LAN Technologies - Spanning Tree Protocol (STP) PDF

Chapter 1: LAN Technologies 1. 0 L A N T E C H N O LO G I ES 1.2.4 STP 1.2.5 ETHERCHANNEL Spanning Tree Protocol (STP) Purpose of Spanning Tree Redundancy at OSI Layers 1 and 2 Redundancy is an important part of the hierarchical design for preventing disruption of network services to users. Redu...

Chapter 1: LAN Technologies 1. 0 L A N T E C H N O LO G I ES 1.2.4 STP 1.2.5 ETHERCHANNEL Spanning Tree Protocol (STP) Purpose of Spanning Tree Redundancy at OSI Layers 1 and 2 Redundancy is an important part of the hierarchical design for preventing disruption of network services to users. Redundant networks require the addition of physical paths, but logical redundancy must also be part of the design. However, redundant paths in a switched Ethernet network may cause both physical and logical Layer 2 loops. Purpose of Spanning Tree Redundancy at OSI Layers 1 and 2 Multiple cabled paths between switches: Provide physical redundancy in a switched network. Improves the reliability and availability of the network. Enables users to access network resources, despite path disruption. Purpose of Spanning Tree Issues with Layer 1 Redundancy: MAC Database Instability Ethernet frames do not have a time to live (TTL) attribute. Frames continue to propagate between switches endlessly, or until a link is disrupted and breaks the loop. Results in MAC database instability. Can occur due to broadcast frames forwarding. If there is more than one path for the frame to be forwarded out, an endless loop can result. 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. Purpose of Spanning Tree Issues with Layer 1 Redundancy: 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. It is also known as denial of service A broadcast storm is inevitable on a looped network. As more devices send broadcasts over the network, more traffic is caught within the loop; thus consuming more resources. This eventually creates a broadcast storm that causes the network to fail. Purpose of Spanning Tree Issues with Layer 1 Redundancy: Broadcast Storms Purpose of Spanning Tree Issues with Layer 1 Redundancy: Duplicate Unicast Frames 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. Layer 2 LAN protocols, such as Ethernet, lack a mechanism to recognize and eliminate endlessly looping frames. Purpose of Spanning Tree Issues with Layer 1 Redundancy: Duplicate Unicast Frames  STP Operation Spanning Tree Algorithm: Introduction 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. This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops. 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. STP Operation Spanning Tree Algorithm: Introduction STP Operation Spanning Tree Algorithm: Introduction STP Operation Spanning Tree Algorithm: Introduction  STP Operation Spanning Tree Algorithm: Introduction STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed "blocking-state" ports. The switches running STP are able to compensate for failures by dynamically unblocking the previously blocked ports and permitting traffic to traverse the alternate paths.  STP Operation Spanning Tree Algorithm: Introduction Spanning Tree Algorithm (STA) used to determine which switch ports on a network must be put in blocking state 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 In the figure, the root bridge (switch S1) is chosen through an election process. The switch with the lowest BID automatically becomes the root bridge for the STA calculations.  STP Operation Spanning Tree Algorithm: Introduction How does the STA create a loop-free topology? Selecting a Root Bridge: This bridge (switch) is the reference point for the entire network to build a spanning tree around. Block Redundant Paths: 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. When a port is blocked, user data is prevented from entering or leaving that port. Create a Loop-Free Topology: A blocked port has the effect of making that link a non- forwarding link between the two switches. This creates a topology where each switch has only a single path to the root bridge, similar to branches on a tree that connect to the root of the tree. Recalculate in case of Link Failure: 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. STP recalculations can also occur any time a new switch or new inter-switch link is added to the network. STP Operation Spanning Tree Algorithm: Introduction  STP Operation Spanning Tree Algorithm: Introduction Root ports - Switch ports closest to the root bridge. Root ports are selected on a per-switch basis. Designated ports - All non-root ports that are still permitted to forward traffic on the network. Designated ports are selected on a per-trunk basis. If one end of a trunk is a root port, then the other end is a designated port. All ports on the root bridge are designated ports. Alternate and backup ports - Alternate ports and backup ports are configured to be in a blocking state to prevent loops. Alternate ports are selected only on trunk links where neither end is a root port. Disabled ports - A disabled port is a switch port that is shut down. STP Operation Spanning Tree Algorithm: Root Bridge STP Operation Spanning Tree Algorithm: Path Cost STP Operation BPDU Propagation and Process STP Operation BPDU Propagation and Process  Overview List of Spanning Tree Protocols STP or IEEE 802.1D-1998 PVST+ IEEE 802.1D-2004 Rapid Spanning Tree Protocol (RSTP) or IEEE 802.1w Rapid PVST+ Multiple Spanning Tree Protocol (MSTP) or IEEE 802.1s  Overview List of Spanning Tree Protocols STP - This is the original IEEE 802.1D version (802.1D-1998 and earlier) that provides a loop-free topology in a network with redundant links. 