CCNA Expl 2. Chapter 3. Dynamic routing protocols

CCNA Exploration 2.
Routing protocols and Consepts.
Chapter 3. Resume. Dynamic routing protocols 

Routing protocols determine the best path to each network which is then added to the routing table. 

Primary benefits to using a dynamic routing protocol:  

             Routers exchange routing information whenever there is a topology change.

             => routers  automatically learn about new networks 

             => find alternate paths when there is a link failure to a current network.

The purpose of a routing protocol includes:

          ~ Discovery of remote networks

          ~ Maintaining up-to-date routing information

          ~ Choosing the best path to destination networks

          ~ Ability to find a new best path if the current path is no longer available

In general, the operations of a dynamic routing protocol can be described as follows: 

          1.The router sends and receives routing messages on its interfaces.

          2. The router shares routing messages and routing information with other routers that are using the 

              same routing protocol.

          3. Routers exchange routing information to learn about remote networks. 

          4. When a router detects a topology change the routing protocol can advertise this change to other 

              routers.

Static routing advantages:	Static routing disadvantages:
·        Minimal CPU processing. ·        Easier for administrator to understand. ·        Easy to configure.	Ё Configuration and maintenance is time-consuming. Ё Configuration is error-prone, especially in large networks. Ё Administrator intervention is required to maintain changing route information. Ё Does not scale well with growing networks; maintenance becomes cumbersome. Ё Requires complete knowledge of the whole network for proper implementation.
Dynamic routing advantages:	Dynamic routing disadvantages:
·        Administrator has less work maintaining the configuration when adding or deleting networks. ·        Protocols automatically react to the topology changes. ·        Configuration is less error-prone. ·        More scalable, growing the network usually does not present a problem.	ЁRouter resources are used (CPU cycles, memory and link bandwidth). Ё More administrator knowledge is required for configuration, verification, and troubleshooting.

An autonomous system (AS) - otherwise known as a routing domain - is a collection of routers under a common administration. 

Interior Gateway Protocols (IGP) are used for intra-autonomous system routing - routing inside an autonomous system.

Exterior Gateway Protocols (EGP) are used for inter-autonomous system routing - routing between autonomous systems.

Interior Gateway Protocols (IGPs):

Distance vector routing protocols  Routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric. work best: 1.    The network is simple and flat and does not require a special hierarchical design. 2.    The administrators do not have enough knowledge to configure and troubleshoot link-state protocols. 3.    Specific types of networks, such as hub-and-spoke networks, are being implemented. 4.    Worst-case convergence times in a network are not a concern.

Link-state routing protocols Router can create a "complete view" or topology of the network by gathering information from all of the other routers. work best: 1.    The network design is hierarchical, usually occurring in large networks. 2.    The administrators have a good knowledge of the implemented link-state routing protocol. 3.    Fast convergence of the network is crucial.

       Classful routing protocols do not send subnet mask information in routing updates.  

Classful routing protocols cannot be used when a network is subnetted using more than one subnet mask,

        L imitations to classful routing protocols 

                          ~  do not support variable length subnet masks (VLSM)

                          ~  do not  support discontiguous networks.

Classless routing protocols include the subnet mask with the network address in routing updates. 

                         ~  support for VLSM, discontiguous networks 

     The network has converged when all routers have complete and accurate information about the network. Convergence time is the time it takes routers to share information, calculate best paths, and update their routing tables.     

       Each routing protocol uses its own metric.The metric is used to determine which path is most preferable when there are multiple paths to the same remote network.    

Metrics used in IP routing protocols include:

• Hop count - A simple metric that counts the number of routers a packet must traverse
• Bandwidth - Influences path selection by preferring the path with the highest bandwidth
• Load - Considers the traffic utilization of a certain link
• Delay - Considers the time a packet takes to traverse a path
• Reliability - Assesses the probability of a link failure, calculated from the interface error count or previous link failures
• Cost - A value determined either by the IOS or by the network administrator to indicate preference for a route. Cost can represent a metric, a combination of metrics or a policy. 

Load balancing is in effect if two or more routes are associated with the same destination.

The packets are forwarded using all equal-cost paths. 

Administrative distance (AD) defines the preference of a routing source. The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed. 

The show ip rip database command shows all RIP routes learned by R, whether or not the RIP route is installed in the routing table.

Directly connected networks appear in the routing table as soon as the IP address on the interface is configured and the interface is enabled and operational.