Introduction to networking (Yarnfield) Introduction to routing

Objectives  Describe a routers primary aim  Differentiate between ports and interfaces  Describe the basic router configuration  Describe the routing table

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

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

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

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

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

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

Example

Routing table principles 1. Every router make sits decision alone, based on the information it has in its own routing table 2. The fact that one router has certain information in its routing table does not mean that other routers have the same information 3. 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

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

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

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

Routing factors  Switching function  Accept packet on one interface and switch it out of another 1. Decapsulate layer 3 packet 2. Examine destination IP address to find best path 3. 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

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

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