11

Routing TableRouting Table The seven fields

Mask: for finding (sub)network address of the destination

Host-specific routing: 255.255.255.255 (/32) Default routing: 0.0.0.0 (/0) Unsubnetted network: default mask of each class

Destination address: either host address or network address

Next-hop address: the address of the next-hop router

22

Routing Table (cont.)Routing Table (cont.) The seven fields (cont.)

Flags:

U (Up): the router is running, the router is down G (Gateway): indirect delivery, direct delivery H (Host-Specific): host-specific address, network address D (Added by redirection): the routing information has been added by a

redirection message from ICMP M (Modified by redirection): the routing information has been modified by

a redirection message from ICMP

Reference count: no. of users using this route at any moment

Use: no. of packets transmitted through router

Interface: the name of the interface

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Routing ModuleRouting Module1. For each entry in the routing table

1. Apply the mask to packet destination address

2. If (the result matches the value in the destination field)

1. If (the G flag is present)

1. Use the next-hop entry in the table as next-hop address

2. If (the G flag is missing)

1. Use packet destination address (direct delivery)

3. Send packet to fragmentation module with next-hop address

4. Stop

2. If no match is found, send an ICMP error message

3. Stop

44

ExamplesExamples Configuration for routing examples

55

Examples (cont.)Examples (cont.) Routing table for router R1 in the figure

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ExamplesExamples Make the routing table for router R1 in the following fig

ure.

There are three explicit destination networks, two class B and one class C with no subnetting. There is also one access to the rest of the Internet (default route) The routing table has four rows

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Examples Examples Solution

Mask Destination Next Hop I/F

255.255.0.0 134.18.0.0 -- m0

255.255.0.0 129.8.0.0 222.13.16.40 m1

255.255.255.0 220.3.6.0 222.13.16.40 m1

0.0.0.0 0.0.0.0 134.18.5.2 m0

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Interior and Exterior RoutingInterior and Exterior Routing

Popular routing protocols

99

Interior and Exterior RoutingInterior and Exterior Routing Autonomous systems

Interior routingInterior routing Exterior routingExterior routing

1010

RIP (Routing Information Protocol)RIP (Routing Information Protocol) Based on distance vector routing, which uses the

Bellman-Ford algorithm.

Distance Vector Routing

each router periodically shares its knowledge about the entire internet with neighbors

the operational principles of this algorithm

1. Sharing knowledge about the entire autonomous system

2. Sharing only with neighbors

3. Sharing at regular intervals

1111

RIPRIP

Routing Table

Distance Vector Routing Table

1212

RIPRIPThe routing table is updated upon receipt of a RIP

response message.

1313

RIPRIP Example of updating a routing table

1414

OSPFOSPF Link State Routing

OSPF uses Link State Routing to update the routing tables in an area

Each router shares its knowledge about its neighborhood with every router in the area.

1. Sharing knowledge about the neighborhood

2. Sharing with every other router by flooding

3. Sharing when there is a change

cf. Distance Vector Routing : sending the information at regular intervals regardless of change

So, every router can calculate the shortest path

between itself and each network

1515

OSPFOSPF Dijkstra Algorithm

calculating the shortest path between two points on a network using a graph made up of nodes and edges

1616

OSPFOSPF Routing Table

Each router uses the shortest path tree method to construct its routing table

Showing the cost of reaching each network in the area

To find the cost of reaching networks outside of the area, the routers use the summary link to network, the summary link to boundary router, and the external link advertisements

Link state routing table for router A