Bridges Routers (1)

Bridges, Routers and SwitchesBridges, Routers and SwitchesPart One Part One

Three things are certain:

Death, taxes, and lost data.

Guess which has occurred.Haiku error message

BridgingBridging

Bridges connect two independent LANs to form internetworks

Bridge

Types of BridgesTypes of Bridges

Transparent Source Routing Transnational

BridgesBridges

Each LAN is referred to as a Network Segment

Bridge

Blue NetworkSegment

Red NetworkSegment

Brige Port

Bridge PortsBridge Ports

Bridge Ports connect the Bridge to the Network

Consist of a Network Interface (Ethernet, Token Ring, FDDI)

Bridges Form One Logical NetworkBridges Form One Logical Network

IP Address178.27.0.32

Bridge

IP Address178.27,0,12







IP Address178.27.0.92 IP Address

178.27.0.2

178.27.0.99

178.27.0.88

One Logical Network

Bridging and ISO ModelBridging and ISO Model

Bridges function at at the Media Access (MAC) layer

APPLICATION

PHYSICAL

NETWORK

TRANSPORT

SESSION

PRESENTATION

Logical Link Control

Media Access Control

PHYSICAL

Bridge


Bridges “see” the network at the MAC layer

Bridges make forwarding decisions based on MAC (Physical) addresses

PHYSICAL

Media Access Control

PHYSICAL

Bridge


Bridges have no knowledge of paths between address

Bridges are transparent to higher level protocol

Bridges are protocol independent, the same bridge can pass IP, IPX, DecNet traffic

Transparent BridgesTransparent Bridges

Usually used on Ethernet networks Called Transparent because nodes are

not aware of its presence Defined by IEEE 802.1 standard Forwards frames between networks Self learning


Maintains a data base of all known node address on each port

Bridge

CA

B

text

Port C Data Base5ae4ff0b....39873ad...a2367b3...

text

Port B Data Base11e4fa0b....2d873ad...40367b3...

text

Port C Data Base09e4ff0b....86873ad...a4367b3...


Transparent Bridges are self learningExamine every frame which passes a portDetermines the source addressCompares each address to its data baseIf address is not in data base then the bridge

adds the address

Transparent BridgesTransparent BridgesExamines Source

address to determineif it is known

NoAdds to

DataBase

DeterminesDestination Address

Is DestinationAddress on sameLAN as Interface

Ignores FrameLooks up Destinationaddress in Data Base

Is address known?

Transmits Frame Outthe Approrate Port

Yes

Yes

No

Yes

Transmits Frame Outall PortsNo

Transparent Bridge Frame ForwardingTransparent Bridge Frame Forwarding

Examines Destination addressIf address is on the same LAN as the incoming

port, ignores the frame

If NotLooks up destination address in its Data Base

If Address is foundForward out the appropriate Port

If Address is not foundForwards out all Ports

Spanning Tree AlgorithmSpanning Tree Algorithm

Bridge Bridge

Bridge

Address 11

Address 22

Address Data Base11

Address Data Base11


Bridge Bridge

Bridge

Address 11

Address 22

Address Data Base11

Address Data Base11

Address Data Base11

Address Data Base11


Bridge Bridge

Bridge

Address 11

Address 22

Address Data Base11

Address Data Base11

Address Data Base11

Address Data Base11

Address Data Base11

Address Data Base11


Bridge Bridge

Bridge

Address 11

Address 22

Address Data Base11

Address Data Base11

Address Data Base11

Address Data Base11

Address Data Base11

Address Data Base11

Eliminates all active loops in the network by creating a set of paths with only one path between each node

Bridges negotiate which paths are to remain open and which paths will be blocked

If a path becomes inaccessible, the bridges will reconfigure the network, creating a new spanning tree and new set of paths


Source Route BridgingSource Route Bridging

Only used in Token Ring environments

Source RouteBridge

Ring 1 Ring 2

Ring 3

Source RouteBridge

Bridge 1

Bridge 2


NOTE: Despite its name Source Routing is Bridging and takes place at the MAC layer


