CCNA3-1 Chapter 5-1 Chapter 5 Chapter 5 Spanning Tree Protocol Spanning Tree Protocol (STP) (STP) Part I Part I CCNA3-2 Chapter 5-1 Spanning Tree Protocol (STP) Spanning Tree Protocol (STP) Redundant Layer 2 Topologies Redundant Layer 2 Topologies CCNA3-3 Chapter 5-1 Redundant Layer 2 Topologies Redundant Layer 2 Topologies • As businesses become increasingly dependent on the As businesses become increasingly dependent on the network, the availability of the network infrastructure network, the availability of the network infrastructure becomes a critical business concern. becomes a critical business concern. • Redundancy Redundancy is the solution for achieving the necessary is the solution for achieving the necessary availability. availability. • Layer 2 redundancy improves the availability of the Layer 2 redundancy improves the availability of the network by implementing network by implementing alternate network paths alternate network paths by by adding equipment and cabling. adding equipment and cabling. • Having multiple paths for data to traverse the network Having multiple paths for data to traverse the network allows for a single path to be disrupted allows for a single path to be disrupted without impacting without impacting the connectivity of devices on the network. the connectivity of devices on the network. CCNA3-4 Chapter 5-1 Redundancy Redundancy CCNA3-5 Chapter 5-1 Redundancy Redundancy Redundant paths create Redundant paths create loops loops in the network. in the network. Redundant paths create Redundant paths create loops loops in the network. in the network. How are they controlled? How are they controlled? Spanning Tree Protocol Spanning Tree Protocol How are they controlled? How are they controlled? Spanning Tree Protocol Spanning Tree Protocol CCNA3-6 Chapter 5-1 Redundancy Redundancy • The Spanning Tree Protocol The Spanning Tree Protocol (STP) (STP) is enabled on all is enabled on all switches. switches. • STP has placed some switch ports in STP has placed some switch ports in forwarding forwarding state and state and other switch ports in other switch ports in blocking blocking state. state. Forward Forward Forward Forward Blocked Blocked Blocked Blocked CCNA3-7 Chapter 5-1 Issues with Redundancy Issues with Redundancy • Redundancy Redundancy is an important part of the hierarchical design. is an important part of the hierarchical design. • When multiple paths exist between two devices on the When multiple paths exist between two devices on the network and network and STP has been disabled STP has been disabled on those switches, a on those switches, a Layer 2 loop Layer 2 loop can occur. can occur. • If If STP is enabled STP is enabled on these switches, on these switches, which is the default which is the default , , a a Layer 2 loop would not occur Layer 2 loop would not occur . . CCNA3-8 Chapter 5-1 Issues with Redundancy Issues with Redundancy • Ethernet frames do not have a Ethernet frames do not have a Time-To-Live (TTL) Time-To-Live (TTL) parameter parameter like IP packets. like IP packets. • As a result, if they are not terminated properly on a As a result, if they are not terminated properly on a switched network, they continue to bounce from switch to switched network, they continue to bounce from switch to switch endlessly. switch endlessly. CCNA3-9 Chapter 5-1 Issues with Redundancy Issues with Redundancy • Remember that switches use the Remember that switches use the Source MAC address Source MAC address to to learn where the devices are and learn where the devices are and enters this information enters this information into into their MAC address tables. their MAC address tables. • Switches will Switches will flood the frames for unknown destinations flood the frames for unknown destinations until until they learn the MAC addresses of the devices. they learn the MAC addresses of the devices. CCNA3-10 Chapter 5-1 Issues with Redundancy Issues with Redundancy • Additionally, Additionally, multicasts and broadcasts multicasts and broadcasts are also flooded out are also flooded out all ports except the receiving port. all ports except the receiving port. (Multicasts will not be (Multicasts will not be flooded if the switch has been specifically configured to flooded if the switch has been specifically configured to handle multicasts.) handle multicasts.) [...]... • In other words, each switch considers itself as the root bridge when it boots CCNA3-22 Chapter 5-1 Root Bridge • Election Process: • As the switches forward their BPDU frames, 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... Priority: • A customizable value that you can use to influence which switch becomes the root bridge (Another rigged election!) • The switch with the lowest priority, which means lowest BID, becomes the root bridge • The lower the priority value, the higher the priority CCNA3-35 Chapter 5-1 Bridge ID • Bridge Priority: • Notice that the addition of the VLAN ID leaves fewer bits available for the bridge... Bridge-ID (BID) Bridge Priority 2 CCNA3-21 MAC Address 6 Chapter 5-1 Root Bridge • Election Process: • All switches in the broadcast domain participate • After a switch boots, it sends out Bridge Protocol Data Units (BPDU) frames containing the switch BID and the root ID every 2 seconds • The root ID identifies the root bridge on the network • By default, the root ID matches the local BID for all switches... Protocol Data Unit • STP determines a root bridge for the spanning-tree instance by exchanging Bridge Protocol Data Units (BPDU) CCNA3-28 Chapter 5-1 BPDU Process • Root Bridge Election Process: CCNA3-29 S3 believes S2 is the root bridge S3 believes S2 is the root bridge S1 still thinks it is the root bridge S1 still thinks it is the root bridge Chapter 5-1 BPDU Process • Root Bridge Election Process:... election process begins again process begins again Chapter 5-1 Bridge ID Early STP implementation – no VLANs Early STP implementation – no VLANs That means that there is a separate That means that there is a separate instance of STP for each VLAN instance of STP for each VLAN CCNA3-33 Changed to include VLAN ID Changed to include VLAN ID Chapter 5-1 Bridge ID CCNA3-34 Chapter 5-1 Bridge ID • Bridge Priority:... manually configured to specify that a specific path is the preferred path instead of allowing the STA to choose the best path • Realize, however, that changing the cost of a particular path will affect the results of the STA • The ‘no’ form of the following command will return the cost to its default value switch(config)#interface fa0/1 switch(config-if)#spanning-tree cost [value] switch(config-if)#end... 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 default port costs are specified by the IEEE and defined by the speed at which the port operates Link Speed 10Gbps 2 1Gbps 4 100Mbps 19 10Mbps CCNA3-24 Cost 100 Chapter 5-1 Best Path • You are not restricted to the defaults... in a blocking state to prevent loops CCNA3-20 Chapter 5-1 Spanning-Tree Algorithm (STA) • STP uses the Spanning Tree Algorithm (STA) to determine which switch ports on a network need to be configured for blocking to prevent loops • Through an election process, the algorithm designates a single switch as the root bridge and uses it as the reference point for all calculations • The election process is... Verifying the port and path cost Port Cost Port Cost Path Cost Path Cost CCNA3-26 Chapter 5-1 STP Bridge Protocol Data Unit • STP determines a root bridge for the spanning-tree instance by exchanging Bridge Protocol Data Units (BPDU) Identifies the root Identifies the root bridge and the bridge and the cost of the path to cost of the path to the root bridge the root bridge CCNA3-27 Chapter 5-1 STP Bridge... Affects all of the traffic on S1 traffic on S1 CCNA3-15 Chapter 5-1 Spanning Tree Protocol (STP) Introduction to STP CCNA3-16 Chapter 5-1 Introduction to STP • Redundancy: • Increases the availability of the network topology by protecting the network from a single point of failure • In a Layer 2 design, loops and duplicate frames can occur, having severe consequences • The Spanning Tree Protocol (STP) . achieving the necessary availability. availability. • Layer 2 redundancy improves the availability of the Layer 2 redundancy improves the availability. disrupted allows for a single path to be disrupted without impacting without impacting the connectivity of devices on the network. the connectivity