03 dll bridges by apcoms

8/14/2019 03 DLL Bridges By Apcoms

1/4

Computer NetworksCPS 422

DATA LINK LAYER

Faisal Amjad CPS 422

BRIDGES


Bridges are Devices used tointer-connect multiple LANs


Bridges - The RequirementDifferent departments of an organization may

have different types of LANs suited for their ownrequirements - a need almost always arises toconnect them

Due to larger distances between buildings it maybe cheaper to interconnect using bridges and awireless medium

It is logical to split a very large LAN (as ourcollege network) into a smaller LAN, restrictingtraffic within a smaller LAN if it is not requiredoutside

Bridges are also amplifiers with error correctioncapability. Can be used to increase the distance.


Bridges - The Requirement (Contd)Bridges partition the LANs and so increase

network reliability. If a station continuouslytransmits garbage (intentionally or otherwise)complete network will be spared

The above feature provides security as well, bypartitioning the network and not allowing trafficwhere it is not required

Faisal Amjad CPS 422802.3 to 802.5 Bridge


The problems of BridgingDifferent frame formats bridging requires

reformatting, checksum recalculation andundetected errors induced by a bridge (due to badbuffer bits in a bridge)

Different data rates of different LANs, same

data rate specification but different on ground(due to different collision characteristics).Values of timers in higher layers specially affect

(upper layers) fragmented packet transmission.Any one fragment lost due to a delayed frame willresult in retransmission of entire packet (evenaborting a transmission)

Difference in max frame length


2/4


Source Routing Bridges


The Source Routing BridgeThe sender of a frame is assumed to know

whether or not the destination is on the same LANor not

When sending a frame for another LAN, thesender sets the high order bit of source addressto 1 and the frame contains the exact path for theframe to follow

Path construction is as fol:o Each LAN has a unique 12-bit numbero Each bridge has a unique 4-bit numbero Two bridges may have same numbers but should not be

connected between the same two LANso The path is a sequence of numbers as LAN, bridge, LAN,

bridge,.........)


A B C D E F

G H

B1 B2

LAN 1 LAN 2 LAN 3

LAN 4

BridgesStations


Source Routing Bridge-Functioning

Source routing bridge is interested only in frameswith high order bit of source address set to 1

After this, the bridge scans for the LAN numberon which the frame arrived

If the LAN number is followed by its own bridgenumber, the frame will be forwarded on the LANwhose number follows its own bridge number as:LAN number, Bridge number (own), LAN number

(frame received from) (to forward

frame)


A B C D E F

G H

B1 B2

LAN 1 LAN 2 LAN 3

LAN 4

BridgesStations

Source Routing Bridge-Functioning(Transmission from station A to station H)A sets the higher order bit of source address to 1

Sets the path as LAN1, B1, LAN2, B2,LAN4

B1 receives the frame and inspects high order bitof source address

Sees that it has received the frame from LAN 1 andfinds its own number B1

Forwards the frame on the LAN whose numberfollows its own bridge number (in this case LAN2)


Source Routing - Route Discovery

When a station wants to send a frame to a station whosepath is not known, it FLOODS a Discovery frame.

Every bridge forwards this frame on every LAN to which itis connected.

The discovery frame ultimately reaches the destinationwhich sends back a reply frame through flooding

When the reply comes back, all bridges record their ownidentity in it, so that a route is constructed.

The selection of the best path is done at the sourcestation by running an algorithm designed to do so.

Although All the paths are discovered, this techniqueresults in a frame explosion due to flooding.

To limit flooding, routes are stored in a cache for aspecified duration

Caching induces significant administrative intervention


3/4


Transparent Bridges


Transparent Bridges

As the name suggests, its primary objective is to ensurecomplete transparency

Transparency means simply a Plug-and-Play operation ofthe bridge

A transparent bridge operates in Promiscuous mode, inwhich it accepts every frame that it receives and forwardsit by reading its source/destination fields

Whenever a frame arrives at a bridge, it must decidewhether to discard or to forward it

If a frame has its destination on a LAN other than the onefrom which it arrived, it must be forwarded

Otherwise it must be discarded


Transparent Bridges Route Discovery

A technique called Backward Learning is used for routediscovery

Every bridge has a hash table containing all possibledestinations and which output line (LAN) to which itbelongs

Initially there are no entries in the table The bridge resorts to flooding whenever a route is not

known (not in the hash table)Whenever a frame arrives whose destination is not known,

the frame is flooded and the senders address (e.g. A) isnoted in the hash table along with the (output port) LANfrom which it arrived

Next time if a frame destined for station A arrives, it willbe forwarded to its known output line



Every entry in the hash table has an associatedtime value

To accommodate topology changes, whenever aframe is received from a particular station, itstime entry in the table is also updated

If a specific time entry in the table has not beenupdated for a few minutes, it is purged by thealgorithm

For the next time flooding will be used



The routing procedure for a frame, can besummarized as :o If source and destination LANs are same, discard it

o If source and destination LANs are different, forward

ito If source and destination LANs are unknown, use

flooding


Spanning Tree Bridges

Flooding induces a problem of frame forwarding ininfinite loops

A spanning tree algorithm calculates a loop freetopology

Loops can be removed from the topology by

constructing a spanning tree of the graph, whichhas only one route for a given source destinationpair

Links in the graph causing loops are ignored andframes not forwarded through them

Algorithm is run after the lapse of specified timeinterval

Spanning Tree Bridges implement this technique


4/4


Assignment # 3Study the issues related to bridging 802 LANs

A comprehensive coverage of problems for bridging802.x to 802.y LANs is given in fig 4-37

Study the comparison of Transparent bridges andSource Routing bridges

03 dll bridges by apcoms

Documents