1. Exploration 3 Chapter 5

2. Redundancy in Switched Networks Switches learn the MAC addresses of devices on their ports so that data can be properly forwarded to the destination. Switches will flood frames for unknown destinations until they learn the MAC addresses of the devices. Switches learn the MAC addresses of devices on their ports so that data can be properly forwarded to the destination. Switches will flood frames for unknown destinations until they learn the MAC addresses of the devices. Broadcasts and multicasts are also flooded. Broadcasts and multicasts are also flooded. A redundant switched topology may cause broadcast storms, multiple frame copies, and MAC address table instability problems. A redundant switched topology may cause broadcast storms, multiple frame copies, and MAC address table instability problems.

3. L2 Loops Broadcasts and Layer 2 loops can be a dangerous combination. Broadcasts and Layer 2 loops can be a dangerous combination. Ethernet frames have no TTL field 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. After an Ethernet frame starts to loop, it will probably continue until someone shuts off one of the switches or breaks a link. Physical loops without STP can be disastrous Physical loops without STP can be disastrous

4. Layer 2 Loops When multiple paths exist between two devices on the network and STP has been disabled on those switches, a Layer 2 loop can occur. If STP is enabled on these switches, which is the default, a Layer 2 loop would not occur. When multiple paths exist between two devices on the network and STP has been disabled on those switches, a Layer 2 loop can occur. If STP is enabled on these switches, which is the default, a Layer 2 loop would not occur. Ethernet frames do not have a time to live (TTL) like IP packets traversing routers. As a result, if they are not terminated properly on a switched network, they continue to bounce from switch to switch endlessly or until a link is disrupted and breaks the loop. Ethernet frames do not have a time to live (TTL) like IP packets traversing routers. As a result, if they are not terminated properly on a switched network, they continue to bounce from switch to switch endlessly or until a link is disrupted and breaks the loop.

5. 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. Consequently, no bandwidth is available bandwidth for legitimate traffic, and the network becomes unavailable for data communication. A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. Consequently, no bandwidth is available bandwidth for legitimate traffic, and the network becomes unavailable for data communication. A broadcast storm is inevitable on a looped network. As more devices send broadcasts out on the network, more and more traffic gets caught in the loop, eventually creating a broadcast storm that causes the network to fail. A broadcast storm is inevitable on a looped network. As more devices send broadcasts out on the network, more and more traffic gets caught in the loop, eventually creating a broadcast storm that causes the network to fail. There are other consequences for broadcast storms. Because broadcast traffic is forwarded out every port on a switch, all connected devices have to process all broadcast traffic that is being flooded endlessly around the looped network. This can cause the end device to malfunction because of the high processing requirements for sustaining such a high traffic load on the network interface card. There are other consequences for broadcast storms. Because broadcast traffic is forwarded out every port on a switch, all connected devices have to process all broadcast traffic that is being flooded endlessly around the looped network. This can cause the end device to malfunction because of the high processing requirements for sustaining such a high traffic load on the network interface card.

6. Spanning Tree Protocol (STP) The algorithm used to create this loop free logical topology is the spanning-tree algorithm. The algorithm used to create this loop free logical topology is the spanning-tree algorithm. Allows redundancy without loops Allows redundancy without loops The loop free logical topology created is called a tree. The loop free logical topology created is called a tree. This topology is a star or extended star logical topology, the spanning tree of the network. This topology is a star or extended star logical topology, the spanning tree of the network. It is a spanning tree because all devices in the network are reachable or spanned. It is a spanning tree because all devices in the network are reachable or spanned. Slow to converge, so now there is Rapid Spanning Tree Slow to converge, so now there is Rapid Spanning Tree

7. STP Topology Redundancy increases the availability of the network topology by protecting the network from a single point of failure, such as a failed network cable or switch. Redundancy increases the availability of the network topology by protecting the network from a single point of failure, such as a failed network cable or switch. When redundancy is introduced into a Layer 2 design, loops and duplicate frames can occur. When redundancy is introduced into a Layer 2 design, loops and duplicate frames can occur. Loops and duplicate frames can have severe consequences on a network. The Spanning Tree Protocol (STP) was developed to address these issues. Loops and duplicate frames can have severe consequences on a network. The Spanning Tree Protocol (STP) was developed to address these issues. 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. 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 network traffic is prevented from entering or leaving that port. A port is considered blocked when network traffic 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. This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops. Blocking the redundant paths is critical to preventing loops on the network. Blocking the redundant paths is critical to preventing loops on the network. The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring. 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. 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.

