Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Spanning Tree Protocol

Cisco Networking Academy Program © Cisco Systems, Inc Spanning Tree Protocol 10BaseT Ports (12)100BaseT Ports 10BaseT Ports (12) 100BaseT Ports A Redundant Paths and No Spanning Tree. So, whats the problem? Moe Larry Host Kahn Host Baran A Hub Cisco Networking Academy Program

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc BaseT Ports (12) A Moe Larry Host Kahn Host Baran A Hub 100BaseT Ports Host Kahn sends an Ethernet frame to Host Baran. Both Switch Moe and Switch Larry see the frame and record Host Kahns Mac Address in their switching tables.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc BaseT Ports (12) 100BaseT Ports A Moe Larry Host Baran A SAT (Source Address Table) Port 1: SAT (Source Address Table) Port 1: D-FE Hub Host Kahn

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc SAT (Source Address Table) Port 1: SAT (Source Address Table) Port 1: BaseT Ports (12) 100BaseT Ports A Moe Larry A D-FE Hub Both Switches do not have the destination MAC address in their table so they flood it out all ports. Host Baran Host Kahn

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc SAT (Source Address Table) Port 1: BaseT Ports (12) 100BaseT Ports A Moe Larry A D-FE Hub SAT (Source Address Table) Port 1: Port A: Switch Moe now learns, incorrectly, that the Source Address is on Port A. Host Baran Host Kahn

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc SAT (Source Address Table) Port 1: Port A: SAT (Source Address Table) Port 1: Port A: BaseT Ports (12) 100BaseT Ports A Moe Larry A D-FE Hub Switch Larry also learns, incorrectly, that the Source Address is on Port A. Host Baran Host Kahn

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc SAT (Source Address Table) Port A: BaseT Ports (12) 100BaseT Ports A Moe Larry A D-FE Hub SAT (Source Address Table) Port A: Now, when Host Baran sends a frame to Host Kahn, it will be sent the longer way, through Switch Larrys port A. Host Baran Host Kahn

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Then the same confusion happens, but this time with Host Baran. Okay, maybe this is not the end of the world. Frames will just take a longer path and you may also see other unexpected results. But what about broadcast frames, like ARP Requests?

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc BaseT Ports (12) 100BaseT Ports A Moe Larry Host Kahn A D-FE Hub Lets, leave the switching tables alone and just look at what happens with the frames. Host Kahn sends out a Layer 2 broadcast frame, like an ARP Request. Host Baran

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc BaseT Ports (12) 100BaseT Ports A Moe Larry Host Kahn A D-FE Hub Because it is a Layer 2 broadcast frame, both switches, Moe and Larry, flood the frame out all ports, including their port As. Host Baran

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc BaseT Ports (12) 100BaseT Ports A Moe Larry Host Kahn A D-FE Hub Duplicate frame Duplicate frame Both switches receive the same broadcast, but on a different port. Doing what switches do, both switches flood the duplicate broadcast frame out their other ports. Host Baran

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc BaseT Ports (12) 100BaseT Ports A Moe Larry A D-FE Hub Duplicate Frame Duplicate Frame Here we go again, with the switches flooding the same broadcast again out its other ports. This results in duplicate frames, known as a broadcast storm! Host Kahn Host Baran

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc BaseT Ports (12) A Moe Larry A D-FE Hub Remember, that Layer 2 broadcasts not only take up network bandwidth, but must be processed by each host. This can severely impact a network, to the point of making it unusable. Host Kahn Host Baran

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Spanning Tree to the Rescue!

