interface. Rick Graziani graziani@cabrillo.edu

  42. show spanning-tree detail Access1# show spanning-tree detail VLAN0001 is executing the ieee compatible Spanning Tree Protocol Bridge Identifier has priority of 32768, sysid 1, 0003.E461.46EC Configured hello time 2, max age 20, forward delay 15 Current root has priority 32769 Root port is 5 (FastEthernet0/5), cost of root path is 19 Topology change flag not set, detected flag not set Number of topology changes 0 last change occurred 00:00:00 ago from FastEthernet0/1 Times: hold 1, topology change 35, notification 2 hello 2, max age 20, forward delay 15 Timers: hello 0, topology change 0, notification 0, aging 300 Path Cost Use this command to view the Root Path Cost of an interface. Rick Graziani graziani@cabrillo.edu

  43. show spanning-tree detail Distribution2# show spanning-tree detail VLAN0001 is executing the ieee compatible Spanning Tree Protocol Bridge Identifier has priority of 32768, sysid 1, 0060.47B0.5850 Configured hello time 2, max age 20, forward delay 15 Current root has priority 32769 Root port is 3 (FastEthernet0/3), cost of root path is 19 Topology change flag not set, detected flag not set Number of topology changes 0 last change occurred 00:00:00 ago from FastEthernet0/1 Times: hold 1, topology change 35, notification 2 hello 2, max age 20, forward delay 15 Timers: hello 0, topology change 0, notification 0, aging 300 Path Cost Use this command to view the Root Path Cost of an interface. Rick Graziani graziani@cabrillo.edu

  44. show spanning-tree detail Access2# show spanning-tree detail VLAN0001 is executing the ieee compatible Spanning Tree Protocol Bridge Identifier has priority of 32768, sysid 1, 0001.964E.7EBB Configured hello time 2, max age 20, forward delay 15 Current root has priority 32769 Topology change flag not set, detected flag not set Number of topology changes 0 last change occurred 00:00:00 ago from FastEthernet0/1 Times: hold 1, topology change 35, notification 2 hello 2, max age 20, forward delay 15 Timers: hello 0, topology change 0, notification 0, aging 300 Use this command to view the Root Path Cost of an interface. No Root port – This switch is the Root Bridge! Rick Graziani graziani@cabrillo.edu

  45. Switches now send BPDUs with their Root Path Cost out other interfaces. • Access 1 uses this value of 19 internally and sends BPDUs with a Root Path Cost of 19 out all other ports. (For simplicity we will not include BPDU to root.) • Switches receive BPDU and add their path cost. • Note: STP costs are incremented as BPDUs are received on a port, not as they are sent out a port. Path Cost BPDU Cost=4+19=23 BPDU Cost=4+19=23 19 19 BPDU Cost=19 BPDU Cost=19 0 0 19 0 Root Bridge Rick Graziani graziani@cabrillo.edu

  46. Distribution 1 and Distribution 2 receive the BPDUs from Access 1, and adds the Path Cost of 4 to those interfaces, giving a Root Path Cost of 23. • However, both of these switches already have an “internal” Root Path Cost of 19 that was received on another interface. (Fa0/3 for each with a Root Path Cost of 19.) • Distribution 1 and Distribution 2 use the better BPDU of 19 when sending out their BPDUs to other switches. BPDU Cost=4+19=23 BPDU Cost=4+19=23 19 19 BPDU Cost=19 BPDU Cost=19 0 0 19 0 Root Bridge Rick Graziani graziani@cabrillo.edu

  47. Path Cost • Distribution 1 now sends BPDUs with its Root Path Cost out other interfaces (Best BPDU). • Again, STP costs are incremented as BPDUs are received on a port, not as they are sent out a port. BPDU Cost=4+19=23 BPDU Cost=19+19=38 BPDU Cost=19 19 23 23 19 0 Root Bridge 19 0 19 BPDU Cost=4+19=23 0 Rick Graziani graziani@cabrillo.edu

  48. Final Results • Ports show BPDU ReceivedRoot Path Cost + Path Cost = Root Path Cost of Interface, after the “best” BPDU is received on that port from the neighboring switch. • This is the cost of reaching the Root Bridge from this interface towards the neighboring switch. • Now let’s see how this is used! Path Cost 19+4=23 19+4=23 23+4=27 23+4=27 19+19=38 19+19=38 19 19+4=23 19 19+4=23 19+4=23 19+4=23 0 0 19 Root Bridge 0 Rick Graziani graziani@cabrillo.edu

  49. show spanning-tree Core# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.964E.7EBB Cost 4 Port 25(GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0001.C945.A573 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi0/1 Root FWD 4 128.25 P2p Gi0/2 Altn BLK 4 128.26 P2p Which port is the Root Port? Path Cost Rick Graziani graziani@cabrillo.edu

  50. show spanning-tree detail Core# show spanning-tree detail VLAN0001 is executing the ieee compatible Spanning Tree Protocol Bridge Identifier has priority of 32768, sysid 1, 0001.C945.A573 Configured hello time 2, max age 20, forward delay 15 Current root has priority 32769 Root port is 25 (GigabitEthernet0/1), cost of root path is 4 Topology change flag not set, detected flag not set Number of topology changes 0 last change occurred 00:00:00 ago from FastEthernet0/1 Times: hold 1, topology change 35, notification 2 hello 2, max age 20, forward delay 15 Timers: hello 0, topology change 0, notification 0, aging 300 Which port is the Root Port? Path Cost Rick Graziani graziani@cabrillo.edu

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