CCNA 2.1 Configure routing protocols given user requirements
Now we are in the meat of it. Part one of the objectives tended toward theoretical information. Now we are into the objectives that uses verbs like implement, configure, and customize. This is going to be a long one, so get comfortable.
Configure routing protocols given user requirements
The principle job of a router is to take in a packet, figure out where to send it next to get it closer to its destination, and send it out the appropriate interface. To do this, a router is configured with or builds a routing table containing network addresses and how to route to those networks (either by IP address or router interface). The majority of this objective, and a good part of the CCNA exam, is related to configuring a router to dynamically build the routing table.
There are four routing protocols in addition to static routing that you need to be comfortable with, all with their own jargon and configuration steps. Ultimately, the configuration steps for each are rather simple. The trick is understanding when to use one over the others and remembering the configuration nuances of each.
Multiple routing protocols may be in use at the same time on the same router. Each routing protocol is assigned an administrative distance, a number determining the reliability of the information. When multiple routes from different protocols are available to a destination, the router will select the route from the protocol with the lowest administrative distance.
Static Routes
Administrative distance: 1
Routing protocols are either static or dynamic. Static routes change only when manually edited. Changes to the network will not be reflected in the routing table of a router. Static routing alone is rarely used in large networks because of the administrative overhead, but their ease of configuration make them attractive for small networks with few or rarely changing routes or between networks connected by an expensive or slow WAN link. Static routes require no conversation between routers, so utilize no bandwidth. All dynamic routing protocols require some exchange of routing information between routers, thus utilizing the bandwidth between them.
Configuring a static route is discussed here.
Default route
The “gateway of last resort” is another term for the default route, an IP address that serves as the route used when a destination is not included in the routing table. The default route is a specialized form of static route and it’s configuration is similar.
Dynamic routing
Static routing does not scale well as networks increase in size. Dynamic routing protocols provide the scalability required to support large networks. There are numerous protocols, but for the exam it is sufficient to know RIP, IGRP, EIGRP, and OSPF. Each routing protocol has its own merits and configuration steps.
For each of these protocols, the important information is highlighted in the text (the benefits and drawbacks of each), a link is provided for basic configuration instructions, and another link for detailed information about the protocol. It is recommended not to get too deep into the behind-the-scenes information about the protocols. The important information here is to be able to select the appropriate routing protocol based on simplistic scenarios and to be able to perform the few commands necessary to configure each protocol. Avoid links to the more involved whitepapers on these topics (until you’re going for your CCNP.. or maybe CCIE).
RIP
Administrative distance: 120
Distance-vector (maximum hop count 15)
Route with fewest hops wins
High bandwidth use (full routing table updates every 30 seconds)
Configuration steps
Detailed information
OSPF
Administrative distance: 110
Configuration steps
Detailed information
IGRP
Administrative distance: 100
Proprietary (developed by Cisco)
Distance-vector (maximum hop-count 255)
Classful protocol (no subnet masks)
Configuration steps
Detailed information
EIGRP
Administrative distance: 90
Distance-vector and link-state hybrid
Classless protocol (supports variable length subnet masks)
Configuration steps
Detailed information