Routing and routing protocols
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Routing and Routing Protocols. Introduction to Static Routing. Routing Decisions. Routing is the process that a router uses to forward packets toward a packet’s destination. Routing decisions are based on the destination IP address of a packet. Routing Methods.

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Routing and routing protocols

Routing and Routing Protocols

Introduction to Static Routing


Routing decisions

Routing Decisions

  • Routing is the process that a router uses to forward packets toward a packet’s destination.

  • Routing decisions are based on the destination IP address of a packet.


Routing methods

Routing Methods

  • Routers must learn the direction to remote networks in order to forward packets.

  • Two ways to learn this information:

    • Dynamically

      • Information is learned from other routers

        • Often through RIP, OSPF, or EIGRP routing protocols

    • Statically

      • Configured manually

        • Requires the network administrator to add and delete static routes when topology changes

        • In large networks it requires a tremendous amount of administrative time

        • On small, or unchanging networks, it requires very little maintenance


Static route operation

Static Route Operation

  • Static route operations can be divided into 3 stages:

    • A network administrator manually configures the static route on the router

    • The router installs the route in its routing table

    • Packets are routed using the static route


Configuring static routes

Destination Network

Subnet Mask

Local Router’s Outgoing Interface

Subnet Mask

Destination Network

Next Hop IP Address (address of next router)

Configuring Static Routes

Send traffic through an interface:

Send traffic to the next router’s address:


Configuring the outbound interface

Configuring the Outbound Interface


Configuring the next hop address

Configuring the Next-Hop Address


Routes and interfaces

Routes and Interfaces

  • If 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 distance

Administrative Distance

  • The 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

Default Administrative Distances


Multiple routes to the same destination

Multiple Routes to the Same Destination

  • When 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 backups

Static Routes as Backups

  • Often static routes are used for backup purposes, such as when the dynamically learned route fails.


Backup route example

Backup Route Example

  • For 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.

No Routing Protocol

192.168.0.0/24

192.168.2.1

192.168.2.2

192.168.1.2

192.168.1.1


Router0 routing table

Router0 Routing Table

No Routing Protocol

192.168.0.0/24

192.168.2.1

192.168.2.2

192.168.1.2

192.168.1.1


Backup route creation

Backup Route Creation

  • To configure a static route for the 56 kbps backup serial line:

No Routing Protocol

192.168.0.0/24

192.168.2.1

192.168.2.2

192.168.1.2

192.168.1.1


Router0 new routing table

Router0 New Routing Table

No Routing Protocol

192.168.0.0/24

192.168.2.1

192.168.2.2

  • Now, the static route shows up in the routing table, instead of the better route learned through RIP.

192.168.1.2

192.168.1.1


Fixing the routing table problem

Fixing the Routing Table Problem

No Routing Protocol

192.168.0.0/24

192.168.2.1

192.168.2.2

  • RIP’s 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.

192.168.1.2

192.168.1.1


The backup route in action

The Backup Route In Action

No Routing Protocol

192.168.0.0/24

192.168.2.1

192.168.2.2

  • When the RIP-learned route is unavailable, the static backup route is placed in the routing table.

192.168.1.2

192.168.1.1


The backup route in action1

The Backup Route In Action

No Routing Protocol

192.168.0.0/24

192.168.2.1

192.168.2.2

  • Once the better route is available, the RIP route will be re-entered into the routing table, automatically.

192.168.1.2

192.168.1.1


Configuring default route forwarding

Configuring Default Route Forwarding

  • Default 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 example

Default Routing Example

  • In this example, no routing protocol is used.

  • Instead, a default route will be used to allow Router0 to direct traffic to the ISP router.

No Routing Protocol

192.168.0.1

192.168.0.2

10.0.0.1

10.0.0.0/24


Router0 routing table prior to default route

Router0 Routing Table Prior to Default Route

  • Router0 can see the two directly connected networks and cannot route traffic to the Internet.

No Routing Protocol

192.168.0.1

192.168.0.2

10.0.0.1

10.0.0.0/24


Adding a default route

Adding a Default Route

  • Entering the quad-zero route on Router0 allows the router to send all remote traffic to the next-hop.

No Routing Protocol

192.168.0.1

192.168.0.2

10.0.0.1

10.0.0.0/24


Viewing the default route

Viewing the Default Route

No Routing Protocol

  • Entering the quad-zero route on Router0 allows the router to send all remote traffic to the ISP.

192.168.0.1

192.168.0.2

10.0.0.1

10.0.0.0/24


Default route note

Default Route Note

  • The 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.

No Routing Protocol

192.168.0.1

192.168.0.2

10.0.0.1

10.0.0.0/24


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