8. STP Ethernet switches can implement the IEEE 802.1D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network. Ethernet switches can implement the IEEE 802.1D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network. Shortest path is based on cumulative link costs. Link costs are based on the speed of the link. Shortest path is based on cumulative link costs. Link costs are based on the speed of the link. STP establishes a root node, called the root bridge. The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node. STP establishes a root node, called the root bridge. The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node. The resulting tree originates from the root bridge. The resulting tree originates from the root bridge. Redundant links that are not part of the shortest path tree are blocked, causing a loop free topology Redundant links that are not part of the shortest path tree are blocked, causing a loop free topology

9. Spanning Tree Link Costs

10. BPDUs The Spanning-Tree Protocol requires network devices to exchange messages to detect bridging loops. The Spanning-Tree Protocol requires network devices to exchange messages to detect bridging loops. Links that will cause a loop are put into a blocking state. Links that will cause a loop are put into a blocking state. The message that a switch sends, allowing the formation of a loop free logical topology, is called a Bridge Protocol Data Unit (BPDU). The message that a switch sends, allowing the formation of a loop free logical topology, is called a Bridge Protocol Data Unit (BPDU). BPDUs continue to be received on blocked ports. This ensures that if an active path or device fails, a new spanning tree can be calculated. BPDUs continue to be received on blocked ports. This ensures that if an active path or device fails, a new spanning tree can be calculated.

11. What ’ s in a BPDU?

12. STP Algorithm STP uses the Spanning Tree Algorithm (STA) to determine which switch ports on a network need to be configured for blocking to prevent loops from occurring. The STA designates a single switch as the root bridge and uses it as the reference point for all path calculations. STP uses the Spanning Tree Algorithm (STA) to determine which switch ports on a network need to be configured for blocking to prevent loops from occurring. The STA designates a single switch as the root bridge and uses it as the reference point for all path calculations. All switches participating in STP exchange BPDU frames to determine which switch has the lowest bridge ID (BID) on the network. The switch with the lowest BID automatically becomes the root bridge for the STA calculations. All switches participating in STP exchange BPDU frames to determine which switch has the lowest bridge ID (BID) on the network. The switch with the lowest BID automatically becomes the root bridge for the STA calculations.

13. The BPDU is the message frame exchanged by switches for STP. Each BPDU contains a BID that identifies the switch that sent the BPDU. The BID contains a priority value, the MAC address of the sending switch, and an optional extended system ID. The lowest BID value is determined by the combination of these three fields. The BPDU is the message frame exchanged by switches for STP. Each BPDU contains a BID that identifies the switch that sent the BPDU. The BID contains a priority value, the MAC address of the sending switch, and an optional extended system ID. The lowest BID value is determined by the combination of these three fields. After the root bridge has been determined, the STA calculates the shortest path to the root bridge. After the root bridge has been determined, the STA calculates the shortest path to the root bridge. Each switch uses the STA to determine which ports to block. Each switch uses the STA to determine which ports to block. While the STA determines the best paths to the root bridge for all destinations in the broadcast domain, all traffic is prevented from forwarding through the network. While the STA determines the best paths to the root bridge for all destinations in the broadcast domain, all traffic is prevented from forwarding through the network. The STA considers both path and port costs when determining which path to leave unblocked. The STA considers both path and port costs when determining which path to leave unblocked. The path costs are calculated using port cost values associated with port speeds for each switch port along a given path. The path costs are calculated using port cost values associated with port speeds for each switch port along a given path. The sum of the port cost values determines the overall path cost to the root bridge. The sum of the port cost values determines the overall path cost to the root bridge. If there is more than one path to choose from, STA chooses the path with the lowest path cost. If there is more than one path to choose from, STA chooses the path with the lowest path cost. When the STA has determined which paths are to be left available, it configures the switch ports into distinct port roles. The port roles describe their relation in the network to the root bridge and whether they are allowed to forward traffic. When the STA has determined which paths are to be left available, it configures the switch ports into distinct port roles. The port roles describe their relation in the network to the root bridge and whether they are allowed to forward traffic.