Node maintain their own routing table When a frame is transmitted to a node

on another ring, the packet must contain the route or path

Node “discovers” the route by sending out an Explorer Frame with the address of the destination Node

Depending on the protocols used the node may send an All Route Exploder Frame or a Singe Route Explorer Frame


The route explorer frame records the Bridge Number and Ring Number of all bridges and networks crossed

When it reaches the destination node, the destination node sets a flag and transmits the frame back to the sending node

The Route Explorer Frame now has the route to the destination station

If two Routes return to the sending node, it usually uses the first route received


Source RouteBridge

Ring 1 Ring 2

Ring 3

Source RouteBridge

Bridge 1

Bridge 2

Destination Node

Ring 6

Source Node

Ring3,Bridge2

Ring3,Bridge2:Ring2Bridge1Ring3,Bridge2:Ring2Bridge1:Ring1

Ring3,Bridge2:Ring2Bridge1:Ring6


Route


Source Route Bridges can connect rings running at different speeds

Source RouteBridge

Ring 14Mbps

Ring 24Mbps

Bridge 1

What’s Wrong with Source Routing?What’s Wrong with Source Routing?

Takes processing resources of end nodes

Each Ring and Bridge must be assigned a Ring Number and Bridge Number - Any error in assigning these numbers causes fatal errors

Fatal errors happen often


Source RouteBridge

Ring 1 Ring 2

Ring 3

Source RouteBridge

Bridge 1

Bridge 2

Destination Node

Ring 6

Source Node

Ring3,Bridge2

Ring3,Bridge2:Ring2Bridge1Ring3,Bridge2:Ring2Bridge1:Ring1



Route

Frame duplicates Itself andcrosses explores both

Rings


Allows loops How many paths though this network?

Source RouteBridge

Ring 1 Ring 2

Ring 3

Source RouteBridge

Bridge 1

Bridge 2

Destination Node

Ring 6

Source Node

Route

Source RouteBridge

Bridge 3


What else is wrong with Source Routing?

What is good about Source Routing?

Source Route and Transparent Bridges in Source Route and Transparent Bridges in Same NetworkSame Network

Usually not very reliable - may work in some simple networks

Source Routed traffic can only cross Source Route Bridges

Transnational BridgesTransnational Bridges

Connect two dissimilar topologies

text

Translational Birdge

Advantages of BridgingAdvantages of Bridging

Easy to install and configure Inexpensive Transparent to Network Automatically adapt to Network

Changes Bridge unroutable protocols

Disadvantages of BridgesDisadvantages of Bridges

Can not take advantage of redundant paths in the network

Can not prevent Broadcast Storms Because bridges form a single logical

network the network becomes large and therefore difficult to manage

Routers What makes a Protocol RoutableRouters What makes a Protocol Routable

To be routable a protocol must assign addresses so that the address contain Network and Node address

Routable Protocols includeIPIPXDecNetVines

Non-Routable ProtocolsNetBios

RoutersRouters

Function of a Router is to efficiently forward packets between networksVery often more than one path exists, a router

must chose the “best” path

Routers function at the Network layer of the OSI Model, routing decisions are made using the address assigned by a Network layer protocol such as IP

Network Address Node Address

RoutersRouters

Since routers function at the Network layer routers are protocol dependent devicesMust support IP, IPX, AppleTalk, DecNet, etc.

