Objectives. The Role of Redundancy in a Switched Network. Layer 2 Loops. Broadcast Storms. More problems with Layer 2 loops

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1 ITE I Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 1 Objectives Implement Spanning Tree Protocols LAN Switching and Wireless Chapter 5 Explain the role of redundancy in a converged network Summarize how STP works to eliminate Layer 2 loops in a converged network Explain how the STP algorithm uses three steps to converge on a loop-free topology Implement rapid per VLAN spanning tree (rapid PVST+) in a LAN to prevent loops between redundant switches. 2 The Role of Redundancy in a Switched Network Need to eliminate single points of failure in the LAN, as far as possible Redundant links connecting switches in the three layers provides more than one path between hosts Layer 2 Loops Redundant links between switches results in Layer 2 loops Ethernet frames do not have a time-to-live (TTL) like IP packets 3 4 Broadcast Storms A switch will flood a frame out of all ports except the one it arrived on, if - the frame is a broadcast - the destination MAC address is not in the switch table In a loop, broadcasts will - loop endlessly - increase in number exponentially - bring down the LAN (usually within seconds) A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. More problems with Layer 2 loops Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device. - some network protocols cannot handle duplicate frames Loops can cause MAC address tables to become unstable Loops result in high CPU load on all switches caught in the loop. Network loops that are a result of accidental duplicate connections in the wiring closets are a common occurrence. 5 6

2 Spanning Tree Protocol (STP) STP prevents Layer 2 loops. STP runs on all Layer 2 switches and bridges by default straight out of the box 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 Switches and bridges running STP cooperate to produce a logical loop-free layer 2 topology If a link goes down, the STP algorithm will automatically be run again to determine a new spanning tree topology. Hence, redundancy is maintained. The Spanning Tree Topology Nodes are connected as a tree if - each node has one and only one parent node - except for the root node Change a general structure into a tree structure by removing some of the links A spanning tree means all nodes remain connected A tree structure will not contain any loops Root node 7 8 STP in action STP puts port F0/2 on S3 into blocking mode, thus removing a loop Now there is only one path between any source and destination device BPDUs Switches running STP need to cooperate with each other They use Bridge Protocol Data Units (BPDUs) to exchange messages An Ethernet frame encapsulates the BPDU Uses an Ethernet multicast address for the spanningtree group Each BPDU contains a BID number that identifies the switch that sent the BPDU 9 10 The BPDU Fields Bridge ID (BID) Each switch has a unique Bridge ID number (BID) made up of: Bridge priority 16-bit customizable, default MAC address guarantees uniqueness The BID identifies the switch and is used in elections. The admin can determine the outcome of an election by setting the value of the Bridge Priority field 11 12

3 STP Convergence Steps When bootup has completed, each switch determines the logical spanning tree topology by running the Spanning Tree Algorithm (STA) The STP Algorithm uses three steps to converge on a loop-free topology: Step 1: Elect a Root Bridge Step 2: Elect the Root Ports Step 3: Elect the Designated and Non-Designated ports Step 1: Elect a Root Bridge The first step is to elect a Root Bridge. The switch with the lowest BID wins. All switches continuously transmit BPDUs out of all ports Each BPDU contains the BID of the sender and the BID of the current Root Bridge Each switch initially assumes that it is the Root Bridge When a BPDU is received with a lower Root Bridge BID, it replaces the current one in future BPDU transmissions. After no more than 20 seconds, a single Root Bridge will have been identified by all switches. 15 Port Roles Each switch port connecting to another switch (i.e. that receives BPDUs) is assigned one of the following port roles by the STA: Root port This is the port closest to the Root Bridge This port will forward frames Designated port If two or more switches connect to the same segment, only one will be the designated port This port will forward frames Non-designated port The port is in blocking mode; it will not forward user frames 16 Step 2: Elect Root Ports All of the Root Bridge ports become Designated Ports A Root Port exists on each non-root bridge. It is the switch port with the best path to the Root Bridge Choosing the Root Port: - The Root Bridge continues to send BPDUs which are relayed through the network by the other switches - As each BPDU is sent out of a port, the cost field is updated, in accordance with the port bandwidth - As a switch receives BPDUs from the Root Bridge, the one with the lowest cost identifies the Root Port - Each switch will have one and only one Root Port 17 18

