bridging principles - ringdale · 4 token ring bridging 4 nconnects two physical rings nsingle...

1

Bridging Principles

2

Token Ring Bridging 2

By the end of this sessionyou will be able to...

n Define bridging modes Source Routing Transparent Source Route Transparent (SRT)

n Describe how Spanning Tree functions

3


Flexible Frame ForwardingChoice of Techniques

n Transparentu Ethernet and Token Ring

u simple to implementu not easy to manage in a complex network

n Source Routingu Token Ringu requires management effort to implementu trouble shooting is simplified

n SRTu short term combination solution

Source RouteBridging

TransparentBridging

Source RouteTransparent

Bridging

Bridging Techniques

Transparent

Can be used on both Token Ring and Ethernet networks

Nothing is identified so implementation is simple

Nothing is identified so locating problems can be difficult on complexnetworks

Source Route Bridging

Designed for Token Ring networks

Requires each ring and bridge to be identified

Locating potential and actual trouble spots is simplified

SRT

Useful when combing transparent and source routing networks, e.g. whenadding a department using the other method to a company network.Allows bridges/switches to forward both source routed and transparentframes appropriately. Also allows the bridges/switches to communicatewith each other.

A short term solution, ultimately MAKE UP YOUR MIND; use sourcerouting OR transparent for the whole network.

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n Connects two physical rings

n Single logical network

What is the purpose of a Bridge ?

Ring A Ring B

2 4

3 5

16

n Forwards or Filters Frames

n Keeps local traffic local

Bridges are used to physically connect two rings. It is invisible to theworkstations on the rings so, in effect, it makes a single logical ring. Itsjob is to inspect each frame arriving on either ring and to decide whetherit needs to be forwarded to the other or not (i.e. remain on it home ring).This will depend on where the destination address in the frame is.

The bridge not only keeps local data traffic local, but MAC managementis kept local too.

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Transparent Bridge Operation

Addr Port

ABCDEF

111222

n Each bridge builds a table containingu Destination addresses it knowsu Port on which that address can be found

B

C

A

E

F

1 2 21

D

Addr Port

ABCDEF

111122

Transparent Bridging

A Transparent bridge takes a note of every SOURCE address in eachframe that arrives and stores it in a table against the port number at whichit arrived.

Over a period of time it LEARNS all the MAC addresses on eachsegment (if only from the AMP and SMPs). Thus if station A above issending a frame to station F, the first bridge knows that F is on its port 2.Now we know that F is on the third ring which is over another bridge, butas far as the first bridge is concerned frames with a source address of Fhave always arrived at port 2. The first bridge knows nothing of what ishappening on the other side of the second bridge.

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Source Route Bridging

In this form of bridging the bridge doesnt need to store anything.

Each ring is numbered (3 digits of hex) and each bridge has a 1 digit hexid. As the frame crosses each bridge, the Routing Information Field (RIF)is built up inside the frame.


Source Route Bridge Operation

n Ring and bridge numbers stored in RIFu Routing Information Field

n Routing Information held in Frameu not in Bridge Table

DATALLCRIFHDR DA SA Trailer

Ring 00A

B

C

A Ring 00C

E

F

11 2 21

2

D

Ring 00B

7

Workstations use explorer frames to find the address of an intendeddestination and as the frame crosses the network the RIF is incrementedat each bridge. When the frame arrives at its destination the completepath will be stored in the RIF. The destination station can use this to routethe reply.

The workstation sends out a basic source routing frame. This has thestart of the routing information field (RIF) but no actual routing information(since the workstation knows none!)

When the frame arrives at the first bridge/switch the lack of routinginformation tells the bridge that it is the first bridge to see this frame. Itputs in the originating ring number, its own bridge number and the ring towhich it is forwarding the frame. On a bridge this, of course, can only beone ring, but on a switch it could be one of many. A broadcast (explorer)frame will be forwarded to all output ports in a switch so each will have adifferent RIF.

When the frame reaches the next, and every subsequent, bridge thebridge number and next ring number are added. Thus when the framearrives at the destination machine it will have a complete path in the RIF.


Source Route Bridge Operation

n RIF built up as explorer frame is broadcast acrossthe rings

n Frame reaches server with complete RIFn Server uses RIF to get back to station

Ring/bridge pairsHDR DA SA TrailerB

C

A

E

11 2 21

2

D

LLC DATA

RIF

Station

ServerF

Ring 00A Ring 00CRing 00B

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Flagging the framefor Source Routing

Example:0000F6123456 No RIF8000F6123456 RIF present

Individual/Group bit in theSource Address is always Individual ...

