2 12/18/ :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 12/18/ :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
12 12/18/ :21 Chapter 12Bridges12 Spanning Tree Routing 0 Transparent Bridge Standards * Dissimilar LANs based on the MAC standards (802.3, 802.4, , 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.
18 12/18/ :21 Chapter 12Bridges18 Spanning Tree Algorithm
19 12/18/ :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 12/18/ :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 12/18/ :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 12/18/ :21 Chapter 12Bridges22 Example Configuration for Spanning Tree Algorithm
23 12/18/ :21 Chapter 12Bridges23 Spanning Tree Algorithm application
24 12/18/ :21 Chapter 12Bridges24 Configuration of Bridge Protocol Data Units (BPDUs)
26 12/18/ :21 Chapter 12Bridges26 Effects of Various Combinations of Addressing and Source Routing
27 12/18/ :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.