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Routing

Institute of IT

University of Peshawar

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Connectionless Forwarding

Each router (switch) makes a LOCALdecision to forward the packet towards B

A B

R1 R4

R2

R3

R6

R7

R5

R8

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Connectionless Forwarding

This is termed destination-basedconnectionless forwarding

How does each router know the correct local forwarding decision for any possible destination

address?

Knowledge of the topology state of the network

AND

knowledge is maintained by a routing protocol

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Routing Protocols

Distribute the knowledge ofthe currenttopology state ofthe networktoallrouters

This knowledge is used by each router togenerate a forwarding table, which

contains the local switching decisionfor each known destination address

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Routing Protocols

ManagementofqualitynetworkServicesrequires:

accuracyofthe routinginformation

dynamic adjustmentofthe routinginformation

matchingaggregate traffic flow tonetworkcapacity

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Exterior routing

ISP Routing Tasks

customers

internal

peer / upstream

Interior routing

Customer routing

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Interior Routing

Discovers the topology of a networkthrough the operationof a distributedrouting protocol

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A

20

10

4

10

40

5

6

10

15

10

5

B

5

5

45

R1 R4

R2

R3

R6

R

R5

R8

Path Selection

Minimum cost from A to B is 39 units

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A

20

10

4

10

40

5

6

15

10

5

B

5

5

45

R1 R4

R2

R3

R6

R

R5

R8

Dynamic Path Adjustment

IfR5 R7 breaks, minimum cost path from A to B is

Now 46 units

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Interior Routing Protocols

Describe the currentnetwork topology

Routing protocols distribute how to reachaddress prefix groups where

Routing protocols function through either Distributed computing model (distance vector)

Parallel computing model (link state)

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Routing Protocols

Distance Vector Routing Protocols

Each node sends its routing table (dest,distance) to all neighbors every 30 seconds

Lower distances are updated with the

neighbor as next hop

cannot scalecannot resolve routing loops

quicklyRIPis the main

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Routing Protocols

Link State Routing Protocols Each link, the connected nodes and the metric is flooded

to all routers

Each link up/down status change is incrementallyflooded

Each router re-computes the routing table in parallelusing the common link state database

OSPF is the main protocol in use today

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Interior and Exterior

Routing Protocols

Exterior Routing Space

Interior

Route

Space

InteriorRoute

Space

Interior

Route

Space

Interior

Route

Space

Interior

Route

Space

AS1 AS1221

AS2402

AS3561

AS701

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The Routing Process(NOT A Simple Process)

HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

A simple scenario of two host and one

router where HOST Apings Hosts B IPAddress

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HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

The Routing Process(NOT A Simple Process)

STEP 01: ICMP creates an ECHO Message Payload

STEP 02: ICMP hands over the payload to Internet Protocol for creating a

packet(source ,destin address and protocolfield) that tell the host to

whom it should hand the payload when the destination is reached

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HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

The Routing Process(NOT A Simple Process)

STEP 03: After packet creation IP works with ARP to determine whether

the destination IP host is on the same network or other(Remote in this case)

STEP 04: Since result of step 03 is remote host, the packet is send to the

default gateway The registry in the Windows is parsed to find the

configured default gateway.

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HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

The Routing Process(NOT A Simple Process)

STEP 05: The default gateway of host A(172.16.10.2) is configured to 172.16.10.1but to send this packet HARDWARE Address of router Ethernet 0 is to be

known so that the packet can be handed over to Data Link Layer

STEP 06: So address Step 05, ARP cache is checked to see if the IP

address of the default gateway is resolved to hardware address

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HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

The Routing Process(NOT A Simple Process)

STEP 06 (a): if have packet is send to Data link for framing

STEP 06(b): if not ARP broadcast of Local network for h/w addressof 172.16.10.1. The router responds and the host caches the address.

The router also caches the h/w address of Host A in ARP Cache

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HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

The Routing Process(NOT A Simple Process)

STEP 7: At DLink Layer , LAN driver is used to provide media access A FRAis generated

STEP 8: Once frame is completed . Handed over to physical layer to be

put on the physical medium

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HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

The Routing Process(NOT A Simple Process)

STEP 9: Bits received by every device in the network and builds the frame andbuild CRC to check answer in the FCS.. If crc matched then H/w destination

address is checked to see if it is matched and if so. Then ethernet type field is

checked to find the protocol used

STEP 10: Packet is pulled from the frame and frame is discarded

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HOST A

172.16.10.2

HOST B

172.16.20.2

Ethernet 0

172.16.10.1

Ethernet 1

172.16.20.1

The Routing Process(NOT A Simple Process)

STEP 11: IP receives check IP address since doesnt match router looks

For the destination network address in its routing table

STEP 12:if no entry of the network in the routing table an ICMP messag

to HOST A with message of destination Network not available