Enabling IP Routing
IP Routing
WAN connects different LANs with each other to communicate and to share the data and resources. For this, routers must be used on each network or at backbone to route the IP packets. For this, IP Routing should be enabled on the routers. Routers must learn the destinations that are not directly connected by building and maintaining routing tables. Once the routing table is built, the router switches packets by matching the destination address of an incoming packet with the "longest match" in the routing table.
The routing table can be constructed with entries from multiple sources, including:
- Connected routes
- Static routes
- Default routes
- Distance Vector learned routes (RIP, IGRP)
- Link State learned routes (OSPF)
- Learned routes from other dynamic routing protocols (IS-IS, EIGRP, BGP4, etc)
There are seven columns in a Cisco IP routing table:
The "show ip route" command allows you to selectively search for a specific network entry, or range of entries, in a routing table. For existing entries in a routing table, type "show ip route" with existing IP network or subnet and view the details about specific network.
For maintaining security of the network, ip aliasing routing tool can be used. To verify does ip provide routing services or not and to trace the events involve with building and maintaining an IP routing table, used the "debug ip routing" tool.
Types of IP Routing
A router can find best route to the destination by exchanging the routing information. This is possible only when any kind of IP Routing is enabled on the routers.
There are two types of IP Routing:
Static Routing
The Cisco IOS provides two types of static route statements. The first references a destination address with a local interface. This type of static route will appear as a connected route in the routing table. Since it appears as a connected route, it will be redistributed automatically into routing protocols whose network statements are associated with the destination address referenced in the static route. The second type of static route specifies the next hop address, instead of local interface. It will appear in the routing table with an administrative distance of one. It must be manually redistributed into dynamic routing processes.
A static route is appropriate when the Cisco IOS software cannot dynamically build a route to a specified destination prefix. Static routes are also useful when it is undesirable to use dynamic routing protocols.
Dynamic Routing
Dynamic routing basically uses a route that a routing protocol adjusts automatically for maintaining routing information and for topology or traffic changes.
Routing Protocols
A router uses routing protocols in order to maintain the routing tables and to communicate network changes with other routers. This routing information is used for sending the routed traffic. Routing protocols such as RIP and IGRP are used between routers to determine paths and maintain routing table. Once the path is determined a router can route a routed protocol such as IP.
The selection of routing protocol that is going to be employed in a network is based on the capabilities and features of that routing protocol. Each routing protocol has its own set of features and capabilities. A network administrator can select and deploy the routing protocol that meets with the network requirements.
Enabling IP Routing
By default, there is no type of IP Routing enabled on the Cisco IOS. You have to enable IP Routing manually on the router either by configuring static routing or by enabling a routing protocol.
Configuring Static Routing
Static routing is the most reliable type of routing, although it is not very scalable. The static IP Routing enabled no extra overhead that's why the cost of the network is comparatively reduced. The syntax of configuring an IP static route in global configuration mode is displayed below:
Router (config) # ip route prefix mask {address | interface} [distance] [permanent]i
Syntax Description:
Enabling a Routing Protocol
By default, there is no IP routing protocol is enabled on the Cisco IOS. All IP dynamic routing protocols are enabled by the following command of global configuration mode.
Router (config) # router ?
Bgp Border Gateway Protocol
Egp Exterior Gateway Protocol
Eigrp Enhanced Interior Gateway Routing Protocol
Igrp Interior Gateway Routing Protocol
IS-IS ISO IS-IS
Ospf Open Shortest Path First
Rip Routing Information Protocol
Beside RIP, you have to define a routing number in the configuration. The routing numbers are also called autonomous system number or process id. The routing number should be same as the routing protocol, because during the process of routing routers check routing number also.
At very least, dynamic IGP routing protocols must have at least one network statement in a given routing protocol mode. This network statement must match the network addresses configured on local interface to enable routing and remote access services.
The complete set of routing commands is:
Router (config) # router Eigrp 1
Router (config-router) # network 172.16.0.0
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