1. © 2008 Cisco Systems, Inc. All rights reserved.Cisco ConfidentialPresentation_ID 1 Chapter 2: LAN Redundancy Scaling Networks

2. Presentation_ID 2 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Chapter 2 2.0 Introduction 2.1 Spanning Tree Concepts 2.2 Varieties of Spanning Tree Protocols 2.3 Spanning Tree Configuration 2.4 First-Hop Redundancy Protocols 2.5 Summary

3. Presentation_ID 3 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Chapter 2: Objectives  Describe the issues with implementing a redundant network.  Describe IEEE 802.1D STP operation.  Describe the different spanning tree varieties.  Describe PVST+ operation in a switched LAN environment.  Describe Rapid PVST+ operation in a switched LAN environment.  Configure PVST+ in a switched LAN environment.  Configure Rapid PVST+ in a switched LAN environment.  Identify common STP configuration issues.  Describe the purpose and operation of first hop redundancy protocols.  Describe the different varieties of first hop redundancy protocols.  Use Cisco IOS commands to verify HSRP and GLBP implementations.

4. © 2008 Cisco Systems, Inc. All rights reserved.Cisco ConfidentialPresentation_ID 4 2.1 Spanning Tree Concepts

5. Presentation_ID 5 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Purpose of Spanning Tree Redundancy at OSI Layers 1 and 2 Multiple cabled paths between switches:  Provide physical redundancy in a switched network.  Improves the reliability and availability of the network.  Enables users to access network resources, despite path disruption.

6. Presentation_ID 6 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Purpose of Spanning Tree Issues with Layer 1 Redundancy: MAC Database Instability  Ethernet frames do not have a time to live (TTL) attribute. Frames continue to propagate between switches endlessly, or until a link is disrupted and breaks the loop. Results in MAC database instability. Can occur due to broadcast frames forwarding.  If there is more than one path for the frame to be forwarded out, an endless loop can result. When a loop occurs, it is possible for the MAC address table on a switch to constantly change with the updates from the broadcast frames, resulting in MAC database instability.

7. Presentation_ID 7 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Purpose of Spanning Tree Issues with Layer 1 Redundancy: Broadcast Storms  A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. It is also known as denial of service  A broadcast storm is inevitable on a looped network. As more devices send broadcasts over the network, more traffic is caught within the loop; thus consuming more resources. This eventually creates a broadcast storm that causes the network to fail.

8. Presentation_ID 8 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Purpose of Spanning Tree Issues with Layer 1 Redundancy: Broadcast Storms

9. Presentation_ID 9 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Purpose of Spanning Tree Issues with Layer 1 Redundancy: Duplicate Unicast Frames  Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device.  Most upper layer protocols are not designed to recognize, or cope with, duplicate transmissions.  Layer 2 LAN protocols, such as Ethernet, lack a mechanism to recognize and eliminate endlessly looping frames.

10. Presentation_ID 10 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Purpose of Spanning Tree Issues with Layer 1 Redundancy: Duplicate Unicast Frames

11. Presentation_ID 11 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Spanning Tree Algorithm: Introduction  STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop.  A port is considered blocked when user data is prevented from entering or leaving that port. This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops.  The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring.  If the path is ever needed to compensate for a network cable or switch failure, STP recalculates the paths and unblocks the necessary ports to allow the redundant path to become active.

12. Presentation_ID 12 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Spanning Tree Algorithm: Introduction

13. Presentation_ID 13 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Spanning Tree Algorithm: Introduction

14. Presentation_ID 14 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Spanning Tree Algorithm: Introduction

15. Presentation_ID 15 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Spanning Tree Algorithm: Port Roles

16. Presentation_ID 16 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Spanning Tree Algorithm: Root Bridge

17. Presentation_ID 17 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Spanning Tree Algorithm: Path Cost

18. Presentation_ID 18 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation 802.1D BPDU Frame Format

19. Presentation_ID 19 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation BPDU Propagation and Process

20. Presentation_ID 20 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation BPDU Propagation and Process

21. Presentation_ID 21 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Extended System ID STP was enhanced to include support for VLANs, requiring the VLAN ID to be included in the BPDU frame through the use of the extended system ID

22. Presentation_ID 22 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Operation Extended System ID In the example, the priority of all the switches is 32769. The value is based on the 32768 default priority and the VLAN 1 assignment associated with each switch (32768+1).

