Introduction to networking ( Yarnfield )

1. Introduction to networking(Yarnfield) Introduction to routing

2. Objectives • Describe a router’s primary aim • Differentiate between ports and interfaces • Describe the basic router configuration • Describe the routing table

3. Router overview • Routers are the centre of a network • They can connect multiple networks • They are responsible for the efficient delivery of packets • Routers have interfaces that belong to different networks • Routers can forward packets to the network of the final destination, or to another router that is used to reach the final destination • Each network typically requires a separate physical interface

4. Routes determine the best path • The primary activity of the router is to forward packets to local or remote networks • This is done by • Determining the best path to send packets • Forwarding packets to their destination • This is achieved via the routing table • Received packets are referenced with the routing table • When a match is found the router will encapsulate the packet into the corresponding data-link frame of the outgoing interface and forward the packet toward its destination

5. Ports and interfaces • Ports usually refer to management ports, such as the console port • Interfaces usually refer to the connectors that send and receive user data • Interfaces come in two major categories • LAN interfaces • E.g. Fast Ethernet – must have an IP and subnet mask • WAN interfaces • ISDN, Frame relay – must have an IP address

6. AUX Task • You are looking at the back of a router and find a network cable which has a RJ-45 connector plugged into one of the ports/interfaces on the router. You read the label where the cable is plugged in and it says AUX above it. • Is AUX a port or an interface? • What is the purpose of AUX? • You have 10 minutes to come up with an answer

7. Forwarding packets • Routers are considered layer 3 devices because their forwarding decision is based upon the IP packet • Routing tables are searched for matching IP addresses based upon a received packet, and forwarded to the next router if no match has been found • Eventually a router will receive a packet that can forward it to a directly connected interface, which will be the packets final destination • Routers operate at layers 1, 2 and 3 of the OSI model

8. Switches/Hubs Task • You are using switches and hubs within your organisation. Your manager has approached you to ask you the following: • At what OSI layer(s) do switches work? • At what OSI layer(s) do hubs operate? • How do switches/hubs differ from routers? • You have 10 minutes to come up with the answers

9. Basic router configuration • Name the router • Set passwords • Console, AUX, telnet and secret • Configure interfaces • IP address, subnet mask, description (240 chars), clock rate, turn it on! • Banner • Save changes • Verify configuration • Show running-config • Show ip route • Show ip interface brief • Show interfaces

10. The routing table • Routing tables store information about directly connected and remote networks • They tell a router what the association between the ‘next hop’ and a network is • A directly connected network is one that is directly attached to the routers interface • This happens when the interface is configured with an IP address and subnet mask of a given network • A remote network is one where the router cannot connect to it without the help of another router • This is possible via static and dynamic routing protocols • Directly connected routes – visiting a neighbour • Static routes – used by trains • Dynamic routes – driving a car

11. Example

12. Routing table Task • Using the routing table from the previous slide, could you explain what will happen when the router receives a packet destined to the 172.16.3.0 network? • You have 10 minutes to come up with the answer and provide justifications for your logic

13. Routing table principles • Every router makes its decision alone, based on the information it has in its own routing table • The fact that one router has certain information in its routing table does not mean that other routers have the same information • Routing information about a path from one network to another does not provide routing information about the reverse, or return path • Asymmetric routing – packets can traverse a network in one direction, using one path and return through another path

14. Routing table principles 1 3 2 R2 R1 PC1 PC2 5 4 • PC1 sends ping to PC2 • RA has a route to PC2’s network • R2 is directly connected to PC2’s network • PC2 sends a reply ping to PC1 • R2 does NOT have a route to PC1’s network, so it drops the packet

15. Routing factors • Equal cost load balancing • Unequal cost paths (IGRP and EIGRP) • Packet forwarding involves • Path determination via metrics • Directly connected network – packet sent to host directly • Remote network – packet sent to next router • No route determined – packet discarded

16. Metrics • Bandwidth – the data capacity of a link • Delay – the length of time required to move a packet along each link from source to destination • Load – the amount of activity on a network resource such as a router or a link • Reliability – an error rate of each network link • Hop count – the number of routers that a packet must travel through before breaching its destination • Cost – an arbitrary value, usually base don bandwidth, monetary expense, or other measurement, that is assigned by an administrator

17. Routing factors • Switching function • Accept packet on one interface and switch it out of another • De-encapsulate layer 3 packet • Examine destination IP address to find best path • Encapsulate layer 3 packet into new layer 2 frame and forward frame out of exit interface • If TTL field reaches 0 then packet is discarded • Layer 3 IP addresses do not change during routing, but layer 2 data-link addresses change at every hop

18. Summary • Routers have multiple network interfaces representing multiple networks • The primary activity of a router is to forward packets via the best path • Routers use a routing table to determine the best path • Ports and interfaces refer to two different aspects • Routers work at layers 1, 2 and 3 of the OSI model, with layer 3 being the primary operating layer • Show ip route – shows the routing table • Each router interface must belong to a different network

19. Questions... • ...are there any?