14. Switches use BPDUs to: Select a single switch that will act as the root of the spanning tree Select a single switch that will act as the root of the spanning tree Calculate the shortest path from itself to the root switch Calculate the shortest path from itself to the root switch Designate one of the switches as the closest one to the root, for each LAN segment. This bridge is called the “ designated switch ”. The designated switch handles all communication from that LAN towards the root bridge. Designate one of the switches as the closest one to the root, for each LAN segment. This bridge is called the “ designated switch ”. The designated switch handles all communication from that LAN towards the root bridge. Choose one of its ports as its root port, for each non-root switch. This is the interface that gives the best path to the root switch. Choose one of its ports as its root port, for each non-root switch. This is the interface that gives the best path to the root switch. Select ports that are part of the spanning tree, the designated ports. Non-designated ports are blocked. Select ports that are part of the spanning tree, the designated ports. Non-designated ports are blocked.

15. Root ports - Switch ports closest to the root bridge. Root ports - Switch ports closest to the root bridge. Designated ports - All non-root ports that are still permitted to forward traffic on the network. Designated ports - All non-root ports that are still permitted to forward traffic on the network. Non-designated ports - All ports configured to be in a blocking state to prevent loops. Non-designated ports - All ports configured to be in a blocking state to prevent loops.

16. Electing the Root Bridge When a switch is turned on, the spanning-tree algorithm is used to identify the root bridge. BPDUs are sent out with the Bridge ID (BID). When a switch is turned on, the spanning-tree algorithm is used to identify the root bridge. BPDUs are sent out with the Bridge ID (BID). The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address (from PROM). By default BPDUs are sent every two seconds. The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address (from PROM). By default BPDUs are sent every two seconds. When a switch first starts up, it assumes it is the root switch and sends “ inferior ” BPDUs. These BPDUs contain the switch MAC address in both the root and sender BID. All switches see the BIDs sent. As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out. All bridges see these and decide that the bridge with the smallest BID value will be the root bridge. When a switch first starts up, it assumes it is the root switch and sends “ inferior ” BPDUs. These BPDUs contain the switch MAC address in both the root and sender BID. All switches see the BIDs sent. As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out. All bridges see these and decide that the bridge with the smallest BID value will be the root bridge.

17. The Root Bridge All switches in the broadcast domain participate in the election process. All switches in the broadcast domain participate in the election process. After a switch boots, it sends out BPDU frames containing the switch BID and the root ID every 2 seconds. After a switch boots, it sends out BPDU frames containing the switch BID and the root ID every 2 seconds. By default, the root ID matches the local BID for all switches on the network. By default, the root ID matches the local BID for all switches on the network. The root ID identifies the root bridge on the network. Initially, each switch identifies itself as the root bridge after bootup. The root ID identifies the root bridge on the network. Initially, each switch identifies itself as the root bridge after bootup. As the switches forward their BPDU frames, adjacent switches in the broadcast domain read the root ID information from the BPDU frame. As the switches forward their BPDU frames, adjacent switches in the broadcast domain read the root ID information from the BPDU frame. If the root ID from the BPDU received is lower than the root ID on the receiving switch, the receiving switch updates its root ID identifying the adjacent switch as the root bridge. If the root ID from the BPDU received is lower than the root ID on the receiving switch, the receiving switch updates its root ID identifying the adjacent switch as the root bridge. Note: It may not be an adjacent switch, but any other switch in the broadcast domain. Note: It may not be an adjacent switch, but any other switch in the broadcast domain. The switch then forwards new BPDU frames with the lower root ID to the other adjacent switches. The switch then forwards new BPDU frames with the lower root ID to the other adjacent switches. Eventually, the switch with the lowest BID ends up being identified as the root bridge for the spanning-tree instance. Eventually, the switch with the lowest BID ends up being identified as the root bridge for the spanning-tree instance.

18. Best Paths to the Root Bridge When the root bridge has been designated for the spanning-tree instance, the STA starts the process of determining the best paths to the root bridge from all destinations in the broadcast domain. When the root bridge has been designated for the spanning-tree instance, the STA starts the process of determining the best paths to the root bridge from all destinations in the broadcast domain. The path information is determined by summing up the individual port costs along the path from the destination to the root bridge. The path information is determined by summing up the individual port costs along the path from the destination to the root bridge. The default port costs are defined by the speed at which the port operates. The default port costs are defined by the speed at which the port operates.

