Routing Basics By Craig Lindstrom

Overview Routing Process Routing Process Default Routing Default Routing Static Routing Static Routing Dynamic Routing Dynamic Routing Routing Protocols Routing Protocols

The Router A router is a device that forwards data packets from one network to another. A router is a device that forwards data packets from one network to another. Hardware Based Routers Hardware Based Routers Device’s primary function is to route network traffic. Device’s primary function is to route network traffic. Usually have specialized ASICs to increase performance. Usually have specialized ASICs to increase performance. Cisco Systems Inc. Is major supplier. Cisco Systems Inc. Is major supplier. Software Based Routing Software Based Routing Routing is performed by a software add-on to a generic system (LINUX, Windows2000, etc.) Routing is performed by a software add-on to a generic system (LINUX, Windows2000, etc.) All routers must have more than one network interface. All routers must have more than one network interface.

Routing Process Detail 1. A data packet is received at a network interface. 2. The destination address is decoded 3. The address is compared to the routers addresses to see if the packet’s destination is the router. 4. If YES the router passes the data up the protocol stack.

Routing Process Detail cont. 5. If NO the router checks it’s routing table for the best route to the destination IP address a) If the destination network is directly connected to the router. The router forwards the packet to the destination host b) If the destination network is not directly connected to the router the packet is forwarded to the “best” next-hop router as determined by the routing table

Routing Process Detail cont. a) The routing process continues from router to router until the destination network is found. If the destination network is never found an error message is returned to the sending host.

Three Types of Routing Static Static All routes must be manually configured All routes must be manually configured Default Default One way street routing One way street routing Dynamic Dynamic Let the computer do the work Let the computer do the work

Static Routing Route table is manually built by the administrator. Route table is manually built by the administrator. Each router knows the networks that it is physically connected to. Each router knows the networks that it is physically connected to. The route to all networks not directly connected to the router must be configured by the administrator. The route to all networks not directly connected to the router must be configured by the administrator.

What Static Routes Are Needed? Lab Server TR / T0 T1 S0 S Host TR1 T0 S0 S / /

Default Routing Lets say we connect our small network to the internet. The internet has thousands of networks. It is not possible for an administrator to enter a route to every possible network Lets say we connect our small network to the internet. The internet has thousands of networks. It is not possible for an administrator to enter a route to every possible network Default routing is the answer. Default routing is the answer. A default route is the route used when a route to the destination network is not found in the route table A default route is the route used when a route to the destination network is not found in the route table

Default Route Example Router /24 IP Mask Default Gateway E0 DSL Host IP Mask Default Gateway

When to use a default route Default routes are used to “push” data from a stub network to a number of networks. Default routes are used to “push” data from a stub network to a number of networks. Most routers use a combination of a default route and other routes. Most routers use a combination of a default route and other routes. Hosts use a default route (the default gateway) so they don’t have to maintain a large route table. Hosts use a default route (the default gateway) so they don’t have to maintain a large route table. A network with only two routers can be configured using only default routes A network with only two routers can be configured using only default routes

How Can You Configure This Network Using Only Default Routes? Lab Server TR / T0 T1 S0 S Host TR1 T0 S0 S / /

Dynamic Routing Overview Dynamic routing uses routing protocols to exchange route information between routers. Dynamic routing uses routing protocols to exchange route information between routers. Each router always knows it’s connected interfaces. Each router always knows it’s connected interfaces. If two routers share a network they can exchange their known routes with each other. Combining the new routes with the router’s existing route table. After a few exchanges all routes are known. If two routers share a network they can exchange their known routes with each other. Combining the new routes with the router’s existing route table. After a few exchanges all routes are known.

Routing Protocols A routing protocol is a protocol that automatically discovers known routes and exchanges the route information with neighbor routers. A routing protocol is a protocol that automatically discovers known routes and exchanges the route information with neighbor routers. Some examples are: Some examples are: Routing Information Protocol (RIP) Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP) Interior Gateway Routing Protocol (IGRP) Open Shortest Path First (OSPF) Open Shortest Path First (OSPF)

Administrative Distance (Best Route) Route SourceDefault Distance Connected interface0 Static route1 Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5 External Border Gateway Protocol (BGP)20 Internal EIGRP90 IGRP100 OSPF110 Intermediate System-to-Intermediate System (IS-IS)115 Routing Information Protocol (RIP)120 Exterior Gateway Protocol (EGP)140 External EIGRP170 Internal BGP200 Unknown255

Summary All devices within a LAN can communicate directly with all other devices in the LAN. All devices within a LAN can communicate directly with all other devices in the LAN. Congestion occurs when too much data is trying to be sent on a given network. Congestion occurs when too much data is trying to be sent on a given network. Congestion is often solved by segmenting the network. Congestion is often solved by segmenting the network. Routing is the process of moving data packets between networks. Routing is the process of moving data packets between networks. Routers must maintain a route table (network map) to determine best route. Routers must maintain a route table (network map) to determine best route. Routing protocols are used to automatically maintain route tables. Routing protocols are used to automatically maintain route tables.