UNH-IOL BFC Knowledgebase Bridging

7/29/2019 UNH-IOL BFC Knowledgebase Bridging

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Bridging Protocols Overview

Bridge Functions Consortium


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Bridging Protocols

Filtering Database (802.1Q/802.1D)

Spanning Tree Protocol (802.1D clauses 8 & 9)VLANs (802.1Q)

GARP/GVRP (802.1D clause 12/802.1Q clause 11)

GARP/GMRP (802.1D clause 10 & 12)

Link Aggregation (802.3ad)


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Bridging History Back in the days before Ethernet was the

clear winning technology on the LAN,

Token Ring and FDDI were popular

This meant two different methods ofbridging

1) Source Route Bridginga. Used by Token Ring and FDDI

2) Transparent Bridging

a. Used by Ethernet


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Source Route Bridging Source Route Bridging allows load balancing to

avoid congestion. This is done by routing packets

over two or more routes to a destination.

Switch 3

Switch 1

Switch 2Source LAN

Server

Destination

LAN


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Transparent Bridging The transparent bridging method follows the plug and

play philosophy.

Each bridge contains one (or more) Filtering Databasesthat learn and remember MAC addresses on its networks.

Forwarding decisions are then made with consultation ofthe Filtering Database. If a destination MAC address hasbeen learned, the packet is then forwarded out of that

port. These addresses then will be cleared from the Filtering

Database if they are not active for a specific amount oftime. This range is defined by Aging Time, which can beset in the management.


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Filtering Database One database

contains MAC

addresses, whichport theyre on, andif theyre active ordisabled

Duplicate MACaddresses notallowed (the second onewould replace the first)

Entry MAC Addr Port active

1 0800900A2580 1 yes

2 002034987AB1 1 yes

3 00000C987C00 2 yes

4 00503222A001 2 yes5

6

7

8

9

1011

12


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Learning of Addresses The Filtering Database learns a stations location

from the source address on an incoming frame

Switch

Frame with destination address

00 22 22 33 33 44 is received

on Port 4.

Port 1

Port 4

Frames with the destination

address 00 22 22 33 33 44 are

only forwarded on port 1

Frame with destination address

00 22 22 33 33 44 is received

on Port 4.

Frame with source address

00 22 22 33 33 44 is

received on Port 1.

This source address islearned by the filtering

database. All future frames

destined for this MAC address

will be forwarded ONLY out ofthis Port.

Destination address not yet learned.

Packet is forwarded out all ports.


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Multicast Frames Multicast Frames originate from one source and

have the possibility of going to more than one

destination. An example of this is the SpanningTree BPDU.

Switch 4

Switch 1

Shared LAN

Switch 3Switch 2


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The Permanent Database Upon Bridge Initialization, a reserved block of Multicast

Addresses is transferred to the Filtering Database

Currently only 3 of these multicast addresses arestandardized. The rest are reserved for future use.Frames containing these addresses in the source are never

learned or forwarded.

Assignment ValueBridge Group Address (Span. Tree) 01 80 C2 00 00 00

IEEE Std. 802.3, Full Duplex Pause Operation 01 80 C2 00 00 01

Slow Protocols Multicast Address 01 80 C2 00 00 02

Reserved for future standardization 01 80 C2 00 00 03

To

01 80 C2 00 00 0F


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Basic/Extended Filtering Services Bridges that support Basic Filtering Services

can dynamically learn all MAC addressesexcept those from the Permanent Database

These addresses can also be staticallyconfigured so that they do not age out

Switches filtering frames from the Permanent

Database are said to support Basic FilteringServices

Extended Filtering Services are implementedby devices that support advanced features

like GARP


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Aging Time Aging time is defined as a range of 10 to one million

seconds

One million seconds = 11 days 13 hrs 46 min and 40 sec The default time is 300 seconds

The Filtering Database starts aging time when an address islearned and resets it whenever another frame arrives on

that port Why is aging time important?

When aging time expires, the address and port are discarded fromthe Filtering Database.


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Filtering Database Review Every bridge has a table called a Filtering

Database

Entries in this table are updated upon receiptof frames, the source addresses and theports they arrive on are learned

Once a MAC address is associated with aport, frames containing that destinationaddress are only forwarded out of that port


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Filtering Database Review(cont.)

In real switches these tables vary insize, most have the capability of holding

several thousand MAC addresses. Iveseen one that has the capacity to learnmore than 150,000 addresses

(3Com9100).


