Post on 20-Dec-2015
Dec 6, 2007 CS573: Network Protocols and Standards
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Transparent Bridging
Network Protocols and Standards
Winter 2007-2008
Dec 6, 2007 CS573: Network Protocols and Standards 2
Reasons for Bridges On a single LAN, there are
limitations: Number of stations Size of segment Bandwidth per segment
Bridges connect LAN segments to make “extended” LANs LANs, LAN Segments, Extended LANs
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Example: Bridging Benefits
Consider a LAN segment with average traffic R pkts/sDivide it into two segments and connect with a BridgeAverage traffic on each segment is R/2 pkts/s
Bridge
Stations Stations
R/2 pkts/s R/2 pkts/s
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Example: Bridging Benefits On average:
Each segment generates a traffic of R/2 pkts/s
Half of the traffic is for “local” stations Half of the traffic is for “other” segment Traffic on each segment is R/2+(1/2) R/2
Average traffic on each segment is 3R/4 This traffic must not exceed the capacity of
the segment
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Example: Bridging Benefits Therefore 3R/4 < C
C is the capacity of the physical link R < 4C/3
Effective R exceeds the capacity i.e. Rmax < 4C/3 rate on any segment must not exceed the capacity
What was the maximum rate allowed when the LAN was not segmented?
(Rmax < C) Does the maximum effective R (i.e., Rmax)
increase when three segments are used? Depends how the segments are connected!
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Can we use a router instead? The answer is “It depends” Inter-segment traffic may be handled by routers
if all stations understand layer 3 Older machines did not understand layer 3, but new
ones do Does this mean that with newer stations, we did
not need bridges? Not really! Bridges handle all layer 3 protocols while
early routers usually handled a single layer 3 protocol Don’t multiprotocol routers do address this
issue? And what about convergence to IP? Does that not eliminate the need for multiprotocol routers
An IP router can replace a bridge then, right?
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Do we still need a Bridge? What if stations want to move on
the “extended” LAN without reconfiguring their IP addresses? Bridges can help! Bridges have high performance Bridges are simple (less expensive)
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Transparent Bridging
…
Bridge
For stations, the two topologies are the same transparent bridging
stations
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Transparent Bridge Functions Promiscuous Listening
Every packet passed up to software Store and Forward
Based on a forwarding database Filtering
Also based on forwarding database
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Can a Bridge act smart? For the two segment-one bridge topology for
which the maximum rate was 4/3 of the link capacity, was Bridge doing something smart?
Yes, the Bridge forwarded the traffic smartly Manual entry of station addresses? Stations use addresses from a range? Station addresses are assigned such that a
portion indicates the LAN number? Bridges can also “learn” on their own!!!
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Forwarding Database (FDB):Creation and Maintenance
The bridge promiscuously listens to every packet/frame received on each port
For each received frame, address in the source field is stored together with the port on which the frame is received. The FDB is created in Station Cache.
Each entry in the FDB is deleted if no traffic is received from that source address for a given period of time (Aging time). Why?
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Forwarding Frames For each received frame, the bridge looks
at the destination address: If the address is multicast or broadcast (all 1’s)
then the frame is forwarded to all the interfaces (ports) except for the one on which it is received
For unicast addresses: If the address is not found in FDB, the frame is
forwarded to all the ports except for the one on which it is received
If the address is found in FDB, the frame is forwarded to the port in FDB entry. If the FDB entry has same port on which the frame is received, frame is dropped (filtered)
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Example 1: Learning and Forwarding
Transmission order A D
Ports 2, 3 D A
Port 1 Q A
Filtered Z C
Ports 1, 3
BPort 1
Port 2
Port 3
A Q
Z C
D M
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Example 2: Two Bridges
B1Port 1 Port 2
B2Port 1 Port 2
A Q D M K T
What are the Station Caches after “complete” learning?
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Topologies with Loops Problems
Frames proliferate Learning process unstable Multicast traffic loops forever
B1 B2 B3
LAN 1
LAN 2
A
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Topologies with Loops Solutions
Require that the topologies be loop-free through careful deployment of segments and bridges
Design Bridges to detect loops and complain and, perhaps, stop working
Not a good idea because loops provide redundancy Design into the bridges an algorithm that
prunes the topology into a loop-free subset (a spanning tree)
Blocking of some ports may be required Automatically adapt to the changes in topology
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Reconfiguration Algorithm Configures an arbitrary topology into a
spanning tree Automatic reconfiguration in case of
topology changes The algorithm should converge for any size
LAN; the stability should be achieved within a short, bounded time
Active topology should be reproducible and manageable
Transparency to end-stations is required Must not use a lot of bandwidth
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Spanning Tree Algorithm A distributed Algorithm
Elects a single bridge to be the root bridge Calculates the distance of the shortest path
from each bridge to the root bridge (cost) For each LAN segment , elects a
“designated” bridge from among the bridges residing on that segment
The designated bridge for a LAN segment is the one closest to the root bridge
And…
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Spanning Tree Algorithm For each bridge
Selects ports to be included in spanning tree The ports selected are:
The root port --- the port that gives the best path from this bridge to the root
The designated ports --- ports connected to a segment on which this bridge is designated
Ports included in the spanning tree are placed in the forwarding state
All other ports are placed in the blocked state
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Forwarding frames along the spanning tree
Forward and Blocked States of Ports
Data traffic (from various stations) is forwarded to and from the ports selected in the spanning tree
Incoming data traffic is always discarded (this is different from filtering frames. Why?) and is never forwarded on the blocked ports
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Root Selection: Bridge ID Each port on the Bridge has a unique LAN
address just like any other LAN interface card. Bridge ID is a single bridge-wide identifier that could be: A unique 48-bit address Perhaps the LAN address of one of its ports
Root Bridge is the one with lowest Bridge ID
BPort Address