1 Version 3.0 Module 7 Spanning Tree Protocol

2 Version 3.0 Redundancy Redundancy in a network is needed in case there is loss of connectivity in one segment. But redundancy in itself presents problems – loops. The Spanning-Tree Protocol is used in switched networks to create a loop free logical topology from a physical topology that has loops. Links, ports, and switches that are not part of the active loop free topology do not participate in the forwarding of data frames.

3 Version 3.0 Redundancy Companies want 100% uptime, but % (5 nines) is the goal. Remember the goal is reliability without faults. Fault tolerance is achieved by redundancy. Example of having 1 car versus 2 cars – 1 is always available – redundancy So companies should: –eliminate single points of failure and –design alternate routes to a destination

4 Version 3.0 Reliability and 24x7 network demands have compelled LAN designers to construct multiple paths between user and resource

5 Version 3.0 Redundant Switched Topologies Again, if one path fails, the other path or device can take over. This is good, but there is a downside that has to be accounted for: –Broadcast storms –Multiple (or duplicate) frame copies –MAC address table instabilities

6 Version 3.0 Redundant Paths and No Spanning Tree...

10 Version 3.0 Or, A Broadcast Storm...

11 Version 3.0 Broadcast Storms, like ARP requests 10BaseT Ports (12) 100BaseT Ports A Switch A Switch B Host A A D-FE Hub Host B

12 Version BaseT Ports (12) 100BaseT Ports A Switch A Switch B Host A A D-FE Hub Because it is a Layer 2 broadcast frame, both switches, Switch A and Switch B, flood the frame out all ports, including their port A’s. Host B

13 Version BaseT Ports (12) 100BaseT Ports A Switch A Host A A D-FE Hub Duplicate frame Duplicate frame Both switches receive the same broadcast, but on a different port. Doing what switches do, both switches flood the duplicate broadcast frame out their other ports. Host B

14 Version BaseT Ports (12) 100BaseT Ports A Switch A Switch B A D-FE Hub Duplicate Frame Duplicate Frame Here we go again, with the switches flooding the same broadcast again out its other ports. This results in duplicate frames, known as a broadcast storm! Host A Host B

15 Version BaseT Ports (12) A Switch A Switch B A D-FE Hub Layer 2 broadcasts not only take up network bandwidth, but must be processed by each host. This can severely impact a network, to the point of making it unusable. Host A Host B

16 Version 3.0 Redundant Topology The traffic that switches flood out all ports can be caught in a loop, because in the Layer 2 header there is no TTL. (Remember that in Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0) You need switching (bridging) for reliability, but now the problem of loops – a switched network cannot have loops if it is to do what it is supposed to do. Solution? Allow physical loops, but create a loop- free topology

17 Version 3.0 Spanning Tree Protocol

18 Version 3.0 Broadcast Frame Standby Link Switches forward broadcast frames Prevents loops Loops can cause broadcast storms and duplicate frames Allows redundant links Prunes topology to a minimal spanning tree Resilient to topology changes and device failures Main function of the Spanning Tree Protocol (STP) is to allow redundant switched/bridged paths without suffering the effects of loops in the network Spanning Tree Protocol

19 Version 3.0 Root Bridge Root Bridge Server = Backup Link = Forwarding Path The Spanning-Tree Protocol specifies an algorithm (Spanning- Tree Algorithm) that ultimately creates a logical loop-free topology A BCHJIEGFD

20 Version 3.0 The STA is used to calculate a loop-free logical topology. Spanning-tree frames called bridge protocol data units (BPDUs) are sent and received by all switches in the network at regular intervals and are used to determine the spanning tree topology. These BPDUs are used to determine the shortest path to the root bridge, and which ports will forward frames as part of the spanning tree – BPDUs sent out every 2 seconds A separate instance of STP runs within each configured VLAN. Spanning Tree Algorithm

21 Version 3.0 Spanning Tree For every switched network: One root bridge One root port per non root bridge One designated port per segment Unused, non- designated ports

22 Version 3.0 Step 1: Electing a Root Bridge Bridge Priority Bridge ID Root Bridge Step 2: Electing Root Ports Path Cost or Port Cost Root Path Cost Root Port Step 3: Electing Designated Ports Path Cost or Port Cost Root Path Cost 3 Steps to Spanning Tree

23 Version 3.0 Step 1: Electing a Root Bridge The first step is for switches to select a Root Bridge. The root bridge is the bridge from which all other paths are decided. Only one switch can be the root bridge. Election of a root bridge is decided by: 1. Lowest Bridge Priority 2. Lowest Bridge ID (tie-breaker)

24 Version 3.0 Bridge Priority This is a numerical value. The switch with the with the lowest bridge priority is the root bridge. The switches use BPDU’s to accomplish this. All switches consider themselves as the root bridge until they find out otherwise. All Cisco Catalyst switches have the default Bridge priority of

25 Version 3.0 A B 1 1 A B C 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Bridge Priorities

26 Version 3.0 Switch A: Bridge Priority

27 Version 3.0 In case of a tie, the Bridge ID is used… Bridge ID The Bridge ID is the MAC address assigned to the individual switch. The lower Bridge ID (MAC address) is the tiebreaker. Because MAC addresses are unique, this ensures that only one bridge will have the lowest value. NOTE: There are other tie breakers, if these values are not unique, but we will not cover those situations.

28 Version 3.0

29 Version 3.0 A B 1 1 A B C 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 Bridge Priorities and Bridge Ids Which one is the lowest?

30 Version 3.0 A B 1 1 A B C 10BaseT Ports (12) 10BaseT Ports (24) 100BaseT Ports Priority: ID: 00-B D-00 Priority: ID: 00-B CB-80 Priority: ID: 00-B DC-00 Lowest: A becomes the root bridge A B

31 Version 3.0 States initially set, later modified by STP Server ports can be configured to immediately enter STP forward mode Understanding STP States Blocking Listening Learning Forwarding Disabled

32 Version 3.0  Blocking - No frames forwarded, BPDUs received  Listening - No frames forwarded, listening for frames  Learning - No frames forwarded, but learning MAC addresses  Forwarding – Receiving BPDUs, Forwarding data traffic, receiving data traffic, learns MAC addresses  Disabled - No frames forwarded, no BPDUs heard Understanding STP States 50 seconds from blocking to forwarding

33 Version 3.0 Rapid Spanning Tree Protocol IEEE 802.1w Will eventually replace 802.1d Port states and roles will be clarified A set of link types will be defined that will allow going to a forwarding stage quicker All switches will generate their own BPDUs instead of relying on the root bridge. Link types would be: –Point to point –Edge-type –Shared Can go to forward state immediately

34 Version 3.0 Module 7 Spanning Tree Protocol