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Spanning Tree Protocol (STP) An algorithm,, used to prevent logic loops in

a bridged network by creating a spanning tree

When multiple paths exist,, STA lets a bridgeuse only the most efficient one. If that pathfails, STA automatically reconfigures thenetwork to make another path become active,

sustaining network operationsDefinition ofSpanning Tree Algorithm from Newtons Telecom Dictionary.


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The Spanning Tree PoemI think that I shall never see

A graph more lovely than a tree.

A tree whose crucial property

Is loop-free connectivity.

A tree that must be sure to span

So packets can reach every LAN.

First, the root must be selected.

By ID, it is elected.

Least-cost paths from root are traced.

In the tree, these paths are placed.

Amesh is made by folks like me,

Then bridges find a spanning tree.

-Radia Perlman


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What is a Spanning Tree? Only one active path

exists between any

two devices.

Resembles a familytree. (problems arise in bothwhen loops occur)


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Why Spanning Tree? The purpose of Spanning Tree is to

have bridges dynamically discover a

subset of the topology that is loop-freeand yet has just enough connectivity sothat there is a path between every pair

of nodes in the LAN.


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How does Spanning Tree work? The basic idea behind the Spanning

Tree Protocol is that bridges transmit

special messages to each other thatallow them to calculate a spanning tree

Configuration Bridge Protocol Data

Units (BPDUs) Sometimes referred to a Config. BPDUs


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STP ExampleRoot

BA

D EC F


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Port States Bridge ports operate the Spanning Tree

Algorithm using the following states: Blocking incoming frames are discarded

Listening incoming frames are discarded, but theport is in the process of transitioning to Learning

Learning incoming frames are discarded, buttheir source addresses and ports are placed in the

Filtering Database Forwarding incoming frames are forwarded,

source addresses are learned

Disabled the port is disabled by management


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Configuration BPDUs The Configuration BPDU contains enough info so

that bridges can do the following:1) Elect a single bridge to be Root Bridge

2) Calculate the distance of the shortest path fromthemselves to the Root Bridge

3) Elect a Designated Bridge for each LAN segment,which is the bridge in the LAN segment closest to the

Root Bridge, to forward packets from that LANsegment toward the Root Bridge.

4) Choose the port, called the root port, that gives thebest path from themselves to the Root Bridge.

5) Select ports to be included in the spanning tree.These include only root ports and designated ports.


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Inside Config BPDUs Destination MAC Address: 01 80 C2 00 00 00

Special Multicast address for Spanning

Tree Root ID

ID of the bridge assumed to be root

Bridge ID

ID of the bridge transmitting BPDU Cost

Cost of least-cost path to the root fromthe transmitting bridge (at least the bestpath of which the transmitting bridge iscurrently aware of)


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Inside Config BPDUs Protocol ID = 0x0000

Protocol Version ID and BPDUType = 0x00

If transmitting bridge is Root,Message Age = Zero, otherwiseit is set to the value of the RootPorts Message Age timer plusan increment of one*


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Path Cost Path costs are designed to be

associated with the speed of the link

Link Speed Recommended

value

Recommended

range

Range

4 Mb/s 250 1001000 165 535

10 Mb/s 100 50600 165 535

16 Mb/s 62 40400 165 535

100 Mb/s 19 1060 165 535

1 Gb/s 4 310 165 535

10 Gb/s 2 15 165 535


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Bridge Initialization Root ID set to Bridge ID

Root Path Cost set to zero

All ports on bridge become designatedports

Configuration BPDU transmitted on eachdesignated port

Hello Timer is started


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How this all works togetherA bridge continuously receives

Configuration BPDUs on each of its ports

and saves the best configurationmessage from each port. The bridgedetermines the best configurationmessage by comparing not only the

Configuration BPDUs received on aparticular port, but also the configurationmessage that the bridge would transmiton that port.


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How is best determined? Given two Configuration BPDUsC1 and C2

C1 is the best if:

the root ID in C1 is numerically lower then the rootID in C2

If the root IDs are equal, then if the cost in C1 isnumerically lower than the cost in C2

If the root IDs and cost are equal, then if the Bridge

ID in C1 is numerically lower than the Bridge ID inC2

The final tiebreaker is the port ID. Each porton a switch has a port ID. Useful if two ports

from the same switch are on one LAN segment.