4 Step 3: Elect Designated and Non-Designated Ports The remaining ports on a switch which connect to other switches will be either Designated Ports or Nondesignated Ports If two switches connect to the same segment, the port on the switch with the lowest BID becomes a Designated Port. The port on the other switch becomes a Non-designated Port. Designated Ports will forward user frames. Non-designated Ports will be blocked Result is each segment will have only one switch forwarding frames onto it Port States While the STA is running, each switch port will be in one of these port states: Blocking - A non-designated port; does not participate in frame forwarding. Receives BPDU frames only Listening - STP has determined that the port can participate in frame forwarding. Receives BPDU frames and also transmits its own BPDU frames Learning - The port prepares to participate in frame forwarding and begins to populate the MAC address table. Forwarding - The port is part of the active topology and forwards frames, sends and receives BPDU frames. Disabled - The switch port is administratively disabled Port States The spanning tree is determined immediately after a switch has finished booting up. All ports are initially put in Blocking mode (LEDs on switch port will be amber) When the STA has completed, each port will be in either Blocking mode or Forwarding mode A port that becomes part of the final spanning tree topology will transition between modes in this order: 1. Blocking 2. Listening 3. Learning 4. Forwarding 23 BPDU Timers Network diameter is the number of devices that a packet has to cross before it reaches its destination. Default convergence times are based on a seven-switch diameter network Hello time Default 2 secs. Forward delay time spent in listening and learning state. Default 15 secs each. Maximum age Default 20 secs. Optimize timers by reconfiguring the network diameter, not the BPDU timers. On root bridge only: spanning-tree vlan vlan-id root primary diameter value 24

5 Summary Spanning Tree Protocol (STP) STP Variants Standard IEEE 802.1D STP Only one spanning tree instance in a network (i.e. broadcast domain). BID is 2-byte bridge priority + MAC address. Only one Root Bridge elected in the network. Each non-root switch has one Root Port shortest path to the Root Bridge Each segment is connected by no more than one Designated Port All other ports on a switch are non-designated ports and are in blocking mode. Convergence time is 50 seconds 25 STP s lengthy convergence time (50 seconds) facilitated the development of: RSTP IEEE standard (IEEE 802.1w) convergence time is slightly over 6 seconds Rapid PVST+ Cisco proprietary technology This is the preferred STP on a Cisco switched network Adds VLAN support to RSTP Separate Root Bridge for each instance means better redundancy. Can load balance VLANs on trunks 26 PVST+ Feature: PortFast PVST+: Bridge ID (BID) PortFast is a Cisco proprietary technology. When an access switch port is configured with PortFast it transitions from blocking to forwarding state immediately. Use only on access ports connected to a single workstation,etc. to allow those devices to connect to the network immediately. If a port configured with PortFast receives a BPDU frame, spanning tree can put the port into the blocking state using a feature called BPDU guard. PortFast technology can be used to support DHCP. 27 The Bridge ID number (BID) is made up of three fields: Bridge priority 4-bit customizable Extended System ID 12-bit VLAN ID number MAC address guarantees uniqueness The first two fields are displayed as a single number. Therefore changing the Bridge Priority changes the number in steps of 4096 Default is i.e VLAN 1 28 RSTP (IEEE 802.1w) RSTP - speeds the recalculation of the spanning tree when the Layer 2 network topology changes. RSTP supersedes STP (802.1D) while retaining backward compatibility. RSTP keeps the same BPDU format with version set to 2. Edge port - corresponds to the Cisco PortFast feature Non-edge ports are categorized into two link types, point-to-point and shared. Possible RSTP port states: discarding, learning, and forwarding 29