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Broadcast Frame Standby Link Switches forward broadcast frames Prevents loops Loops can cause broadcast storms, exponentially proliferate frames Allows redundant links Prunes topology to a minimal spanning tree Resilient to topology changes and device failures Main function of the Spanning Tree Protocol (STP) is to allow redundant switched/bridged paths without suffering the effects of loops in the network Introducing Spanning-Tree Protocol

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc The STA is used to calculate a loop-free path. Spanning-tree frames called bridge protocol data units (BPDUs) are sent and received by all switches in the network at regular intervals and are used to determine the spanning tree topology. A separate instance of STP runs within each configured VLAN. (VLANs are later)

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc States initially set, later modified by STP Server ports can be configured to immediately enter STP forward mode Understanding STP States Blocking Listening Learning Forwarding Disabled

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Blocking - No frames forwarded, BPDUs heard Listening - No frames forwarded, listening for frames Learning - No frames forwarded, learning addresses Forwarding - Frames forwarded, learning addresses Disabled - No frames forwarded, no BPDUs heard Understanding STP States

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Part of 802.1d standard Simple principle: Build a loop-free tree from some identified point known as the root. Redundant paths allowed, but only one active path. Developed by Radia Perlman Spanning Tree Algorithm (STA)

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Spanning Tree Process Step 1: Electing a Root Bridge Step 2: Electing Root Ports Step 3: Electing Designated Ports All switches send out Configuration Bridge Protocol Data Units (Configuration BPDUs) BPDUs are sent out all interfaces every two seconds (by default - tunable) All ports are in Blocking Mode during the initial Spanning Tree is process.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc. 2000

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Spanning Tree Algorithm (STA): Bridge Protocol Data Units Fields (BPDU) (FYI) The fields used in the STA BPDU are provided for your information only. During the discussion of STA you may wish to refer to this protocol to see how the information is sent and received.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Protocol Identifier (2 bytes), Version (1 byte), Message Type (1 byte): Not really utilized (N/A here) Flags (1 byte): Used with topology changes (N/A here) Root ID (8 bytes): Indicates current Root Bridge on the network, includes: Bridge Priority (2 bytes) Bridge MAC Address (6 bytes) Known as the Bridge Identifier of the Root Bridge

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Cost to Root (4 bytes): Cost of the path from the bridge sending the BDPU to the Root Bridge indicated in the Root ID field. Cost is based on bandwidth. Bridge ID (8 bytes): Bridge sending the BDPU –2 bytes: Bridge Priority –6 bytes: MAC Address Port ID (2 bytes): Port on bridge sending BDPU, including Port Priority value

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Message Age (2 bytes): Age of BDPU (N/A here) Maximum Age (2 bytes): When BDPU should be discarded (N/A here) Hello Time (2 bytes): How often BDPUs are to be sent (N/A here) Forward Delay (2 bytes): How long bridge should remain in listening and learning states (N/A here)

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports 3 Switches with redundant paths Can you find them?

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Steps to Spanning Tree Step 1: Electing a Root Bridge Bridge Priority Bridge ID Root Bridge Step 2: Electing Root Ports Path Cost or Port Cost Root Path Cost Root Port Step 3: Electing Designated Ports Path Cost or Port Cost Root Path Cost

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Step 1: Electing a Root Bridge The first step is for switches to select a Root Bridge. The root bridge is the bridge from which all other paths are decided. Only one switch can be the root bridge. Election of a root bridge is decided by: 1. Lowest Bridge Priority 2. Lowest Bridge ID (tie-breaker)

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Bridge Priority This is a numerical value. The switch with the with the lowest bridge priority is the root bridge. The switches use BPDUs to accomplish this. All switches consider themselves as the root bridge until they find out otherwise. All Cisco Catalyst switches have the default Bridge priority of Its a tie! So then what?

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Bridge Priorities

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Switch Moe: Bridge Priority

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc In case of a tie, the Bridge ID is used… Bridge ID The Bridge ID is the MAC address assigned to the individual switch. The lower Bridge ID (MAC address) is the tiebreaker. Because MAC addresses are unique, this ensures that only one bridge will have the lowest value. NOTE: There are other tie breakers, if these values are not unique, but we will not cover those situations.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc. 2000

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 Bridge Priorities and Bridge Ids Which one is the lowest?

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 Lowest: Moe becomes the root bridge You got it! A B

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Step 2: Electing Root Ports After the root bridge is selected, switches (bridges) must locate redundant paths to the root bridge and block all but one of these paths. The switches use BPDUs to accomplish this. How does the switch make the decision on which port to use, known as the root port, and which one should be blocked?

