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Rivier College
CS575: Advanced LANs
Chapter 12: Bridges
Chapter 12
Bridges
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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
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Bridges
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
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Bridge Operation
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Protocol Architecture for Bridge
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Connection of Two LANs by a Bridge
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Bridge over a Point-to-Point Link
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Bridge over an X.25 Network
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Configuration of Bridges and LANs
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Configuration of Bridges and LANs,
with Alternate Routes
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Fixed Routing
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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.
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IEEE 802.1D Bridge Architecture
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IEEE 802.1D MAC Bridge Internal Sublayer
Service Primitives
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Definition of IEEE 802.1D MAC Bridge Internal
Sublayer Service Parameters
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Outbound Access Priorities
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Bridge Frame Forwarding and Address Learning
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Spanning Tree Algorithm
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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.”
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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.
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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.
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Example Configuration for Spanning Tree Algorithm
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Spanning Tree Algorithm application
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Configuration of Bridge Protocol Data Units (BPDUs)
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Topology Change Notification Bridge PDUs
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Effects of Various Combinations of Addressing
and Source Routing
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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.
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IEEE 802.1D Traffic Class Operation
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Recommended User Priority to Traffic Class Mapping
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Suggested Traffic Types
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