Token RingFrame

Frame logger will show the true MAC address rather than the bit sequence

DATALLCRIFHDR DA Trailer

So use it to indicatepresence of a RIF

SA

UL

Manufacturer IDIG

Serial #

UL

Manufacturer ID Serial #RIF

We need a way of indicating that there is a RIF, i.e. the source station isusing Source Routing. This is done by the driver software when the frameis assembled.

Although we can send to multiple addresses, we cant send from multipleaddresses, so the way we flag that the frame in a source routing frame isto make the source address a group address - this is obviously nevergoing to happen genuinely.

It has the added bonus that should the frame arrive at a transparentbridge, which reads the source address for its routing table, it will bediscarded as having an invalid address.

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Routing Information Field (RIF)

Routing InformationField

RingBridge

RingBridge

RingBridge

Control2 bytes B 0

Maximum of 18 bytes

001 A 002 003

DATALLCRIFHDR DA SA Trailer

n 2 bytes minimumu Control information only

n 18 bytes maximum:u Control + Ring & Bridge pairsu i.e. maximum hop count (7 bridges ) reached

n Last bridge number always 0u destination node is on a ring

The Routing Information Field

The RIF consists of two bytes of control information (more later) and anumber of ring-bridge combinations.

If we are using the IBM definition of source routing this allows only 7hops i.e. allows a frame to cross 7 bridges.

This means this part can be 7 ring-bridge combinations = 14 bytes plusthe final ring number and a final bridge id of 0 - a further two bytes.Adding in the control byte we have a RIF of 18 bytes.

If we are using the IEEE source routing specification, this allows 13 hops,i.e. a RIF of 30 bytes (26 +2 +2).

The last bridge id is always 0 since the destination must be on a ring. Ifwe stopped on the ring the RIF would be half a byte short so the 0 isadded to complete to a sensible size. Since 0 is not a valid bridge id thiscould also act as an end of field character if necessary.

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Constantly Circulating FramesUnique Ring Numbers

n Ring number not allowedto be in RIF more thanonce

n Stops constantlycirculating frames

Control 002 1 004 2 002

Ring 002

Ring 004

321

0

If we have more than one bridge between two rings, each bridge must benumbered differently so that the ring number-bridge number is a uniquecombination.

The ring number is also only allowed to be in the RIF once. When theframe from ring 002 crosses bridge 1 and gets to ring 004 the RIFcontains:

[control] 002 1 004 0

Bridge 2 (or bridge 3) will not allow the frame back on to ring 002 becausethat ring number is already in the RIF. This check prevents endlesslycirculating frames.

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Source Route BridgingAll Routes Explorer (ARE) Frames

2

3

B

2

3

1

001 002 003

1031101 102

A

ARE

There are two types of explorer frames stations can use to find adestination.

The first of these is the All Routes Explorer which is exactly what it does.It will be propagated (copied) to all output ports of all bridges/switches sothat copies will arrive at the destination having covered all possibleroutes.

In the above example the single ARE will become 4 by the time it reachesthe server on ring 3 using the top route and a further one frame will getthrough the bottom route - one starts, five finish.

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Source Route BridgingAll Routes Explorer (ARE) Frames

001 002 003

103101 102

2

3

B

2

3

11

A

ARE: 001-A-101-1-102-1-103-0

ARE: 001-2-002-2-003-B-103-0

ARE: 001-3-002-3-003-B-103-0

ARE: 001-2-002-3-003-B-103-0

ARE: 001-3-002-2-003-B-103-0

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Source Route BridgingExample Spanning Tree

B

2

001 002 003

1031101 102

2

A

Only one route from any ring to any other ring

= Standby Bridge

3 3

1

3

The second type of explorer is the Spanning Tree Explorer (STE), alsoknown as Single Route Explorer (SRE), but before we can use this theSpanning Tree protocol must be activated. This entails the bridges talkingto each other and deciding which will be designated and which standby- under Source Routing, or forwarding and blocking under transparentbridging.

Once this election is complete, there will be only one route between anytwo stations. In the picture above the striped bridges are in standby (orblocking), so a spanning tree explorer frame from a workstation in ring001 can only use the route 001-2-002-2-003 (- 0). From 102 the route willbe 102-1-101-A-001-2-002-2-003.

