Chapter Three

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Networking and Routing Concepts

This chapter will walk you through the basic concepts of networking and routing in cisco. The administrative distance is an important aspect in networking and routing. There are some default administrative distances (AD) in the world of cisco, which are as under:

• The administrative distance for the connected interface is 0.
• The administrative distance for the static route is 1.
• The administrative distance for the internal EIGRP is 90.
• The administrative distance for the EIGRP summary route is 5.
• The administrative distance for the external border gateway protocol (eBGP) is 20.
• The administrative distance for the internal BGP(iBGP) is 200.
• The administrative distance for the external EIGRP is 170.
• The administrative distance for the Interior Gateway Routing Protocol (IGRP) is 100.
• The administrative distance for the Intermediate System-to-Intermediate System (IS-IS) Protocol is 115.
• The administrative distance for the RIP is 120.
• The administrative distance for the unknown is 255.

Change the Default Settings of the Administrative Distance

You can use some commands to change the OSPF route's administrative distance from the default settings with the help of the following commands.

• The command Georgia(config)#router ospf 1 will let you kick off the process of OSPF routing.
• The command Georgia(config-router)#distance 85 will let you change the OSPF distance from 110 to 85.
• The command Georgia(config-router)#distance 85 192.168.10.2 0.0.0.0 will let you apply the administrative distance of 85 to the OSPF routes that you receive from 192.169.10.2. This newly assigned administrative distance will be locally significant, and other routers will use the default administrative distance.
• The command Georgia(config-router)#distance 103 172.16.10.2 0.0.0.0 will let you change the OSPF distance from 110 to 103 for all the routes that come through 182.16.10.2.
• The command Georgia(config-router)#distance 85 172.16.20.0 0.0.0.255 2 will let you change the distance from 110 to 85 for all the routes that match ACL 2.
• The command Georgia(config-router)#exit will bring you back to the mode of global configuration.
• The command Georgia(config)#access-list 2 permit 192.168.30.0 0.0.0.255 will let you create the ACL that will help you understand which route will have an administrative distance of 85. You can use a named ACL and replace its number with the ACL name that is usually in command.

Permanent Keywords

The command Georgia(config)#ip route 192.168.50.0 255.255.255.0 serial0/0/0/0 permanent will let you create a static route that you cannot remove from the table even if you have shut down the interface. In the absence of a permanent keyword in the static route statement, the static route will stand removed, if your interface that is specified in the command moves down. An interface that is down will trigger the directly connected network and the associated static routes to get deleted from the table. When the interface is back up, the routes will definitely be returned. When you have added a permanent keyword to a static route statement, you will keep the static routes in the table even if the interface goes down. The interface remains down but the routes remain in the table. Its benefit is that when the interface gets back up, little need is usually left for the reprocessing of the static routes. This saves time and also the power that is usually consumed on processing.

IPv6 Address Assignment to Interface

You can use a bunch of commands for the assignment of different types of Ipv6 addresses to your interface.

• The command Georgia(config)#ipv6 unicast-routing will allow you to turn on the forwarding of the IPv6 unicast datagrams on a global scale across the router.
• The command Georgia(config)#interface gigabitetehernet0/0 will allow you to shift on to the configuration mode of your interface.
• The command Georgia(config-if)#ipv6 enable will allow you to kick off an IPv6 link-local address's automatic configuration process. It also enables the processing of IPv6 processing on your interface. The link-local address can be used to communicate with the nodes present on the same link.
• The command Georgia(config-if)#ipv6 address autoconfig will allow the router to configure itself automatically with the help of a link-local address. It does this with the help of a stateless auto configuration process.
• The command Georgia(config-if)#ipv6 address 2001::1/64 will allow you to configure a kind of global IPv6 address on your interface. It also lets you start the IPv6 processing on your interface. If you happen to add the global IPv6 address to your interface before you enter the ipv6 enable command, you will see witness the creation of a link-local address. In the end, the IPv6 will stand enabled on your interface.
• The command Georgia(config-if)#ipv6 unnumbered type/number will specify the unnumbered interface. It will enable IPv6 processing on your interface. The global IPv6 address of your interface that is usually specified by type/number will only be used as a source address for the packets that are sent from your interface.