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Module 7

Module 7. Spanning Tree Protocol. Redundancy. Redundancy in a network is needed in case there is loss of connectivity in one segment. But redundancy in itself presents problems – loops.

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Module 7

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  1. Module 7 Spanning Tree Protocol

  2. 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. Redundancy • Companies want 100% uptime, but 99.999% (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. Reliability and 24x7 network demands have compelled LAN designers to construct multiple paths between user and resource

  5. 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. Redundant Paths and No Spanning Tree. . .

  7. Or, A Broadcast Storm. . .

  8. Broadcast Storms, like ARP requests 1 10BaseT Ports (12) Switch A A Host A Hub 00-90-27-76-96-93 A 10BaseT Ports (12) Switch B 1 2 100BaseT Ports Host B 00-90-27-76-5D-FE

  9. 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. 1 10BaseT Ports (12) Switch A A Host A Hub 00-90-27-76-96-93 A 10BaseT Ports (12) Switch B 1 2 100BaseT Ports Host B 00-90-27-76-5D-FE

  10. 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. 1 10BaseT Ports (12) Switch A A Duplicate frame Host A Duplicate frame Hub 00-90-27-76-96-93 A 10BaseT Ports (12) Switch A 1 2 100BaseT Ports Host B 00-90-27-76-5D-FE

  11. 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! 10BaseT Ports (12) Switch A A Host A Duplicate Frame Hub Duplicate Frame 00-90-27-76-96-93 A 10BaseT Ports (12) Switch B 1 2 100BaseT Ports Host B 00-90-27-76-5D-FE

  12. 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. 10BaseT Ports (12) Switch A A Host A Hub 00-90-27-76-96-93 A 10BaseT Ports (12) Switch B 1 2 Host B 00-90-27-76-5D-FE

  13. 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

  14. Spanning Tree Protocol

  15. Spanning Tree Protocol 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

  16. B C H E J I D F G Root Bridge A Server Server Root Bridge = Forwarding Path = Backup Link The Spanning-Tree Protocol specifies an algorithm (Spanning-Tree Algorithm) that ultimately creates a logical loop-free topology

  17. Spanning Tree Algorithm • 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.

  18. 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

  19. 3 Steps to Spanning Tree • 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

  20. 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)

  21. 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 32768.

  22. Bridge Priorities A 1 A B 10BaseT Ports (12) 100BaseT Ports B A B 10BaseT Ports (24) 100BaseT Ports C A B 1 100BaseT Ports 10BaseT Ports (24)

  23. Switch A: Bridge Priority

  24. 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.

  25. Bridge Priorities and Bridge Ids Which one is the lowest? A 1 A B Priority: 32768 ID: 00-B0-64-26-6D-00 10BaseT Ports (12) 100BaseT Ports B A B Priority: 32768 ID: 00-B0-64-58-CB-80 10BaseT Ports (24) 100BaseT Ports C A B Priority: 32768 ID: 00-B0-64-58-DC-00 1 10BaseT Ports (24)

  26. Lowest: A becomes the root bridge A 1 Priority: 32768 ID: 00-B0-64-26-6D-00 A B 10BaseT Ports (12) 100BaseT Ports B Priority: 32768 ID: 00-B0-64-58-CB-80 A B 10BaseT Ports (24) C Priority: 32768 ID: 00-B0-64-58-DC-00 1 A B 10BaseT Ports (24)

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

  28. Understanding STP States • 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 50 seconds from blocking to forwarding

  29. 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

  30. Module 7 Spanning Tree Protocol

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