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. 802.1D-2004 - This is an updated version of the STP standard, incorporating IEEE 802.1w.  Overview List of Spanning Tree Protocols Rapid Spanning Tree Protocol (RSTP) or IEEE 802.1w - This is an evolution of STP that provides faster convergence than STP. Rapid PVST+ - This is a Cisco enhancement of RSTP that uses PVST+. Rapid PVST+ provides a separate instance of 802.1w per VLAN. Supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard. Multiple Spanning Tree Protocol (MSTP) - This is an IEEE standard inspired by the earlier Cisco proprietary Multiple Instance STP (MISTP) implementation. MSTP maps multiple VLANs into the same spanning tree instance. STP Overview Characteristics of the Spanning Tree Protocols STP Configuration Issues Analyzing the STP Topology EtherChannel Link Aggregation Introduction to Link Aggregation Link aggregation allows the creation of logical links made up of several physical links. EtherChannel is a form of link aggregation used in switched networks. Link Aggregation Advantages of EtherChannel Most configurations are done on the EtherChannel interface ensuring consistency throughout links. Relies on existing switch ports – no need for upgrades. Load-balances between links on the same EtherChannnel. Creates an aggregation viewed as one logical link by STP. Provides redundancy because the overall link is viewed as one logical connection. If one physical link within channel goes down, this does not cause a change in the topology and does not require STP recalculation. EtherChannel Operation Implementation Restrictions EtherChannel implemented by grouping multiple physical ports into one or more logical EtherChannel links. Interface types cannot be mixed. EtherChannel provides full-duplex bandwidth up to 800 Mb/s (Fast EtherChannel) or 8 Gb/s (Gigabit EtherChannel). EtherChannel can consist of up to 16 compatibly-configured Ethernet ports. The Cisco IOS switch currently supports six EtherChannels. EtherChannel Operation Port Aggregation Protocol (PAgP) EtherChannels can be formed through negotiation using one of two protocols, PAgP or LACP. PAgP is a Cisco-proprietary protocol that aids in the automatic creation of EtherChannel links. When an EtherChannel link is configured using PAgP, PAgP packets are sent between EtherChannel- capable ports to negotiate the forming of a channel. When PAgP identifies matched Ethernet links, it groups the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single port. EtherChannel Operation Port Aggregation Protocol (PAgP) PAgP packets are sent every 30 seconds. PAgP checks for configuration consistency and manages link additions and failures between two switches PAgP helps create the EtherChannel link by detecting the configuration of each side and ensuring that links are compatible so that the EtherChannel link can be enabled when needed.. EtherChannel Operation Port Aggregation Protocol (PAgP On - This mode forces the interface to channel without PAgP. Interfaces configured in the on mode do not exchange PAgP packets. PAgP desirable - This PAgP mode places an interface in an active negotiating state in which the interface initiates negotiations with other interfaces by sending PAgP packets. PAgP auto - This PAgP mode places an interface in a passive negotiating state in which the interface responds to the PAgP packets that it receives, but does not initiate PAgP negotiation. EtherChannel Operation Port Aggregation Protocol (PAgP) EtherChannel Operation Link Aggregation Control Protocol (LACP) LACP LACP is part of an IEEE specification (802.3ad) that allows several physical ports to be bundled to form a single logical channel. It performs a function similar to PAgP with Cisco EtherChannel. Because LACP is an IEEE standard, it can be used to facilitate EtherChannels in multivendor environments. On Cisco devices, both protocols are supported. EtherChannel Operation Link Aggregation Control Protocol (LACP) LACP provides the same negotiation benefits as PAgP. LACP helps create the EtherChannel link by detecting the configuration of each side and making sure that they are compatible so that the EtherChannel link can be enabled when needed. On - This mode forces the interface to channel without LACP. Interfaces configured in the on mode do not exchange LACP packets. LACP active - This LACP mode places a port in an active negotiating state. In this state, the port initiates negotiations with other ports by sending LACP packets. LACP passive - This LACP mode places a port in a passive negotiating state. In this state, the port responds to the LACP packets that it receives, but does not initiate LACP packet negotiation. EtherChannel Operation Link Aggregation Control Protocol (LACP)