There is a significant difference to be noted here between Source Routingand Transparent bridges using Spanning Tree. SR bridges which arestandby will only stop Spanning Tree Explorer frames ALL OTHERSWILL PASS. In Transparent a bridge in blocking mode will stop ALLFRAMES.

A: None will arrive back on ring 001 since 001 is already in the RIF ofeach frame and so will be discarded by the bridges on ring 001.

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001 002 003

103101 102

Source Route BridgingSpanning Tree Explorer (STE) Frames

STE: 001-A-101-1-102-0

STE: 001-2-002-2-003-0

3 3

1

3= Standby Bridge

B

2

1

2

A

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3

Source Route BridgingSpecifically Routed Frames

001 002 003

103101 102

SR: 001-2-002-2-003-0

B

2

1

2

A

3

1

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Source Route BridgingRouting Information Control Field

000 Specifically Routed Frame100 All Routes Explorer (SR return)110 Spanning Tree Explorer (ARE return)111 Spanning Tree Explorer (SR return)

000: 516 bytes 100: 8144 bytes001: 1500 bytes 101: 11407 bytes010: 2052 bytes 110: 17800 bytes011: 4472 bytes 111: Initial value

Control 0-8 Ring & Bridge Number fields

3 bits 5 bits 1 bit 3 bits

BroadcastIndicator

Length of Routing info (in bytes)

Directionbit

Maximum Frame size

Unused

Header DA SA RIF LLC Header Data TrailerToken Ring frame

In this slide we see the content of the control bytes.

The first three bits define the broadcast indicator:

000 specifies this frame as Specifically Routed i.e. the RIF is complete

100 says that an ARE will go out and we expect an SR back - if this is goingfrom a workstation to a server, this can mean an enormous number of framesarriving at the server which will have to be processed.

110 says send an STE out and get an ARE back. This means that one framewill arrive at the server which will have traversed the best route, but whichcould become congested; the ARE returning will find all the routes. Thiscould mean a lot of frames arriving at the workstation, but this is less of aproblem since the workstation is far less busy than the server. This is theusual way of finding a destination address these days.

111 says use an STE out and well assume that this is the best route so usean SR back. This is a less efficient way of using source routing since thealternative bridges will be doing nothing.

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n Each bridge has a Bridge Priorityu 4 hex digits (8000h or C000h)

n Each bridge has a number of MAC Addressesu 12 hex digits (0000F6123456)u read in non-canonical (MSB first)

n Bridge ID is Bridge Priority + Mac Addressu (8000)(00006F482C6A)u MAC Address read in Canonical (LSB first)

Lowest Bridge ID becomes Root Bridge

Spanning Tree Formation Election of Root Bridge

Each bridge has a bridge priority and is usually either 8000h or C000h.Bridges also have a number of MAC addresses, at least one for eachport.

The bridge is known by its bridge id which is made up of its priorityconcatenated with its base address (read canonically i.e. in the ethernetform)

The MAC address shown is 0000F61213456

putting this in binary we have:

0000 0000 0000 0000 1111 0110 0001 0010 0011 0100 0101 0110

we read each BYTE in reverse order:

0000 0000 0000 0000 0110 1111 0100 1000 0010 1100 0110 1010

which is in hex

00006F482C6A

The Bridge Id is therefore 800000006F482C6A

When the bridges hold an election the lowest bridge id becomes the rootbridge. All path costs are calculated from the root bridge so you shouldadjust the bridge label if you want to manage which bridge is the root.This should be the top bridge in a hierarchical network and somewherenear the centre in a mesh network. This will minimise path lengths andhence path costs. It is advisable to have one or two extra bridges in thoseareas which can take over should the original fail.

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3

1Ring 101 Ring 102

Ring 104Ring 103 4

2Hello BPDU 100008005aabcdef

800000006f654321

800000006f123456

100000006fabcdef

Hello BPDU

Hello BPDUHello BPDU

Hello BPDU

Spanning Tree Formation Election of Root Bridge

In the above example the lowest id is bridge 1 so after passing round thehello bpdu (Bridge Protocol Data Unit) messages they each know all thebridge ids and the lowest will win. BPDUs have two key pieces of data:the root bridge information (including priority and MAC address) and thesending bridge information (MAC address and path cost).

Assume the bridges power up in numerical order.

1 will send hello BPDU to 101 and 102 with itself as root bridge

2 comes up, sees 1s BPDU and as his own Bridge id is higher willpropagate 1s BPDU to 104 as the root bridge part of the BPDU that itsends

3 comes up and sees 1s BPDU and propagates it to 103

4 comes up and sees BPDUs from 2 and 3 indicating 1 as the RootBridge. As its bridge id is higher 1 remains as Root bridge.

