1. 1 12/18/2014 15:21 Chapter 12Bridges1 Rivier College CS575: Advanced LANs Chapter 12: Bridges

2. 2 12/18/2014 15:21 Chapter 12Bridges2 Overview 0 Bridge Operational Functions & Protocol Architecture 0 Routing with Bridges 0 Spanning Tree Routing: * Frame Forwarding * Address Learning * Spanning Tree Algorithm 0 Source Routing: * Routing Directives and Addressing Modes * Routing Discovery and Selection, Frame Format * Spanning Tree versus Source Routing 0 Traffic Classes and Quality of Service: * The Use of Traffic Classes * Mapping of User Priority to Traffic Class * Internet Traffic Quality of Service

3. 3 12/18/2014 15:21 Chapter 12Bridges3 0 For interconnection of LANs that use identical protocols at the MAC layer (IEEE 802.3, FDDI) 0 Few reasons for using the bridges: -Reliability (the network can be partitioned into self-contained units) -Improve performance -Improve security of communications by allowing different types of traffic (e.g., accounting, personnel, strategic planning) and different types of users with different levels of security -Establish controlled and monitored mechanisms of communication -Geography

4. 4 12/18/2014 15:21 Chapter 12Bridges4 Bridge Operation

5. 5 12/18/2014 15:21 Chapter 12Bridges5 Protocol Architecture for Bridge

6. 6 12/18/2014 15:21 Chapter 12Bridges6 Connection of Two LANs by a Bridge

7. 7 12/18/2014 15:21 Chapter 12Bridges7 Bridge over a Point-to-Point Link

8. 8 12/18/2014 15:21 Chapter 12Bridges8 Bridge over an X.25 Network

9. 9 12/18/2014 15:21 Chapter 12Bridges9 Configuration of Bridges and LANs

10. 10 12/18/2014 15:21 Chapter 12Bridges10 Configuration of Bridges and LANs, with Alternate Routes

11. 11 12/18/2014 15:21 Chapter 12Bridges11 Fixed Routing

12. 12 12/18/2014 15:21 Chapter 12Bridges12 Spanning Tree Routing 0 Transparent Bridge Standards * Dissimilar LANs based on the MAC standards (802.3, 802.4, 802. 5, etc.) can be interconnected 0 The routing mechanism is a technique referred to as the Spanning Tree Algorithm 0 Three Categories of incoming MAC frames: 1. Frame addressed to this bridge includes bridge protocol units (BPDUs) 2. Control Frames: These are handled as part of the MAC protocol 3. User Data Frames containing LLC information.

13. 13 12/18/2014 15:21 Chapter 12Bridges13 IEEE 802.1D Bridge Architecture

14. 14 12/18/2014 15:21 Chapter 12Bridges14 IEEE 802.1D MAC Bridge Internal Sublayer Service Primitives

15. 15 12/18/2014 15:21 Chapter 12Bridges15 Definition of IEEE 802.1D MAC Bridge Internal Sublayer Service Parameters

16. 16 12/18/2014 15:21 Chapter 12Bridges16 Outbound Access Priorities

17. 17 12/18/2014 15:21 Chapter 12Bridges17 Bridge Frame Forwarding and Address Learning

18. 18 12/18/2014 15:21 Chapter 12Bridges18 Spanning Tree Algorithm

19. 19 12/18/2014 15:21 Chapter 12Bridges19 Spanning Tree Algorithm 0 Each bridge is assigned a unique identifier, which consists of a MAC address for the bridge plus a priority level. 0 There is a special group MAC address that means “all bridges on this LAN.” When a MAC frame is transmitted with the group address in the destination address field, all of the bridges on the LAN will capture that frame and interpret it as a frame addressed to itself. 0 Each port of a bridge is uniquely identified within the bridge with a “port identifier.”

20. 20 12/18/2014 15:21 Chapter 12Bridges20 Concepts used in the Spanning Tree Algorithm 0 Root bridge: the bridge with the lowest of bridge identifier is chosen to be the root of the spanning tree. 0 Part cost: the cost of transmitting a frame onto a LAN though a particular port on each bridge. 0 Root port: Each bridge discovers the first hop on the minimum-cost path to the root bridge. The port used for that hop is labeled the root port. 0 Root path cost: For each bridge, the cost of the path to the root bridge with minimum cost is the root path cost for that bridge. 0 Designated bridge, designated: On each LAN, one bridge is chosen to be the designated bridge. This is the bridge on that LAN that provides the minimum cost path to the root bridge. This is the only bridge allowed to forward frames from the LAN for which it is the designated bridge toward the root bridge. The port of the designated bridge that attaches the bridge to the LAN is the designated port. For all LANs to which the root bridge is attached, the root bridge is the designated bridge.

21. 21 12/18/2014 15:21 Chapter 12Bridges21 Spanning Tree Procedures 0 Determine the root bridge. 0 Determine the root port on all other bridges. 0 Determine the designed port on each LAN. This will be the port with the minimum root path cost. In the case of two or more bridges with the same root path cost, then the highest-priority bridge is chosen as the designated bridge. If the designated bridge has two or more ports attached to this LAN, then the port with the lowest value of port identifier is chosen.

22. 22 12/18/2014 15:21 Chapter 12Bridges22 Example Configuration for Spanning Tree Algorithm

23. 23 12/18/2014 15:21 Chapter 12Bridges23 Spanning Tree Algorithm application

24. 24 12/18/2014 15:21 Chapter 12Bridges24 Configuration of Bridge Protocol Data Units (BPDUs)

25. 25 12/18/2014 15:21 Chapter 12Bridges25 Topology Change Notification Bridge PDUs

26. 26 12/18/2014 15:21 Chapter 12Bridges26 Effects of Various Combinations of Addressing and Source Routing

27. 27 12/18/2014 15:21 Chapter 12Bridges27 Priorities to Transmit Frames & Traffic Classes 0 User priority is defined by the priority field in a MAC frame, ranging from a low of 0 to a high of 7. It is a label carried with the frame that communicates the requested priority to downstream nodes (bridges and end systems). It has end-to-end significance across bridged LANs. 0 Access priority is used, on LANs that support priority, to complete for access to the shared LAN with frames from other devices (end systems and other bridges) attached to the same LAN. When both the incoming and outbound LAN are of the same MAC type, the bridge assigns an access priority equal to the incoming user priority. Otherwise, the bridge must perform a mapping (see Table 12.2). 0 Traffic class: If a bridge is configured so that multiple queues are used to hold frames waiting to be transmitted on a given outbound port, then the traffic class is used to determine the relative priority of the queues. Traffic class is assigned by the bridge on the basis of incoming user priority.

28. 28 12/18/2014 15:21 Chapter 12Bridges28 IEEE 802.1D Traffic Class Operation

29. 29 12/18/2014 15:21 Chapter 12Bridges29 Recommended User Priority to Traffic Class Mapping

30. 30 12/18/2014 15:21 Chapter 12Bridges30 Suggested Traffic Types