Routing and Routing ProtocolsIntroduction to Static Routing

Routing DecisionsRouting is the process that a router uses to forward packets toward a packets destination.Routing decisions are based on the destination IP address of a packet.

Routing MethodsRouters must learn the direction to remote networks in order to forward packets.Two ways to learn this information:DynamicallyInformation is learned from other routersOften through RIP, OSPF, or EIGRP routing protocolsStaticallyConfigured manuallyRequires the network administrator to add and delete static routes when topology changesIn large networks it requires a tremendous amount of administrative timeOn small, or unchanging networks, it requires very little maintenance

Static Route OperationStatic route operations can be divided into 3 stages:A network administrator manually configures the static route on the routerThe router installs the route in its routing tablePackets are routed using the static route

Configuring Static RoutesSend traffic through an interface:

Send traffic to the next routers address:

Configuring the Outbound Interface

Configuring the Next-Hop Address

Routes and InterfacesIf a router cannot reach the outgoing interface that is being used in a route, the route will not be installed in the routing table.This means if that interface is down, the route will not be placed in the routing table.

Administrative DistanceThe administrative distance is a number that measures the trustworthiness of the source of the route information.The lower the administrative distance, the more trustworthy the source.If a path has the lowest administrative distance, it is installed in the routing table.

Default Administrative Distances

Multiple Routes to the Same DestinationWhen two or more routes point to the same destination, the administrative distance is used by the router to determine which route is entered into the routing table.All routes are remembered, but only the best route makes it into the routing table.

Static Routes as BackupsOften static routes are used for backup purposes, such as when the dynamically learned route fails.

Backup Route ExampleFor Router0, the preferred path to the 192.168.0.0 network is through the switch.If RIP is used to exchange routes between the two routers, this route will be marked as the best.192.168.0.0/24192.168.1.1192.168.1.2192.168.2.1192.168.2.2No Routing Protocol

Router0 Routing Table192.168.0.0/24192.168.1.1192.168.1.2192.168.2.1192.168.2.2No Routing Protocol

Backup Route CreationTo configure a static route for the 56 kbps backup serial line:192.168.0.0/24192.168.1.1192.168.1.2192.168.2.1192.168.2.2No Routing Protocol

Router0 New Routing Table192.168.0.0/24192.168.1.1192.168.1.2192.168.2.1192.168.2.2No Routing ProtocolNow, the static route shows up in the routing table, instead of the better route learned through RIP.

Fixing the Routing Table Problem192.168.0.0/24192.168.1.1192.168.1.2192.168.2.1192.168.2.2No Routing ProtocolRIPs default administrative distance is 120.Creating the route with an AD higher than 120 will ensure the RIP route is placed in the routing table.

The Backup Route In Action192.168.0.0/24192.168.1.1192.168.1.2192.168.2.1192.168.2.2No Routing ProtocolWhen the RIP-learned route is unavailable, the static backup route is placed in the routing table.

The Backup Route In Action192.168.0.0/24192.168.1.1192.168.1.2192.168.2.1192.168.2.2No Routing ProtocolOnce the better route is available, the RIP route will be re-entered into the routing table, automatically.

Configuring Default Route ForwardingDefault routes are used to route packets to destinations that do not match any of the other routes in the routing table.A default route is a special static route that uses the quad-zero format:ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]If the packet does not match a specific route in the routing table, it will be routed to the 0.0.0.0 network.Any IP address will always yield the network address 0.0.0.0 when ANDed with the mask 0.0.0.0.

Default Routing ExampleIn this example, no routing protocol is used.Instead, a default route will be used to allow Router0 to direct traffic to the ISP router.10.0.0.0/2410.0.0.1192.168.0.2192.168.0.1No Routing Protocol

Router0 Routing Table Prior to Default RouteRouter0 can see the two directly connected networks and cannot route traffic to the Internet.10.0.0.0/2410.0.0.1192.168.0.2192.168.0.1No Routing Protocol

Adding a Default RouteEntering the quad-zero route on Router0 allows the router to send all remote traffic to the next-hop.10.0.0.0/2410.0.0.1192.168.0.2192.168.0.1No Routing Protocol

Viewing the Default RouteEntering the quad-zero route on Router0 allows the router to send all remote traffic to the ISP.10.0.0.0/2410.0.0.1192.168.0.2192.168.0.1No Routing Protocol

Default Route NoteThe routers on the Internet must have a route in their routing tables that points to Router0 (or a default route that accomplishes the same).Otherwise, Router0 will be able to route traffic to servers on the Internet, but the Internet routers would not be able to route the responses back to Router0.10.0.0.0/2410.0.0.1192.168.0.2192.168.0.1No Routing Protocol