19. Electing Root Ports Each switch must form an association with the root bridge. Each switch must form an association with the root bridge. At the conclusion of the root war, the switches move on to selecting Root Ports. At the conclusion of the root war, the switches move on to selecting Root Ports. A bridge ’ s Root Port is the port that is closest to the Root Bridge in terms of Path Cost. Every non-Root Bridge must select one Root Port. Again, bridges use the concept of cost to measure closeness. A bridge ’ s Root Port is the port that is closest to the Root Bridge in terms of Path Cost. Every non-Root Bridge must select one Root Port. Again, bridges use the concept of cost to measure closeness. If a switch receives BPDUs on multiple ports, it has a redundant path to the root bridge (or it is the root bridge!) If a switch receives BPDUs on multiple ports, it has a redundant path to the root bridge (or it is the root bridge!)

20. Electing Root Ports In order to choose which ports will forward data and which ports will block data, the switch looks at three components of the BPDU: In order to choose which ports will forward data and which ports will block data, the switch looks at three components of the BPDU: Lowest path cost to root bridge Lowest path cost to root bridge Lowest sender Bridge ID Lowest sender Bridge ID Lowest port priority/port ID Lowest port priority/port ID

21. Path Cost Lowest cost path to the root preferred Lowest cost path to the root preferred Path cost calculated based on link speed and the number of links that the BPDU crossed downstream from the root. Path cost calculated based on link speed and the number of links that the BPDU crossed downstream from the root. If one port has the lowest cost, it is placed in forwarding mode. All other ports receiving BPDUs are placed in blocking mode. If one port has the lowest cost, it is placed in forwarding mode. All other ports receiving BPDUs are placed in blocking mode.

22. Path Cost

23. Bridge IDs If the path costs of the received BPDUs are equal, the switch looks at the Bridge ID to determine which port should forward. If the path costs of the received BPDUs are equal, the switch looks at the Bridge ID to determine which port should forward. The port receiving the lowest Bridge ID is chosen to forward, all others block. The port receiving the lowest Bridge ID is chosen to forward, all others block.

24. Port Cost/Port ID If the path cost and bridge IDs are equal (as in the case of parallel links), the switch goes to the port priority as a tiebreaker. If the path cost and bridge IDs are equal (as in the case of parallel links), the switch goes to the port priority as a tiebreaker. Lowest port priority wins (default 128). Lowest port priority wins (default 128). You can set the priority from 0 – 255. You can set the priority from 0 – 255. If all ports have the same priority, the port with the lowest port number forwards frames. If all ports have the same priority, the port with the lowest port number forwards frames.

25. Spanning-Tree Port States

26. Blocked Blocked All ports start in blocked mode in order to prevent the bridge from creating a bridging loop. The port stays in a blocked state if Spanning Tree determines that there is a better path to the root bridge. All ports start in blocked mode in order to prevent the bridge from creating a bridging loop. The port stays in a blocked state if Spanning Tree determines that there is a better path to the root bridge.

27. Blocked State Discards frames received from the attached segment or internally forwarded through switching Discards frames received from the attached segment or internally forwarded through switching Receives BPDUs and directs them to the system module Receives BPDUs and directs them to the system module Has no address database Has no address database Does not transmit BPDUs received from the system module Does not transmit BPDUs received from the system module Receives and responds to network management messages but does not transmit them Receives and responds to network management messages but does not transmit them

28. Listening State Discards frames received from the attached segment or frames switched from another port Discards frames received from the attached segment or frames switched from another port Has no address database Has no address database Receives BPDUs and directs them to the system module Receives BPDUs and directs them to the system module Processes BPDUs received from the system module Receives and responds to network management messages Processes BPDUs received from the system module Receives and responds to network management messages It is during the Listening state that the three initial convergence steps take place – elect a Root Bridge, elect Root Ports, and elect Designated Ports. It is during the Listening state that the three initial convergence steps take place – elect a Root Bridge, elect Root Ports, and elect Designated Ports.