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Transmitting BPDUs If Hold Timer is active the Configuration

BPDU will be transmitted upon

expiration. Ensures no more than one

Configuration BPDU is transmitted perHold Time period

Transmit only if Message Age < MaxAge

After transmission Hold Timer is reset


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BPDU Processing Received Configuration BPDU is

checked against stored BPDU

If the received BPDU is better or thesame but with a smaller age, thenstored BPDU is overwritten

Bridge then recalculates root, root pathcost, and root port


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Message Age Each Configuration BPDU contains a

message agefield

Incremented after every unit of time

Ifmessage age= max agethen theBDPU is discarded


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Root or Path to Root Fails Bridge will no longer receive fresh BPDUs

Gradually increases message age on

currently stored Configuration BPDU

When max age occurs bridge will recalculateroot, root path cost, and root port


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Hello Time/Root BPDU Propagation The Root Bridge periodically transmits

Configuration BPDUs every hello time

When the Root Bridge generates aConfiguration BPDU the message age field isset to 0

Upon receipt, Bridge will transmitConfiguration BPDU on each port for which itis the Designated Bridge, and increment themessage age by at least one*


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Designated Bridge


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Topology Change?


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Stopping Loops during Topology Change

Use two substates: Listening and Learning

Data received while in these states is notforwarded

Received Configuration BPDUs are stored

Root, root path cost, and root port arecalculated


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Topology Change Procedure1) Bridge notices that the Spanning Tree

algorithm has caused it to transition a port

into or out of the blocking state2) Bridge periodically transmits a Topology

Change Notification BPDU with same periodas hello time. It continues this until theRoot bridge acknowledges by setting thetopology change bit in its ConfigurationBPDUs.


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Topology Change Procedure(cont.)

3) A bridge that receives a Topology ChangeNotification BPDU on a port for which it is the

Designated Bridge does two things:1) Performs step 2 from previous slide (notifies the

root bridge of topology change)

2) Sets the topology change acknowledgement flag

in the next Configuration BPDU it transmits on theLAN from which the Topology Change NotificationBPDU was received


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4) Root Bridge sets the topology changeflag in its Configuration BPDUs for a

period equal to the sum of forwarddelay and max age, if the Root Bridge

a. Notices a topology change because one

of its ports has changed state, orb. Receives a topology change notification

message



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5) A bridge that is receivingConfiguration BPDUs with the

topology change flag set (or the RootBridge that is setting the topologychange flag in its ConfigurationBPDUs) uses the forward delay timer

until it starts receiving ConfigurationBPDUs without the topology changeflag set



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Networkwide Parameters For correct operation some parameters need

to be uniform throughout the SpanningTree. The Root Bridge includes thefollowing values in its Configuration BPDUs:

1) Max age: time after which Configuration BPDUsare discarded

2) Hello time: interval, used by the Root Bridge,

between issuing Configuration BPDUs3) Forward Delay: amount of time in learning and

listening states (half the time of transition fromblocking to forwarding)


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Management Parameters Bridge priority: a 2-octet value that

allows the network admin. to influence

the choice of the Root Bridge and theDesignated Bridge

Port Priority: a 1-octet value that allowsthe network admin. to influence the

choice of port when a bridge has twoports connected to the same LANsegment


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Why eliminate Loops? Loops cause traffic to build up in a

network until the network no longer

function due to full bandwidth usage

A BLAN Connection

Incoming broadcast

frame


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Performance Issues Two properties make bridge

performance crucial:

1) Lack of receipt of BPDUs causes bridgesto add connectivity. If a bridge does notreceive any Configuration BPDUs onsome port it will take over as theDesignated Bridge on that port.

2) Extra connectivity will cause loops


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What affects Bridge Performance? Network Congestion

Bridge will discard packets before looking

at them if CPU cant keep up

Bridge must be able to transmit BPDUsno matter how congested the network is

This involves being able to move BPDUs tothe front of the queue


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VLANs (Virtual Local Area Network) A means by which LAN users on different

physical LAN segments are afforded priorityaccess privilegesacross the LAN backbone in

order that they appear to be on the samephysical segment on an enterprise-level logicalLAN. VLAN solutions, which are priority innature, are implemented in LAN switches, and

VLAN membership is defined by the LANadministrator on the basis of either port addressor MAC address.

Definition ofVLAN from Newtons Telecom Dictionary.


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How VLANs work:1) LAN Bridge receives tagged data from workstation

2) Bridge reads current tag, and forwards data with a VLANID (tag) corresponding to the VLAN the data came from

(explicit tagging) OR

1) LAN Bridge receives untagged data from workstation

2) Bridge determines the VLAN membership of data by

noting the port on which it arrives (implicit tagging)


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Basic VLAN Concepts Port-based VLANs

Each port on a switch is in one and only one VLAN(except trunk links)

Tagged Frames VLAN ID and Priority info is inserted (4 bytes)

Trunk Links

Allow for multiple VLANs to cross one link Access Links

The edge of the network, where legacy devices attach

Hybrid Links Combo of Trunk and Access Links


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Basic VLAN Concepts(cont.)