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 ? ? ? ? Redundant Paths 100BaseT Ports A B

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Path Cost (or Port Cost) Port Cost is used to help find the cheapest or fastest path to the root bridge. By default, port cost is usually based on the medium or bandwidth of the port. On Cisco Catalyst switches, this value is derived by dividing 1000 by the speed of the media in megabytes per second. Examples: Standard Ethernet: 1,000/10 = 100 Fast Ethernet: 1,000/100 = 10

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Root Path Cost The root path cost is the cumulative port costs (path costs) to the Root Bridge. This value is transmitted in the BPDU cost field.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc However, everything is viewed in relation to the root bridge. Root Ports Ports directly connected to the root bridge will be the root ports. Otherwise, the port with the lowest root path cost will be the root port.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC Path Costs 100BaseT Ports A B

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Curly Even though the Path Cost to the root bridge for Curly is higher using Port 1, Port 1 has a direct connection to the root bridge, thus it becomes the root port. Port 1 is then put in Forwarding mode, while the redundant path of Port A, is put into Blocking mode.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 X Blocking Forwarding 100BaseT Ports Curly A B

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Larry Larry also has a root port, a direct connection with the root bridge, through Port B. Port B is then put in Forwarding mode, while the redundant path of Port A, is put into Blocking mode.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 X Blocking Forwarding 100BaseT Ports X Blocking Forwarding A B Larry

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 X Blocking 100BaseT Ports X Blocking A B Root Port Root Ports

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Step 3: Electing Designated Ports The single port for a switch that sends and receives traffic to and from the Root Bridge. It can also be thought of as the port that is advertising the lowest cost to the Root Bridge. In our example, we only have the two obvious choices, which are on switch Moe. If we had other LAN segments, we could explain designated ports in more detail, but this is fine for now.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 X Blocking Forwarding 100BaseT Ports X Blocking Forwarding A B Designated Port Designated Ports

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Spanning Tree is now complete, and the switches can begin to properly switch frames out the proper ports with the correct switching tables and without creating duplicate frames.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Most LAN and switched internetwork books provide information on Spanning Tree. For more complex examples, you may wish to try these books: Cisco Catalyst LAN Switching, by Rossi and Rossi, McGraw Hill (Very Readable) CCIE Professional Development: Cisco LAN Switching, by Clark and Hamilton, Cisco Press (More Advanced) Interconnections, by Radia Perlman, Addison Wesley (Excellent, but very academic)

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Extra Item! Port Fast Mode (from Cisco documentation) Port Fast mode immediately brings a port from the blocking state into the forwarding state by eliminating the forward delay (the amount of time a port waits before changing from its STP learning and listening states to the forwarding state). Note Port Fast Mode-enabled ports should only be used for end-station attachments.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc When the switch is powered up, the forwarding state, even if Port Fast mode is enabled, is delayed to allow the Spanning- Tree Protocol to discover the topology of the network and ensure no temporary loops are formed. Spanning-tree discovery takes approximately 30 seconds to complete, and no packet forwarding takes place during this time. After the initial discovery, Port Fast-enabled ports transition directly from the blocking state to the forwarding state.

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc A B 1 1 Moe Larry Curly 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 X Blocking Forwarding 100BaseT Ports X Blocking Forwarding A B Spanning Tree Completed

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Moe- Port 1

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Moe- Port B

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Larry

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Larry- Port 1

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Larry- Port B

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Curly

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Curly- Port 1

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc Curly- Port A

Spanning Tree Protocol Cisco Networking Academy Program © Cisco Systems, Inc First, the root must be selected. By ID, it is elected. Least cost paths from root are traced. In the tree, these paths are placed. A mesh is made by folks like me, Then bridges find a spanning tree. I think that I shall never see A graph more lovely than a tree. A tree whose crucial property Is loop-free connectivity. A tree that must be sure to span. So packets can reach every LAN. The Spanning Tree Algorhyme by Radia Perlman