29. Spanning-Tree Port States Learn: The learn state is very similar to the listen state, except that the port can add information it has learned to its address table. Learn: The learn state is very similar to the listen state, except that the port can add information it has learned to its address table. Still not allowed to send or receive data Still not allowed to send or receive data Learns for a period of time called the fwd delay Learns for a period of time called the fwd delay

30. Learning State Discards frames received from the attached segment Discards frames received from the attached segment Discards frames switched from another port for forwarding Discards frames switched from another port for forwarding Receives and responds to network management messages Receives and responds to network management messages Builds the bridging table Builds the bridging table

31. Spanning-Tree Port States Forward: The port can send and receive data. Forward: The port can send and receive data. A port is not placed in the forwarding state unless there are no redundant links or it is determined that it has the best path to the root. A port is not placed in the forwarding state unless there are no redundant links or it is determined that it has the best path to the root.

32. Learning State As the bridge receives a frame, it places the source MAC address and port into the bridging table. Discards frames switched from another port for forwarding As the bridge receives a frame, it places the source MAC address and port into the bridging table. Discards frames switched from another port for forwarding Incorporates station location into its address database Incorporates station location into its address database Receives BPDUs and directs them to the system module Receives BPDUs and directs them to the system module Receives, processes, and transmits BPDUs received from the system module Receives, processes, and transmits BPDUs received from the system module Receives and responds to network management messages Receives and responds to network management messages

33. Spanning-Tree Port States Disabled: The port is shutdown. Disabled: The port is shutdown.

35. BPDU Timers The amount of time that a port stays in the various port states depends on the BPDU timers. Only the switch in the role of root bridge may send information through the tree to adjust the timers. The following timers determine STP performance and state changes: The amount of time that a port stays in the various port states depends on the BPDU timers. Only the switch in the role of root bridge may send information through the tree to adjust the timers. The following timers determine STP performance and state changes: Hello time Hello time Forward delay Forward delay Maximum age Maximum age

36. Cisco PortFast Technology PortFast is a Cisco technology. PortFast is a Cisco technology. When a switch port configured with PortFast is configured as an access port, that port transitions from blocking to forwarding state immediately, bypassing the typical STP listening and learning states. When a switch port configured with PortFast is configured as an access port, that port transitions from blocking to forwarding state immediately, bypassing the typical STP listening and learning states. You can use PortFast on access ports, which are connected to a single workstation or to a server, to allow those devices to connect to the network immediately rather than waiting for spanning tree to converge. You can use PortFast on access ports, which are connected to a single workstation or to a server, to allow those devices to connect to the network immediately rather than waiting for spanning tree to converge. If an interface configured with PortFast receives a BPDU frame, spanning tree can put the port into the blocking state using a feature called BPDU guard. If an interface configured with PortFast receives a BPDU frame, spanning tree can put the port into the blocking state using a feature called BPDU guard.

37. STP Convergence Steps Convergence is an important aspect of the spanning-tree process. Convergence is an important aspect of the spanning-tree process. Convergence is the time it takes for the network to determine which switch is going to assume the role of the root bridge, go through all the different port states, and set all switch ports to their final spanning-tree port roles where all potential loops are eliminated. Convergence is the time it takes for the network to determine which switch is going to assume the role of the root bridge, go through all the different port states, and set all switch ports to their final spanning-tree port roles where all potential loops are eliminated. The convergence process takes time to complete because of the different timers used to coordinate the process. The convergence process takes time to complete because of the different timers used to coordinate the process. Step 1. Elect a root bridge Step 1. Elect a root bridge Step 2. Elect root ports Step 2. Elect root ports Step 3. Elect designated and non-designated ports Step 3. Elect designated and non-designated ports

39. RSTP The Rapid Spanning-Tree Protocol is defined in the IEEE 802.1w LAN standard. The standard and protocol introduce the following: The Rapid Spanning-Tree Protocol is defined in the IEEE 802.1w LAN standard. The standard and protocol introduce the following: Clarification of port states and roles Clarification of port states and roles Definition of a set of link types that can go to forwarding state rapidly Definition of a set of link types that can go to forwarding state rapidly Concept of allowing switches, in a converged network, to generate their own BPDUs rather than relaying root bridge BPDUs Concept of allowing switches, in a converged network, to generate their own BPDUs rather than relaying root bridge BPDUs The “ blocked ” state of a port has been renamed as the “ discarding ” state. The “ blocked ” state of a port has been renamed as the “ discarding ” state.

