© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE I Chapter 6 1 Implement Spanning Tree Protocols LAN Switching and Wireless – Chapter 5 Part II

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 2 Objectives  Portfast  Rapid Spanning Tree Protocol (RSTP)  Rapid Per VLAN Spanning Tree (RPVST)  Per VLAN Spanning Tree (PVST+)  Multiple Spanning Tree Protocol (MSTP) MSTP is also known as Multiple Instance Spanning Tree Protocol (MISTP) on Cisco Catalyst 6500 switches and above  Implement rapid per VLAN spanning tree (rapid PVST+) in a LAN to prevent loops between redundant switches.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 3

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 4 Implement Rapid per VLAN Spanning Tree (rapid PVST+) in a LAN  Summarize the features of the PVST+, RSTP and rapid PVST+ variants of STP

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 5 Implement Rapid per VLAN Spanning Tree (rapid PVST+) in a LAN

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 6 PortFast  Enable PortFast on Layer 2 access ports connected to a single workstation or server to allow those devices to connect to the network immediately, rather than waiting for spanning tree to converge.  The purpose of PortFast is to minimize the time that access ports wait for STP to converge.  The advantage of enabling PortFast is to prevent DHCP timeouts. Host is powered on. Portfast causes port to skip Listening and Learning and immediately go into Forwarding state.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 7 PortFast  By using PortFast, devices can be granted instant access to the Layer 2 network without going through the spanning tree listening and learning stages.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 8 Configuring Portfast Access2(config)#interface range fa 0/ Access2(config-if-range)#switchport mode access Access2(config-if-range)#spanning-tree portfast OR Access2(config)#spanning-tree portfast default  Warning: PortFast should only be enabled on ports that are connected to a single host.  If hubs, concentrators, switches, and bridges. are connected to the interface when PortFast is enabled, temporary bridging loops can occur.  Use with caution.  Use the following command to enable PortFast globally in global configuration mode:

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 9  Portfast is configured on all access mode ports. Portfast

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE I Chapter 6 10 PVST+

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 11 Implement Rapid per VLAN Spanning Tree (rapid PVST+) in a LAN  Describe the features of PVST+

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 12 PVST+  Per VLAN Spanning Tree Plus (PVST+) maintains a separate spanning-tree instance for each VLAN.  By default, a single spanning tree runs on each configured VLAN, provided STP has not been manually disabled.  The plus sign in PVST+ indicates that STP 802.1D has been enhanced by Cisco with proprietary features.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 13 PVST+  If configured, PVST+ provides for load balancing on a per-VLAN basis; PVST+ allows creation of different logical topologies using the VLANs on a switched network to ensure that all links can be used and that one link is not oversubscribed.  Each instance of PVST+ on a VLAN has a single root bridge.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 14 PVST Configuration Root Bridge Configuration

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 15 PVST+ Core(config)#spanning-tree vlan 1 root primary Distribution1(config)#spanning-tree vlan 10 root primary Distribution2(config)#spanning-tree vlan 20 root primary Root 1 Root 10Root 20

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 16 Load Balancing with 3 Root Switches Root 20Root 10 Root 1

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 17 PVST+ verifications

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE I Chapter 6 18 RPVST+

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 19 Rapid Spanning Tree Protocol  The immediate consideration of STP is convergence.  Depending on the type of failure, it takes anywhere from 30 to 50 seconds, to converge the network.  RSTP helps with STP convergence issues  RSTP is based on IEEE 802.1w standard.  RSTP requires full-duplex point-to-point connection between switches to achieve fast convergence.  STP and RSTP also have port designation differences. RSTP has alternate and backup port designations. Ports not participating in spanning tree are known as edge ports. The edge port becomes a nonedge port immediately if a BPDU is heard on the port. The RSTP spanning tree algorithm (STA) elects a root bridge in exactly the same way as 802.1D elects a root.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 20 RSTP Port States

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 21 Port Roles Port RoleDescription Root port (Same as STP) The root port is the switch port on every nonroot bridge that is the chosen path to the root bridge. There can be only one root port on every switch. The root port assumes the forwarding state in a stable active topology. Designated port (Same as STP) Each segment has at least one switch port as the designated port for that segment. In a stable, active topology, the switch with the designated port receives frames on the segment that are destined for the root bridge. There can be only one designated port per segment. The designated port assumes the forwarding state. All switches connected to a given segment listen to all BPDUs and determine the switch that will be the designated switch for a particular segment. Alternative port (Non-Designated Port in STP) The alternative port is a switch port that offers an alternative path toward the root bridge. The alternative port assumes a discarding state in a stable, active topology. An alternative port is present on nondesignated switches and makes a transition to a designated port if the current designated path fails. Backup portThe backup port is an additional switch port on the designated switch with a redundant link to the segment for which the switch is designated. A backup port has a higher port ID than the designated port on the designated switch. The backup port assumes the discarding state in a stable, active topology.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 22 RSTP Port Roles

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 23 What Are Edge Ports? Will never have a switch connected to it Immediately transitions to forwarding Functions similarly to PortFast Configured by issuing the spanning-tree portfast command

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 24 RSTP Link Types Link Type Description Point-to-pointPort 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.

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 25 RPVST+ configuration

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 26 RPVST+ configuration  RPVST+ configuration example

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 27 RPVST+ Verification

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 28 RPVST+ Verification

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 29 Implement Rapid per VLAN Spanning Tree (rapid PVST+) in a LAN  Describe how to design STP to avoid problems

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 30 Implement Rapid per VLAN Spanning Tree (rapid PVST+) in a LAN  Describe how to identify and solve the key STP configuration issues

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 31 Summary  Spanning Tree Protocol (STP) is used to prevent loops from being formed on redundant networks  STP uses different port states & timers to logically prevent loops  There is at least one switch in a network that serves as the root bridge Root bridge is elected using information found in BPDU frames  Root ports are determined by the spanning tree algorithm and are closest to the root bridge

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 32 Summary  STP lengthy convergence time (50 seconds) facilitated the development of: RSTP convergence time is slightly over 6 seconds Rapid PVST+ - adds VLAN support to RSTP - is the preferred spanning-tree protocol on a Cisco switch netowork

© 2006 Cisco Systems, Inc. All rights reserved.Cisco PublicITE 1 Chapter 6 33