APPLICATION

PHYSICAL

DATALINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

PHYSICAL

DATALINK

ROUTER

APPLICATION

PHYSICAL

DATALINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

RoutersRouters

Multiple ProtocolsMost routers can route a verity of protocolsThe ability to route multiple protocols is

implemented in software

RouterOperatingSystems

IPRoutingModule

IPXRoutingModule

RoutersRouters

Token-ring

Ethernet

Router

EthernetRouter

Ethernet

Frame Relay

RoutersRouters

Routers can connect dissimilar LAN topologies

Routers can connect LANs to the WAN

Routers - Why use RoutersRouters - Why use Routers

Segment Networks Congestion Control

Source Quench technique - Router can request a source to stop transmitting until congestion is relieved

Ethernet

Ethernet

Ethernet

Router

RoutersRouters

Bridges examine each frame on the network segment

Routers only receive packets addressed to them

Packets are addressed to routers from workstations or other routers

Routers Node ConfigurationRouters Node Configuration

Router Rest of World

IP Address185.24.36.109

IP Address145.235.21.100

Router


IP ConfigurationMy IP Address:145.235.34.54Supnet Mask255.255.0.0Default Router145.235.21.100

Want to sendmessage to

185.24.36.109Is it on mynetwork?

NoSend to

145.235.21.100

Routers Routing TablesRouters Routing Tables

Routers maintain a Routing Table The Routing Table instructs the router

where (out which interface) to send the packet

Packets are routed one hop at a time

Routers Routing TableRouters Routing Table

Net 3

Net 4

Net 2

Net 5

Net 1

Router1Router2

Net 1

Net 2

Net 3

Net 4Net 5

Int 1

Int 2

Int 3

int 1

Int 2

Int3

Routing Table Router 1Net 1 Int1Net 5 int2Net2 int2Net3 Int 2Net4 int 2

Int 4

Routing Table Router 2

Routers - Multiple RoutesRouters - Multiple Routes

Net 4

Net 3

Net 2Net 1

Router 1

Router 2 Router 3

Routers Multiple RoutesRouters Multiple Routes

Routing Table Router 1Target Net Next Hop Router MetricNet4 Attached 0Net 1 Router 2 1Net 1 Router 3 2Net 3 Router 3 1Net 3 Router 2 2Net 2 Router 3 1Net2 Router 2 2

Net 4

Net 3

Net 2Net 1

Router 1

Router 2 Router 3

Routers Metric ValuesRouters Metric Values

Chose route with lowest Metric value Metric value can represent:

HopsTransmission DelayAdministrative Considerations (Cost)Line Capacity

Routers - Backup LinksRouters - Backup Links

Router Router

DSU/CSU DSU/CSU

Leased Line

ISDN Backup Connection (Dial on Demand)

Metric Leased Line 1Metic ISDN Line 10

Routers - Routing Protocol Routers - Routing Protocol

Routing Tables are dynamically created by the router

Router communicate with other routers to learn all paths through the network

This communication consists of all or portions of each routers routing table

Routers - Routing ProtocolRouters - Routing Protocol

Routers use these Routing Tables to build a map of the network topology

The time it takes for all routers in a network to exchange routing tables is called Convergence

Router to Router communication is implemented according to Routing Protocols

Routers - Routing Protocol Design GoalsRouters - Routing Protocol Design Goals

Ideally Routing Protocols would meet all these criteria:Be able to find the optimal pathSimple enough algorithm to require a minimum

of processing power and overheadRobust enough to function when the

unexpected happens, such as hardware failures

Rapid convergence - failure to converge quickly can cause (temporary) network outages

There a numerous Routing Protocol, each has its strengths and weaknesses

Routers - Routing ProtocolsRouters - Routing Protocols

Two types of Routing ProtocolsDistance-Vector AlgorithmLink-State Algorithm

Routers - Distance-Vector AlgorithmRouters - Distance-Vector Algorithm

Each router calculates the metric value of the links to all its neighbors

Each router then sends a broadcast message containing its entire routing table to its neighboring routers

The receiving station then uses its neighbor’s routing table to calculate a new routing table

The router then broadcasts its new routing table to its neighbors

Routers - Distance-Vector AlgorithmRouters - Distance-Vector Algorithm

Routers use the metric values received to calculate the shortest path through the network