23. © 2008 Cisco Systems, Inc. All rights reserved.Cisco ConfidentialPresentation_ID 23 2.2 Varieties of Spanning Tree Protocols

24. Presentation_ID 24 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Overview List of Spanning Tree Protocols  STP or IEEE 802.1D-1998  PVST+  IEEE 802.1D-2004  Rapid Spanning Tree Protocol (RSTP) or IEEE 802.1w  Rapid PVST+  Multiple Spanning Tree Protocol (MSTP) or IEEE 802.1s

25. Presentation_ID 25 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Overview Characteristics of the Spanning Tree Protocols

26. Presentation_ID 26 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Overview of PVST+ Networks running PVST+ have these characteristics:  A network can run an independent IEEE 802.1D STP instance for each VLAN in the network.  Optimum load balancing can result.  One spanning-tree instance for each VLAN maintained can mean a considerable waste of CPU cycles for all the switches in the network. In addition to the bandwidth that is used for each instance to send its own BPDU.

27. Presentation_ID 27 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Overview of PVST+

28. Presentation_ID 28 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Port States and PVST+ Operation STP introduces the five port states:

29. Presentation_ID 29 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Extended System ID and PVST+ Operation  In a PVST+ environment, the extended switch ID ensures each switch has a unique BID for each VLAN.  For example, the VLAN 2 default BID would be 32770; priority 32768, plus the extended system ID of 2.

30. Presentation_ID 30 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Rapid Spanning Tree Protocol

31. Presentation_ID 31 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP Operation – Port States Port StateDescription Discarding This state is seen in both a stable active topology and during topology synchronization and changes. The discarding state prevents the forwarding of data frames, thus “breaking” the continuity of a Layer 2 loop. Learning This state is seen in both a stable active topology and during topology synchronization and changes. The learning state accepts data frames to populate the MAC table to limit flooding of unknown unicast frames. Forwarding This state is seen only in stable active topologies. The forwarding switch ports determine the topology. Following a topology change, or during synchronization, the forwarding of data frames occurs only after a proposal and agreement process. Operational Status STP Port StateRSTP Port StatePort Included in Active Topology EnabledBlockingDiscardingNo EnabledListeningDiscardingNo EnabledLearning Yes EnabledForwarding Yes Disabled DiscardingNo

32. Presentation_ID 32 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP Operation – Port Roles STP Port RoleRSTP Port Role STP Port State RSTP Port State Root port Forwarding Designated port Forwarding Nondesignated port Alternate or backup port BlockingDiscarding Disabled -Discarding Transition Listening Learning

33. Presentation_ID 33 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP Operation – Rapid Transition to Forwarding – Link Type Link Type Description Point-to- point Port operating in full- duplex mode. It is assumed that the port is connected to a single switch device at the other end of the link. SharedPort operating in half- duplex mode. It is assumed that the port is connected to shared media where multiple switches might exist.

34. Presentation_ID 34 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP Operation – Rapid Transition to Forwarding – Edge Ports  An RSTP edge port is a switch port that is never intended to be connected to another switch device. It immediately transitions to the forwarding state when enabled.  Neither edge ports nor PortFast- enabled ports generate topology changes when the port transitions to disabled or enabled status. Unlike PortFast, an edge port that receives a BPDU immediately loses its edge port status and becomes a normal spanning-tree port. When an edge port receives a BPDU, it generates a topology change notification (TCN).

35. Presentation_ID 35 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP Operation – Proposal and Agreement

36. Presentation_ID 36 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP Operation – Topology Change (TC) Mechanism  Only non-edge ports that are moving to the forwarding state cause a topology change. A port that is moving to blocking does not cause the respective bridge to generate a TC BPDU.

37. Presentation_ID 37 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP Operation – Bridge Identifier for PVRST+  Only four high-order bits of the 16-bit Bridge Priority field affect the priority. Therefore, priority can be incremented only in steps of 4096, onto which are added the VLAN number. For example, for VLAN 11: If the priority is left at default, the 16-bit Priority field will hold 32768 + 11 = 32779.

38. Presentation_ID 38 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential RSTP and 802.1D STP Compatibility  RSTP can operate with 802.1D STP. However, 802.1w’s fast-convergence benefits are lost when interacting with 802.1D bridges.  Each port maintains a variable that defines the protocol to run on the corresponding segment. If the port receives BPDUs that do not correspond to its current operating mode for two times the hello time, it switches to the other STP mode.

