L-2008-01.ppt

WIA2008 Advanced Network Technology Chapter 1 Spanning Tree Protocol (STP) Contents  Introduction  Spanning Tree Protocol  STP Operation  Evolution of STP  STP Configuration Introduction  A well-designed Layer 2 network will have redundant switches and paths to ensure that if one s...

WIA2008 Advanced Network Technology Chapter 1 Spanning Tree Protocol (STP) Contents  Introduction  Spanning Tree Protocol  STP Operation  Evolution of STP  STP Configuration Introduction  A well-designed Layer 2 network will have redundant switches and paths to ensure that if one switch goes down, another path to a different switch is available to forward data. Users of the network would not experience any disruption of service.  Redundant links can also share the traffic load and increase capacity.  Redundant devices, such as multilayer switches or routers, provide the capability for a client to use an alternate default gateway should the primary default gateway fail. Introduction  Redundancy in a hierarchical network design fixes the problem of a single point of failure, yet it can create a different kind of problem called Layer 2 loops.  Ethernet LANs require a loop-free topology with a single path between any two devices.  Spanning Tree Protocol (STP) is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology.  IEEE 802.1D is the original IEEE MAC Bridging standard for STP. Spanning Tree Protocol  Without STP enabled, Layer 2 loops can form, causing broadcast, multicast and unknown unicast frames to loop endlessly. This can bring down a network within a very short amount of time, sometimes in just a few seconds.  Unknown unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device.  When a loop occurs, the MAC address table on a switch will constantly change with the updates from the broadcast frames, which results in MAC database instability. This can cause high CPU utilization, which makes the switch unable to forward frames. Spanning Tree Protocol  A Layer 2 loop can result in three primary issues  MAC Database Instability  Broadcast Storm  Multiple Frame Transmission  MAC Database Instability  Ethernet frames do not have a time to live (TTL) attribute.  The frame continues to propagate between switches endlessly.  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. Spanning Tree Protocol  Broadcast Storm  A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed.  This can cause the end device to malfunction because of the high processing requirements for sustaining such a high traffic load on the NIC.  Multiple Frame Transmission  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. Spanning Tree Protocol Spanning Tree Protocol  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.  If the port is required, STP recalculates the paths and unblocks the necessary ports to allow the redundant path to become active.  STP recalculations can also occur any time a new switch is added to the network. Spanning Tree Protocol Spanning Tree Protocol  STP uses the Spanning Tree Algorithm (STA) to determine the blocking port.  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. Each switch has only a single path to the root bridge.  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. Spanning Tree Protocol Spanning Operation  Using the STA, STP builds a loop-free topology in a four- step process:  Elect the root bridge  Elect the root ports  Elect designated ports  Elect alternate (blocked) ports  Switches use Bridge Protocol Data Units (BPDUs) to share information about themselves and their connections.  BPDUs are used to elect the root bridge, root ports, designated ports, and alternate ports. Each BPDU contains a bridge ID (BID) that identifies which switch sent the BPDU. Spanning Operation  BID contains a priority value, the MAC address of the switch, and an extended system ID. The lowest BID value is determined by the combination of these three fields. Spanning Operation  Bridge Priority  The default priority value for all Cisco switches is the decimal value 32768. The range is 0 to 61440 in increments of 4096. A lower bridge priority is preferable. A bridge priority of 0 takes precedence over all other bridge priorities.  MAC address  When two switches are configured with the same priority and have the same extended system ID, the switch having the MAC address with the lowest value will have the lower BID. Spanning Operation  Extended System ID  The extended system ID (12-bit) value is to identify the VLAN for this BPDU. This ID allows implementations of Rapid STP (RSTP) to have different root bridges for different sets of VLANs.  This can allow for redundant, non-forwarding links in a STP topology for one set of VLANs to be used by a different set of VLANs using a different root bridge.  After a switch boots, it begins to send out BPDU frames every two seconds. The switch with the lowest BID automatically becomes the root bridge for the STA calculations. STP Operation Spanning Operation  When the root bridge has been elected for a given 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, known as the internal root path cost, is determined by the sum of all the individual port costs along the path from the switch to the root bridge.  