The routers do not know how the network segments are connected

This algorithm results in slow convergence

The exchange of Routing Tables uses a high amount of bandwidth

Routers - Link State Algorithm ProtocolsRouters - Link State Algorithm Protocols

Each router broadcasts the portion of its Routing Table which describes its links to its neighboring routersThis description includes links and metric

values

From this information routers can determine the entire network topology - That is they know how all the segments are connected together

Routers - Link State Algorithm ProtocolsRouters - Link State Algorithm Protocols

Router use their knowledge of the network topology to calculate the optimum path though the network

When a router detects any changes in network topology it broadcasts an update to other routers

Converge faster than Distance Vector protocols

More CPU/memory intensive than Distance Vector Protocols

RoutersRoutersInterior Router

Interior RouterInterior Router

Interior Router

Interior RouterInterior Router

Interior Router

Boarder GatewayRouter

Boarder GatewayRouter

AutonomousSystem

AutonomousSystem

Interior Routers routewithin an AutonomousSystemRun Interior Routing Protocols

Boarder Gateway Routersroute between AutonomousSystemsRun Boarder Routing Protocols

Routers - Routing Information Protocol - Routers - Routing Information Protocol - RIPRIP

Distance Vector protocol Interior Gateway Protocol One of the original Routing Protocols -

Very widely used

Routers - Routing Information ProtocolRouters - Routing Information Protocol

Sends routing updates at regular intervals

Uses a Hop Count for metric value Each Hop usually adds one Hop to the

metric value Uses IP address as next hop There is limit of 15 hops between source

and destinationPrevents routing loops

Routers - Interior-Gateway Routing Routers - Interior-Gateway Routing Protocol IGRPProtocol IGRP

Developed by Cisco Systems Distance Vector Interior routing protocol Uses a combination of Inter-network

delay, bandwidth, reliability and load to determine the best pathA metric value is assigned to each of these

factorsThe weight of each factor is user definable

Routers - Interior-Gateway Routing Routers - Interior-Gateway Routing ProtocolProtocol

Permits Multi-path routing Lines of equal-bandwidth will be used in

equal amounts Lines with unequal bandwidth will be

used in proportion to its bandwidth, that is, a line with twice the bandwidth will be given twice the amount of traffic

Only routes with metrics within a specified range of the best route will be used as multiple routes


Router Router

DSU/CSU DSU/CSU

DSU/CSUDSU/CSU

t1

t1

Packet

Packet


Routing updates are sent at regular intervals

When a router goes down its neighbors are able to detect this by their failure to receive a routing update

The router then recalculates it routing table based on this information - all routes though that router are now invalid

The router then sends out its routing table at the scheduled interval


Network Convergence takes time, during this interval a router which is not aware of the invalid routes will broadcast its routing table containing the now invalid routes, this table can find its way to a router which has already received the data about the downed router. This router now replaces its current routing table with an outdated one.


Hold Down TimeHold down times are used to prevent a router

from using a route which may have gone downRouters “hold down” any changes which may

affect routes for a specified period of timeHold down times are usually calculated to be

greater than the network convergence time


RouterHarry

Router"Sam"

Router"Sue"

Router Harryreceives informationthat router "bob" isdown. It updates itsrouting table and at10:00 it broadcasts arouting update

Router "Sam"has not yetheard about"bob" stillincludes routesthrough "bob" inits routingtables. Sendsrouting table torouter "sue" at10:01

At 10:00 receives update fromHarry. Updates it routing tableAt 10:01 receives update form

Sam. Updates its routing Table

RouterBob


Split HorizonRouters never send information about a route

back in the same direction it came fromThere is no reason for Sue to send Harry

information about Bob because Harry is closer to Bob

Routers - Boarder Gateway Protocol BGPRouters - Boarder Gateway Protocol BGP

Routes between autonomous systems Only exchanges information with other

BGP routers When a BGP router first joins the

network routers exchange their entire routing table after that they exchange incremental information

Responsible for exchanging information about the ability to reach other networks

Documents

Bridges Routers (1)