40. RSTP Link Types Link types have been defined as point-to-point, edge-type, and shared. These changes allow failure of links in switched network to be learned rapidly. Link types have been defined as point-to-point, edge-type, and shared. These changes allow failure of links in switched network to be learned rapidly. Point-to-point links and edge-type links can go to the forwarding state immediately. Point-to-point links and edge-type links can go to the forwarding state immediately. Network convergence does not need to be any longer than 15 seconds with these changes. Network convergence does not need to be any longer than 15 seconds with these changes. The Rapid Spanning-Tree Protocol, IEEE 802.1w, will eventually replace the Spanning- Tree Protocol, IEEE 802.1D The Rapid Spanning-Tree Protocol, IEEE 802.1w, will eventually replace the Spanning- Tree Protocol, IEEE 802.1D

41. RSTP Port States

42. RSTP Port Roles The role is now a variable assigned to a given port. The role is now a variable assigned to a given port. The root port and designated port roles remain. The root port and designated port roles remain. The blocking port role is now split into the backup and alternate port roles. The blocking port role is now split into the backup and alternate port roles. The Spanning Tree Algorithm (STA) determines the role of a port based on Bridge Protocol Data Units (BPDUs). The Spanning Tree Algorithm (STA) determines the role of a port based on Bridge Protocol Data Units (BPDUs). To keep things simple, the thing to remember about a BPDU is that there is always a way of comparing any two of them and deciding whether one is more useful than the other. This is based on the value stored in the BPDU and occasionally on the port on which they are received. To keep things simple, the thing to remember about a BPDU is that there is always a way of comparing any two of them and deciding whether one is more useful than the other. This is based on the value stored in the BPDU and occasionally on the port on which they are received.

44. PVST+ Cisco developed PVST+ so that a network can run an STP instance for each VLAN in the network. Cisco developed PVST+ so that a network can run an STP instance for each VLAN in the network. With PVST+, more than one trunk can block for a VLAN and load sharing can be implemented. With PVST+, more than one trunk can block for a VLAN and load sharing can be implemented. However, implementing PVST+ means that all switches in the network are engaged in converging the network, and the switch ports have to accommodate the additional bandwidth used for each PVST+ instance to send its own BPDUs. However, implementing PVST+ means that all switches in the network are engaged in converging the network, and the switch ports have to accommodate the additional bandwidth used for each PVST+ instance to send its own BPDUs. In a Cisco PVST+ environment, you can tune the spanning-tree parameters so that half of the VLANs forward on each uplink trunk. In a Cisco PVST+ environment, you can tune the spanning-tree parameters so that half of the VLANs forward on each uplink trunk. This is accomplished by configuring one switch to be elected the root bridge for half of the total number of VLANs in the network, and a second switch to be elected the root bridge for the other half of the VLANs. This is accomplished by configuring one switch to be elected the root bridge for half of the total number of VLANs in the network, and a second switch to be elected the root bridge for the other half of the VLANs. Creating different STP root switches per VLAN creates a more redundant network. Creating different STP root switches per VLAN creates a more redundant network.

45. PVST+ Bridge ID as you recall, in the original 802.1D standard, an 8-byte BID is composed of a 2-byte bridge priority and a 6-byte MAC address of the switch. as you recall, in the original 802.1D standard, an 8-byte BID is composed of a 2-byte bridge priority and a 6-byte MAC address of the switch. There was no need to identify a VLAN because there was only one spanning tree in a network. There was no need to identify a VLAN because there was only one spanning tree in a network. PVST+ requires that a separate instance of spanning tree run for each VLAN. PVST+ requires that a separate instance of spanning tree run for each VLAN. To support PVST+, the 8-byte BID field is modified to carry a VLAN ID (VID). To support PVST+, the 8-byte BID field is modified to carry a VLAN ID (VID).