Priority-tagged frame tag header carries priority info., but no

VLAN IDVLAN-tagged frame

tag header carries both VLAN ID andpriority info.

Port VLAN ID (PVID) provides the VID for untagged and priority-

tagged frames received on that Port


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Trunk Link

Attaches two VLAN-aware switches

Carries Tagged frames ONLY.


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Access Links

Access Links are Untagged for VLAN unawaredevices

The VLAN switch adds Tags to receivedframes, and removes Tags when transmitting

frames.


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VLAN ID (Tag)

4 Bytes inserted afterDestination and SourceAddress

Length/Type Field

VLANs = 0x8100

Priority Bit

Range: 0-7

VLAN ID

Range: 0-4094


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Tagging Conversions


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Port VLAN ID Each port has a VLAN ID configured on it

Indicates which VLAN untagged data

should be associated with

Does not constrain the port to a specificVLAN, nor does it mean that only

untagged data can be processed


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Sample VLANs


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Traffic Segregation


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Workgroups: Physically Defined A mobile user from

workgroup C, in

building 2, needs todo work in building 1.By physically changingbuildings he must

change the workgroupsection of the LANwhich he/she is in.


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VLANs: Logically Defined

With VLANs he/shecan physically

change buildings,but remain in thesame workgroup.


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Broadcast Domains (Layer 2)

broadcast domain: a network (or portion of anetwork) that will receive a broadcast packet

from any node located within that network broadcast packet: an Ethernet packet sent to

the broadcast address (FF:FF:FF:FF:FF:FF)which designates the packet as destined for

all nodes in the broadcast domain


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Constricting Broadcast Domains

What defines the edge of a layer 2broadcast domain?

Router: does not forward layer 2 broadcastframes

Filtering Database: by configuring thebroadcast address to be not forwarded

VLANs: broadcast packets are tagged sothey do not leave the configured topologyof the VLAN


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Security

Data is contained in the VLANs topology

By allotting sensitive data its own VLAN,

only those nodes in the VLAN will see it.


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GARP/GVRP

GenericAttribute Registration Protocol

GARPVLAN Registration Protocol


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How does GARP work?

Devices declare their desire for a givenattribute by making a declaration

Done by issuing a Joinevent Declarations can be withdrawn by issuing

a Leaveevent

Devices enter a registration for anattribute on a given port when they hear adeclaration for the attribute on that port


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GARP

General-purpose protocol that supportsa specific class of applications within

bridges Defines a subset of the spanning tree

that contains devices interested in a

given network commodity Referred to as an attribute


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GVRP - GARP VLAN Registration Protocol

Disadvantages to Static VLANs

Static VLANs are created via management

Must be maintained by a network admin Static VLANs must be reconfigured for

every network topology change


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GVRP Simplifies All This!

GVRP creates dynamic VLANs

No manual configuration needed

GVRP is maintained by the devicesthemselves

Topology change? No problem, GVRPrecreates the dynamic VLAN automatically


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What can GVRP do for you?

Allows the creation of VLANs with a specificVID and a specific port, based on updates

from GVRP-enabled devices. Advertises manually configured VLANs to

other GVRP-enabled device. As a result ofthis the GVRP-enable devices in the core of

the network need no manual configuration inorder to inter-operate.


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GVRP Info

GVRP is a GARP application thatregisters attributes for dynamic VLANs

GVRP deals only with the managementof dynamic VLANs

Everything that you have learned about

static VLAN packet format andtransmission applies


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How GVRP does all this:

The method of advertisement used byGVRP-enabled devices consists of

sending Protocol Data Units (PDUs),similar to Spanning Tree BPDUs, to aknown multicast MAC address (01 80 C2

00 00 21) to which all GVRP-enableddevices listen to for updates. GVRPadvertisement follows the definition ofGARP.


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What do these PDUs contain?

A single PDU may contain several differentmessages telling the GVRP-enabled device to

perform a specific action. Join: register the port for the specified VLAN

Leave: de-register the port for the specified VLAN

LeaveAll: de-register all VLAN registrations on that port

Empty: request to re-advertise dynamically andstatically configured VLANs

Windows screenshot >


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Industry Implementation Example 3Com manufactures Network Interface Cards that take

advantage of GVRP

Accessed via the Control Panel (DynamicAccess

)

Extremely easy to configure

Windows screenshot>

Vendors (current):

Cisco Systems, 3Com

and Hewlett Packard

Several others are

developing working

implementations also.


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Example: GARP/GVRP

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