39. Presentation_ID 39 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Default STP Configuration on Cisco Switch  PVST+  Bridge priority 32,768 for each VLAN

40. Presentation_ID 40 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Spanning Tree PortFast  Bypass 802.1D STP listening and learning states (blocking state forwarding state)  Ports connected to end stations  Prevents DHCP timeouts  May create bridging loops if enabled on trunk port

41. Presentation_ID 41 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Configuring PortFast on Access Ports  Use the spanning-tree portfast interface command to enable the PortFast feature. Switch# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Switch(config)# interface FastEthernet 3/27 Switch(config-if)# spanning-tree portfast %Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc... to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION %Portfast has been configured on FastEthernet3/27 but will only have effect when the interface is in a non-trunking mode. Switch(config-if)# end Switch# Switch# show spanning-tree interface FastEthernet 3/27 portfast VLAN0001 enabled

42. Presentation_ID 42 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Configuring PortFast Globally  Use the spanning-tree portfast default global configuration mode command to enable the PortFast feature on all nontrunking interfaces. Switch(config)# spanning-tree portfast default

43. Presentation_ID 43 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Configuring PortFast on Trunk Ports  Use the spanning-tree portfast trunk interface command to enable the PortFast feature on a trunk port. Switch(config)# spanning-tree portfast trunk

44. Presentation_ID 44 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Implementing PVRST+ 1.Enable PVRST+ globally. PVRST+ should be configured on all switches in the broadcast domain. 2.Designate and configure a switch to be the root bridge. 3.Designate and configure a switch to be the secondary (backup) root bridge. 4.Ensure load sharing on uplinks using priority and cost parameters. 5.Verify the configuration.

45. Presentation_ID 45 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Verifying PVRST+  The output below illustrates how to verify the RSTP configuration for VLAN2 on a nonroot switch in a topology. Switch# show spanning-tree vlan 2 VLAN0002 Spanning tree enabled protocol rstp Root ID Priority 32768 Address 000b.fcb5.dac0 Cost 38 Port 7 (FastEthernet0/7) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32770 (priority 32768 sys-id-ext 2) Address 0013.5f1c.e1c0 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- -------- -------- --------------- -- Fa0/7 Root FWD 19 128.7 P2p Fa0/8 Root FWD 19 128.8 P2p

46. © 2008 Cisco Systems, Inc. All rights reserved.Cisco ConfidentialPresentation_ID 46 2.3 Spanning Tree Configuration

47. Presentation_ID 47 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration Catalyst 2960 Default Configuration

48. Presentation_ID 48 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration Configuring and Verifying the Bridge ID

49. Presentation_ID 49 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration Configuring and Verifying the Bridge ID

50. Presentation_ID 50 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration PortFast and BPDU Guard  When a switch port is configured with PortFast that port transitions from blocking to forwarding state immediately.  BPDU guard puts the port in an error-disabled state on receipt of a BPDU.

51. Presentation_ID 51 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration PVST+ Load Balancing

52. Presentation_ID 52 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration PVST+ Load Balancing  Another method to specify the root bridge is to set the spanning tree priority on each switch to the lowest value so that the switch is selected as the primary bridge for its associated VLAN.

53. Presentation_ID 53 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration PVST+ Load Balancing  Display and verify spanning tree configuration details.

54. Presentation_ID 54 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential PVST+ Configuration PVST+ Load Balancing

55. Presentation_ID 55 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Rapid PVST+ Configuration Spanning Tree Mode Rapid PVST+ is the Cisco implementation of RSTP. It supports RSTP on a per- VLAN basis.

56. Presentation_ID 56 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Configuration Issues Analyzing the STP Topology

57. Presentation_ID 57 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Configuration Issues Expected Topology versus Actual Topology

58. Presentation_ID 58 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Configuration Issues Overview of Spanning Tree Status

59. Presentation_ID 59 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Configuration Issues Spanning-Tree Failure Consequences  STP erroneously moves one or more ports into the forwarding state.  Any frame that is flooded by a switch enters the loop.

60. Presentation_ID 60 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential STP Configuration Issues Repairing a Spanning Tree Problem  One way to correct spanning-tree failure is to manually remove redundant links in the switched network, either physically or through configuration, until all loops are eliminated from the topology.  Before restoring the redundant links, determine and correct the cause of the spanning-tree failure.  Carefully monitor the network to ensure that the problem is fixed.

61. Presentation_ID 61 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Chapter 2: Summary  IEEE 802.1D is implemented on Cisco switches on a per-VLAN basis in the form of PVST+. This is the default configuration on Cisco switches.  RSTP, can be implemented on Cisco switches on a per-VLAN basis in the form of Rapid PVST+.  With PVST+ and Rapid PVST+, root bridges can be configured proactively to enable spanning tree load balancing.  First hop redundancy protocols, such as HSRP, VRRP, and GLBP provide alternate default gateways for hosts in the switched environment.

62. Presentation_ID 62 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential