Types of Routing Protocols

Distance vector means that routes are advertised by providing two characteristics:

? Distance- Identifies how far it is to the destination network and is based on a metric such as the hop count, cost, bandwidth, delay, and more.

? Vector- Specifies the direction of the next-hop router or exit interface to reach the destination.

For example, in the figure, R1 knows that the distance to reach network 172.16.3.0/24 is one hop and that the direction is out of the interface S0/0/0 toward R2.

A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network. Distance vector protocols use routers as sign posts along the path to the final destination. 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.

There are four distance vector IPv4 IGPs:

? RIPv1- First generation legacy protocol

? RIPv2- Simple distance vector routing protocol

? IGRP- First generation Cisco proprietary protocol (obsolete and replaced by EIGRP)

? EIGRP- Advanced version of distance vector routing

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.

To continue our analogy of sign posts, using a link-state routing protocol is like having a complete map of the network topology. The sign posts along the way from source to destination are not necessary, because all link-state routers are using an identical map of the network. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology.

RIP-enabled routers send periodic updates of their routing information to their neighbors. Link-state routing protocols do not use periodic updates. After the network has converged, a link-state update is only sent when there is a change in the topology.

Link-state protocols work best in situations where:

? The network design is hierarchical, usually occurring in large networks

? Fast convergence of the network is crucial

? The administrators have good knowledge of the implemented link-state routing protocol

There are two link-state IPv4 IGPs:

? OSPF- Popular standards based routing protocol

? IS-IS- Popular in provider networks

The biggest distinction between classful and classless routing protocols is that classful routing protocols do not send subnet mask information in their routing updates. Classless routing protocols include subnet mask information in the routing updates.

The two original IPv4 routing protocols developed were RIPv1 and IGRP. They were created when network addresses were allocated based on classes (i.e., class A, B, or C). At that time, 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.

Note: Only RIPv1 and IGRP are classful. All other IPv4 and IPv6 routing protocols are classless. Classful addressing has never been a part of IPv6.

The fact that RIPv1 and IGRP do not include subnet mask information in their updates means that they cannot provide variable-length subnet masks (VLSMs) and classless interdomain routing (CIDR).

Classful routing protocols also create problems in discontiguous networks. A discontiguous network is when subnets from the same classful major network address are separated by a different classful network address.

Modern networks no longer use classful IP addressing and the subnet mask cannot be determined by the value of the first octet. The classless IPv4 routing protocols (RIPv2, EIGRP, OSPF, and IS-IS) all include the subnet mask information with the network address in routing updates. Classless routing protocols support VLSM and CIDR.

IPv6 routing protocols are classless. The distinction whether a routing protocol is classful or classless typically only applies to IPv4 routing protocols. All IPv6 routing protocols are considered classless because they include the prefix-length with the IPv6 address.

Routing protocols can be compared based on the following characteristics:

? Speed of Convergence- Speed of convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. The faster the convergence, the more preferable the protocol. Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

? Scalability- Scalability defines how large a network can become, based on the routing protocol that is deployed. The larger the network is, the more scalable the routing protocol needs to be.

? Classful or Classless (Use of VLSM)- Classful routing protocols do not include the subnet mask and cannot support VLSM. Classless routing protocols include the subnet mask in the updates. Classless routing protocols support VLSM and better route summarization.

? Resource Usage- Resource usage includes the requirements of a routing protocol such as memory space (RAM), CPU utilization, and link bandwidth utilization. Higher resource requirements necessitate more powerful hardware to support the routing protocol operation, in addition to the packet forwarding processes.

? Implementation and Maintenance- Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.

The table in the figure summarizes the characteristics of each routing protocol.

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. This is accomplished through the use of routing metrics.

A metric is a measurable value that is assigned by the routing protocol to different routes based on the usefulness of that route. In situations where there are multiple paths to the same remote network, the routing metrics are used to determine the overall ?cost? of a path from source to destination. Routing protocols determine the best path based on the route with the lowest cost.

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.

The animation in the figure shows that RIP would choose the path with the least amount of hops; whereas, OSPF would choose the path with the highest bandwidth.

Date: 2016-06-12; view: 245