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Spanning Tree Example

4

1Ring 101 Ring 102

Ring 104Ring 103 5

23

100000006fabcdef

100008005aabcdef

800000006f654321

800000006f123456

800000006fabc123

15

10

5

15 20

n Identify the Root and Designated bridgesn Explain Why & How

A similar process goes on to decide which bridges will be designated(forwarding) and which standby (blocking). The decision is made first onpath costs, if that doesnt resolve it, then on ring number, finally by MACaddress (in Transparent which doesnt use ring numbers).

In the above example Bridge 5 has the lowest bridge id so will becomethe root bridge.

The path through bridge 2 has the highest cost so that will becomedesignated (blocking).

At least one of the bridges between rings 103 and 102 will have to bestandby and bridge 4 has the higher path cost so bridge 3 will bedesignated (forwarding). (If both had had a higher path cost than bridge 2they could both have been made standby.)

This becomes a little more complex when switches are involved sincethey are multiport bridges. With bridges both ports will be designated(forwarding) or standby (blocking); with switches individual ports aremanaged. This means that the path through the switch is designated orstandby and if a port on either end of that internal path is in standby thenthe path is in standby. On a standby path the port nearest to the rootbridge will be designated, the further port will be standby. In the slide, ifthe bridges were switches, then ring 103 port on bridge 4 and ring 104 onbridge 2 will be designated and 101 on bridge 4 and 102 on bridge 2 willbe standby. All other ports on the network will be designated.

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Spanning Tree

The Spanning Tree protocol is fully supported by each logical bridge in aswitch. Note, however, that there are some fundamental differencesbetween the way that Spanning Tree frames are handled in transparent andsource-route bridging:

In a source routed network only Spanning Tree Explorer (Single RouteExplorer) frames are blocked by a standby port. In a transparent network allframes are blocked by a blocking port.

The industry standard is to use the IEEE format for Spanning Tree framesexcept when performing pure source-route bridging, when the IBM standardis generally used. However this is usually configurable.

Because the bridges will communicate using different addresses - SR usesthe BFA, transparent uses the BGA - it is possible that in a mixed networktwo (or more!) spanning trees will be developed. This is not arecommended option.

See next slide.


Spanning Tree SupportSpanning Tree types

Bridging Mode

TransparentSRT, SRT+

SPT Frame Destination

Bridge Group Address

Frames blocked by standby bridges

All frames

Source RoutingBridge Functional Address

Spanning Tree Explorer Frames

n With Transparent, SRT and SRT standby bridge passesNO frames

n Can you have 2 spanning trees?

21

Spanning Tree

Certain customers may wish to deploy a mixture of source routed and non-source routed clients. However the two bridges have different methods forforming the Spanning Tree.

Because different multicast addresses are employed by the two methods,there will be (at least) two separate spanning trees in effect in the casewhere both kinds of bridge are present in a network:

A source routing bridge will not forward transparent spanning tree frames(as there is no RIF)

A transparent bridge will forward source-routing spanning tree framesunchanged (it cannot update the RIF field) since it does not recognise thedestination address (BFA) and therefore will forward to all output ports.

Each ring in the slide will need a ring number since they are connected tosource routing bridges. This will not affect the transparent bridge as it knowsnothing about ring numbers. The SR bridges will see the middle two rings asa single segment - they dont know about the transparent bridge since it usesa different address. So lets say the rings are numbered 101, 102, 103 and104 (left to right). The left hand SR bridge would see the combined ring as102, the right had SR bridge would see it as 103 - this is obviously notallowed. So both rings would have to have the same ring number.


Spanning Tree Support 2 Spanning Trees

SPT frames from Source Routing bridgesforwarded unchanged by Transparent bridges

TSR SR

SPT frames from Transparent bridgesblocked by Source Routing bridges

n How many ring numbers are assigned?n Is this recommended?

22

SRT

With this method the IEEE spanning tree is used for both, so there are nodangers of having two or more spanning trees.

This is a temporary expedient; it is not recommended as a long termsolution. Make up your mind - SR or T!


Spanning Tree Support SRT

SRT

n Supports a mixture of SR and Transparent clientsn Single Spanning Tree

SRTSRT

bridging principles - ringdale · 4 token ring bridging 4 nconnects two physical rings nsingle...

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