Dynamic Routing Protocols

Dynamic Routing Protocols Classification

Routing protocols can be classified into different groups according to their characteristics. The most commonly used routing protocols are:

• RIP - A distance vector interior routing protocol 
• IGRP - The distance vector interior routing developed by Cisco (deprecated from 12.2 IOS and later)
• OSPF - A link-state interior routing protocol 
• IS-IS - A link-state interior routing protocol
• EIGRP - The advanced distance vector interior routing protocol developed by Cisco
• BGP - A path vector exterior routing protocol 

IGP and EGP

An autonomous system (AS) - otherwise known as a routing domain - is a collection of routers under a common administration. Because the Internet is based on the autonomous system concept, two types of routing protocols are required:

• 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.

Distance Vector and Link State

Interior Gateway Protocols (IGPs) can be classified as two types:

• Distance vector routing protocols 
• Link-state routing protocols

Distance vector means that routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric such as hop count and direction is simply the next-hop router or exit interface. Distance vector protocols typically use the Bellman-Ford algorithm for the best path route determination. 

Some distance vector protocols periodically send complete routing tables to all connected neighbors. Distance vector means that routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric such as hop count and direction is simply the next-hop router or exit interface. Distance vector protocols typically use the Bellman-Ford algorithm for the best path route determination. 

Some distance vector protocols periodically send complete routing tables to all connected neighbors. The only information a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Distance vector routing protocols do not have an actual map of the network topology.  

In contrast to distance vector routing protocol operation, a router configured with a link-state routing protocol can create a "complete view" or topology of the network by gathering information from all of the other routers. Using a link-state routing protocol is like having a complete map of the network topology. Link-state routing protocols do not use periodic updates. After the network has converged, a link-state update only sent when there is a change in the topology

Classful and Classless

Classful routing protocols do not send subnet mask information in routing updates. The first routing protocols such as RIP, were classful. This was at a time when network addresses were allocated based on classes, class A, B, or C. A routing protocol did not need to include the subnet mask in the routing update because the network mask could be determined based on the first octet of the network address. 

Classful routing protocols cannot be used when a network is subnetted using more than one subnet mask, in other words classful routing protocols do not support variable length subnet masks (VLSM). 

There are other limitations to classful routing protocols including their inability to support discontiguous networks.

Classless Routing Protocols

Classless routing protocols include the subnet mask with the network address in routing updates. Today's networks are no longer allocated based on classes and the subnet mask cannot be determined by the value of the first octet. Classless routing protocols are required in most networks today because of their support for VLSM, discontiguous networks and other features.

Classless routing protocols are RIPv2, EIGRP, OSPF, IS-IS, BGP

Convergence

Convergence is when all routers' routing tables are at a state of consistency. 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. A network is not completely operable until the network has converged. Convergence is both collaborative and independent. The routers share information with each other but must independently calculate the impacts of the topology change on their own routes. Generally, RIP and IGRP are slow to converge, whereas EIGRP and OSPF are faster to converge. 

Purpose of a Metric

There are cases when a routing protocol learns of more than one route to the same destination. To select the best path, the routing protocol must be able to evaluate and differentiate between the available paths. For this purpose a metric is used. A metric is a value used by routing protocols to assign costs to reach remote networks. The metric is used to determine which path is most preferable when there are multiple paths to the same remote network. Each routing protocol uses its own metric. For example, RIP uses hop count, EIGRP uses a combination of bandwidth and delay. Hop count is the easiest metric to envision. The hop count refers to the number of routers a packet must cross to reach the destination network.

Different routing protocols use different metrics. The metric used by one routing protocol is not comparable to the metric used by another routing protocol. Two different routing protocols might choose different paths to the same destination due to using different metrics.

Example:

RIP would choose the path with the least amount of hops, whereas OSPF would choose the path with the highest bandwidth.

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. 

The metric for each routing protocol is:

• RIP: Hop count - Best path is chosen by the route with the lowest hop count.
• IGRP and EIGRP: Bandwidth, Delay, Reliability, and Load - Best path is chosen by the route with the smallest composite metric value calculated from these multiple parameters. By default, only bandwidth and delay are used. 
• IS-IS and OSPF: Cost - Best path is chosen by the route with the lowest cost. . Cisco's implementation of OSPF uses bandwidth

Routing protocols determine best path based on the route with the lowest metric.

Refer to the example in the figure The routers are using the RIP routing protocol. The metric associated with a certain route can be best viewed using the show ip route command. The metric value is the second value in the brackets for a routing table entry. In the figure, R2 has a route to the 192.168.8.0/24 network that is 2 hops away.

R 192.168.8.0/24 [120/2] via 192.168.4.1, 00:00:26, Serial0/0/1

Load Balancing

We have discussed that individual routing protocols use metrics to determine the best route to reach remote networks. But what happens when two or more routes to the same destination have identical metric values? In this case, the router does not choose only one route. Instead, the router "load balances" between these equal cost paths. The packets are forwarded using all equal-cost paths. To see whether load balancing is in effect, check the routing table. Load balancing is in effect if two or more routes are associated with the same destination. 

Example:

The show ip route command reveals that the destination network 192.168.6.0 is available through 192.168.2.1 (Serial 0/0/0) and 192.168.4.1 (Serial 0/0/1).

R 192.168.6.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0

[120/1] via 192.168.4.1, 00:00:26, Serial0/0/1

Purpose od Administrative Distance

A router might learn of a route to the same network from more than one source. For example, a static route might have been configured for the same network/subnet mask that was learned dynamically by a dynamic routing protocol, such as RIP. The router must choose which route to install. How does a router determine which route to install in the routing table when it has learned about the same network from more than one routing source?

Administrative distance (AD) defines the preference of a routing source. Each routing source - including specific routing protocols, static routes, and even directly connected networks - is prioritized in order of most- to least-preferable using an administrative distance value. Cisco routers use the AD feature to select the best path when it learns about the same destination network from two or more different routing sources.

Administrative distance is an integer value from 0 to 255. 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. An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table.

Example:

N.B: When a static route is configured with an exit interface, the output shows the network as directly connected via that interface. 

The AD value is the first value in the brackets for a routing table entry. Notice that R2 has a route to the 192.168.6.0/24 network with an AD value of 90.

D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0

R2 is running both RIP and EIGRP routing protocols. R2 has learned of the 192.168.6.0/24 route from R1 through EIGRP updates and from R3 through RIP updates. RIP has an administrative distance of 120, but EIGRP has a lower administrative distance of 90. So, R2 adds the route learned using EIGRP to the routing table and forwards all packets for the 192.168.6.0/24 network to router R1. 

What happens if the link to R1 becomes unavailable? Then R2 would not have a route to 192.168.6.0. Actually, R2 still has the RIP route information for 192.168.6.0 stored in the RIP database. This can be verified with the show ip rip database command. This command shows all RIP routes learned by R2, whether or not the RIP route is installed in the routing table.

The AD value can also be verified with the show ip protocols command. This command displays all pertinent information about routing protocols operating on the router