46. The following provides more details on the PVST+ fields: The following provides more details on the PVST+ fields: Bridge priority - A 4-bit field carries the bridge priority. Bridge priority - A 4-bit field carries the bridge priority. Because of the limited bit count, the priority is conveyed in discrete values in increments of 4096 rather than discreet values in increments of 1, as they would be if the full 16-bit field was available. Because of the limited bit count, the priority is conveyed in discrete values in increments of 4096 rather than discreet values in increments of 1, as they would be if the full 16-bit field was available. The default priority, in accordance with IEEE 802.1D, is 32,768, which is the midrange value. The default priority, in accordance with IEEE 802.1D, is 32,768, which is the midrange value. Extended system ID - A 12-bit field carrying the VID for PVST+. Extended system ID - A 12-bit field carrying the VID for PVST+. MAC address - A 6-byte field with the MAC address of a single switch. MAC address - A 6-byte field with the MAC address of a single switch. The MAC address is what makes a BID unique. When the priority and extended system ID are prepended to the switch MAC address, each VLAN on the switch can be represented by a unique BID. The MAC address is what makes a BID unique. When the priority and extended system ID are prepended to the switch MAC address, each VLAN on the switch can be represented by a unique BID. Caution: If no priority has been configured, every switch has the same default priority, and the election of the root bridge for each VLAN is based on the MAC address. Therefore, to ensure that you get the root bridge you want, it is advisable to assign a lower priority value to the switch that should serve as the root bridge. Caution: If no priority has been configured, every switch has the same default priority, and the election of the root bridge for each VLAN is based on the MAC address. Therefore, to ensure that you get the root bridge you want, it is advisable to assign a lower priority value to the switch that should serve as the root bridge.

47. 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. 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. The edge port concept is well known to Cisco spanning-tree users, because it corresponds to the PortFast feature in which all ports directly connected to end stations anticipate that no switch device is connected to them. The edge port concept is well known to Cisco spanning-tree users, because it corresponds to the PortFast feature in which all ports directly connected to end stations anticipate that no switch device is connected to them. The PortFast ports immediately transition to the STP forwarding state, thereby skipping the time-consuming listening and learning stages. The PortFast ports immediately transition to the STP forwarding state, thereby skipping the time-consuming listening and learning stages. Neither edge ports nor PortFast-enabled ports generate topology changes when the port transitions to a disabled or enabled status. Neither edge ports nor PortFast-enabled ports generate topology changes when the port transitions to a disabled or enabled status. Unlike PortFast, an RSTP edge port that receives a BPDU loses its edge port status immediately and becomes a normal spanning-tree port. Unlike PortFast, an RSTP edge port that receives a BPDU loses its edge port status immediately and becomes a normal spanning-tree port. The Cisco RSTP implementation maintains the PortFast keyword using the spanning-tree portfast command for edge port configuration. The Cisco RSTP implementation maintains the PortFast keyword using the spanning-tree portfast command for edge port configuration. Therefore making an overall network transition to RSTP more seamless. Therefore making an overall network transition to RSTP more seamless. Configuring an edge port to be attached to another switch can have negative implications for RSTP when it is in sync state because a temporary loop can result, possibly delaying the convergence of RSTP due to BPDU contention with loop traffic. Configuring an edge port to be attached to another switch can have negative implications for RSTP when it is in sync state because a temporary loop can result, possibly delaying the convergence of RSTP due to BPDU contention with loop traffic.

48. Link Types The link type provides a categorization for each port participating in RSTP. The link type provides a categorization for each port participating in RSTP. The link type can predetermine the active role that the port plays as it stands by for immediate transition to forwarding state if certain conditions are met. The link type can predetermine the active role that the port plays as it stands by for immediate transition to forwarding state if certain conditions are met. These conditions are different for edge ports and non-edge ports. These conditions are different for edge ports and non-edge ports. Non-edge ports are categorized into two link types, point-to-point and shared. Non-edge ports are categorized into two link types, point-to-point and shared. The link type is automatically determined, but can be overwritten with an explicit port configuration. The link type is automatically determined, but can be overwritten with an explicit port configuration. Edge ports, the equivalent of PortFast-enabled ports, and point-to-point links are candidates for rapid transition to a forwarding state. Edge ports, the equivalent of PortFast-enabled ports, and point-to-point links are candidates for rapid transition to a forwarding state. However, before the link type parameter is considered, RSTP must determine the port role. However, before the link type parameter is considered, RSTP must determine the port role. Root ports do not use the link type parameter. Root ports are able to make a rapid transition to the forwarding state as soon as the port is in sync. Root ports do not use the link type parameter. Root ports are able to make a rapid transition to the forwarding state as soon as the port is in sync. Alternate and backup ports do not use the link type parameter in most cases. Alternate and backup ports do not use the link type parameter in most cases. Designated ports make the most use of the link type parameter. Designated ports make the most use of the link type parameter. Rapid transition to the forwarding state for the designated port occurs only if the link type parameter indicates a point-to-point link. Rapid transition to the forwarding state for the designated port occurs only if the link type parameter indicates a point-to-point link.

49. RSTP Port States