Industrial Connectivity Introducing Spanning Tree Outline • Overview • Redundant Switched and Bridged Topologies • Broadcast Storms • Multiple Frame Copies • MAC Database Instability • Summary PROPRIETARY 1 Redundant Topology • Redundant topology eliminates single points of failure. • Redundant topology causes broadcast storms, multiple frame copies and MAC address table instability problems. PROPRIETARY 2 Broadcast Storms • Host X sends a broadcast. • Switches continue to propagate broadcast traffic over and over. PROPRIETARY 3 Multiple Frame Copies • Host X sends a unicast frame to router Y. • The MAC address of router Y has not been learned by either switch. • Router Y will receive two copies of the same frame. PROPRIETARY 4 MAC Database Instability • Host X sends a unicast frame to router Y. • The MAC address of router Y has not been learned by either switch. • Switches A and B learn the MAC address of host X on port 0. • The frame to router Y is flooded. • Switches A and B incorrectly learn the MAC address of host X on port 1. PROPRIETARY 5 Summary • Bridged and switched networks are commonly designed with redundant links and devices, which can introduce problems, such as broadcast storms, multiple frame transmission, and MAC database instability. • A broadcast storm is created when each switch on a redundant network floods broadcast frames endlessly. • Multiple frame transmissions occur when multiple copies of the same frame arrive at the intended host, potentially causing problems with the receiving protocol. • MAC database instability occurs when multiple copies of a frame arrive on different ports of a switch. PROPRIETARY 6 Outline • Overview • Spanning Tree Protocol • Spanning Tree Operation • Root Bridge Selection • Spanning Tree Port States • Spanning Tree Path Costs • Spanning Tree Recalculation • Rapid Spanning Tree Protocol • Summary PROPRIETARY 7 Spanning Tree Protocol Provides a loop-free redundant network topology by placing certain ports in the blocking state PROPRIETARY 8 Spanning Tree Operation • One root bridge per network • One root port per nonroot bridge • One designated port per segment • Nondesignated ports are unused PROPRIETARY 9 Root Bridge Selection • BPDU = Bridge Protocol Data Unit (default = sent every two seconds) • Root bridge = Bridge with the lowest bridge ID • Bridge ID = In this example, which switch has the lowest bridge ID? PROPRIETARY 10 Spanning Tree Port States Spanning tree transits each port through several different states: PROPRIETARY 11 Spanning Tree Port States PROPRIETARY 12 Spanning Tree Operation PROPRIETARY 13 Spanning Tree Path Costs Data Rate STP Cost (802.1t-2001) 4 Mbit/s 5,000,000 10 Mbit/s 2,000,000 16 Mbit/s 1,250,000 100 Mbit/s 200,000 1 Gbit/s 20,000 10 Gbit/s 2,000 PROPRIETARY 14 Spanning Tree Recalculation PROPRIETARY 15 Spanning Tree Convergence • Convergence occurs when all the switch and bridge ports have transitioned to either the forwarding or the blocking state. • When the network topology changes, switches and bridges must recompute STP, which disrupts user traffic. PROPRIETARY Rapid Spanning Tree Protocol PROPRIETARY 17 Rapid Transition to Forwarding PROPRIETARY 18 Summary • STP is a bridge-to-bridge protocol used to maintain a loop-free network. • To maintain a loop-free network topology , STP establishes a root bridge, a root port, and designated ports. • With STP, the root bridge has the lowest BID, which is made up of the bridge priority and the MAC address. PROPRIETARY 19 Summary • When STP is enabled, every bridge in the network goes through the blocking state and the transitory states of listening and learning at power up. If properly configured, the ports then stabilize to the forwarding or blocking state. • If the network topology changes, STP maintains connectivity by transitioning some blocked ports to the forwarding state. • RSTP significantly speeds the recalculation of the spanning tree when the network topology changes. PROPRIETARY 20 Sixnet, LLC 331 Ushers Road Ballston Lake, NY 12019 T +1 518 877 5173 F +1 518 877 8346 [email protected] www.sixnet.com