1 Routing Table  The seven fields Mask: for finding (sub)network address of the destination l Host-specific routing: (/32) l Default routing: (/0) l 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

2 Routing Table (cont.)  The seven fields (cont.) Flags: l U (Up):  the router is running,  the router is down l G (Gateway):  indirect delivery,  direct delivery l H (Host-Specific):  host-specific address,  network address l D (Added by redirection):  the routing information has been added by a redirection message from ICMP l 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

3 Routing Module 1. 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

4 Examples  Configuration for routing examples

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

6 Examples  Make the routing table for router R1 in the following figure. 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

7 Examples  Solution MaskDestinationNext HopI/F m m m m0

8 Interior and Exterior Routing  Popular routing protocols

9 Interior and Exterior Routing  Autonomous systems Interior routing Exterior routing

10 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

11 RIP  Routing Table Distance Vector Routing Table

12 RIP  The routing table is updated upon receipt of a RIP response message.

13 RIP  Example of updating a routing table

14 OSPF  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

15 OSPF  Dijkstra Algorithm calculating the shortest path between two points on a network using a graph made up of nodes and edges

16 OSPF  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