The default port costs are defined by the speed at which the port operates. Spanning Operation  Every non-root switch will select one root port. The root port is the port closest to the root bridge in terms of overall cost (best path) to the root bridge. Spanning Tree Protocol  After each switch selects a root port, the switches will then select designated ports. The designated port is a port on the segment (with two switches) that has the internal root path cost to the root bridge. In other words, the designated port has the best path to receive traffic leading to the root bridge.  All ports on the root bridge are designated ports. Spanning Tree Protocol  Both S2 and S3 have the same path cost to the root bridge. The spanning tree algorithm will use the bridge ID as a tie breaker. S2 has a lower BID. Therefore, the F0/2 port of S2 will be chosen as the designated port. Spanning Tree Protocol  If a port is not a root port or a designated port, then it becomes an alternate (or backup) port. Alternate ports and backup ports are in discarding or blocking state to prevent loops. Spanning Tree Protocol  When a switch has multiple equal-cost paths to the root bridge, the switch will determine a port using the following criteria:  Lowest sender BID  Lowest sender port priority  The default port priority is 128  Lowest sender port ID Spanning Tree Protocol Spanning Tree Protocol  STP convergence requires three timers:  Hello Timer - The hello time is the interval between BPDUs. The default is 2 seconds but can be modified to between 1 and 10 seconds.  Forward Delay Timer -The forward delay is the time that is spent in the listening and learning state. The default is 15 seconds but can be modified to between 4 and 30 seconds.  Max Age Timer -The max age is the maximum length of time that a switch waits before attempting to change the STP topology. The default is 20 seconds but be modified to between 6 and 40 seconds. Spanning Tree Protocol Spanning Tree Protocol Evolution of STP  The varieties of spanning tree protocols include:  PVST+  This is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network.  The separate instance supports PortFast, UplinkFast, BackboneFast, BPDU guard, BPDU filter, root guard, and loop guard.  Rapid Spanning Tree Protocol (RSTP) or IEEE 802.1w  This is an evolution of STP that provides faster convergence than STP. Evolution of STP  Rapid PVST+  Rapid PVST+ provides a separate instance of 802.1w per VLAN. The separate instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.  Multiple Spanning Tree Protocol (MSTP)  Maps multiple VLANs into the same spanning tree instance.  Each instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard. Evolution of STP  RSTP increases the speed of the recalculation of the spanning tree when the Layer 2 network topology changes. RSTP can achieve much faster convergence in a properly configured network, sometimes in as little as a few hundred milliseconds.  If a port is configured to be an alternate port it can immediately change to a forwarding state without waiting for the network to converge.  There are only three port states in RSTP that correspond to the three possible operational states in STP. The 802.1D disabled, blocking, and listening states are merged into a unique 802.1w discarding state. Evolution of STP Evolution of STP Evolution of STP  When a switch powers up, the switch port goes through both the listening and learning states, each time waiting for the Forward Delay timer to expire. This delay is 15 seconds for each state, listening and learning, for a total of 30 seconds. This delay can present a problem for DHCP clients trying to discover a DHCP server.  When a switch port is configured with PortFast, that port transitions from blocking to forwarding state immediately, bypassing the usual 802.1D STP transition states (the listening and learning states) and avoiding a 30 second delay.  You can use PortFast on access ports to allow devices connected to these ports, such as DHCP clients. Evolution of STP  In a valid PortFast configuration, BPDUs should never be received on PortFast-enabled switch ports because that would indicate that another bridge or switch is connected to the port.  BPDU guard immediately puts the switch port in an errdisabled (error-disabled) state on receipt of any BPDU. This protects against potential loops by effectively shutting down the port. The BPDU guard feature provides a secure response to invalid configurations because an administrator must manually put the interface back into service. Evolution of STP Evolution of STP  STP has adapted to the added redundancy and complexity with enhancements as part of RSTP and MSTP.  Although STP will most likely continue to be used as a loop prevention mechanism in the enterprise, on access layer switches, other technologies are also being used, including the following:  Multi System Link Aggregation (MLAG)  Shortest Path Bridging (SPB)  Transparent Interconnect of Lots of Links (TRILL) STP Configuration STP Configuration  Configure Switch Port  Although switch ports have a default port cost associated with them, the port cost is configurable.  interface fa0/1  spanning-tree cost [1 - 200,000,000]  To restore the port cost back to the default value, enter the no spanning-tree cost. STP Configuration  In order to specify a root bridge, the bridge priority value must be adjusted to ensure it is lower than the bridge priority values of all the other switches on the network. STP Configuration STP Configuration STP Configuration