What is routing loop

A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network. A routing loop can occur when two or more routers have routing information that incorrectly indicates that a valid path to an unreachable destination exists.

The loop may be a result of:

• Incorrectly configured static routes
• Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol and is discussed in CCNP-level courses)
• Inconsistent routing tables not being updated due to slow convergence in a changing network
• Incorrectly configured or installed discard routes

Distance vector routing protocols are simple in their operations. Their simplicity results in protocol drawbacks like routing loops. Routing loops are less of a problem with link-state routing protocols but can occur under certain circumstances.

Distance vektor routing protokol

Dynamic routing is the most common choice for large networks like the one shown. Distance vector routing protocols include RIP, IGRP, and EIGRP.

RIP

Routing Information Protocol (RIP) was originally specified in RFC 1058. It has the following key characteristics:
-Hop count is used as the metric for path selection.
-If the hop count for a network is greater than 15, RIP cannot supply a route to that network.
-Routing updates are broadcast or multicast every 30 seconds, by default.

IGRP

Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by Cisco. IGRP has the following key design characteristics:
-Bandwidth, delay, load and reliability are used to create a composite metric.
-Routing updates are broadcast every 90 seconds, by default.
-IGRP is the predecessor of EIGRP and is now obsolete.

EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary distance vector routing protocol. EIGRP has these key characteristics:
-It can perform unequal cost load balancing.
-It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
-here are no periodic updates as with RIP and IGRP. Routing updates are sent only when there is a change in the topology.

Purpose of Administrative Distance

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.

It is possible to modify the administrative distance for static routes and dynamic routing protocols. This is discussed in CCNP.

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.

Purpose of metric

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.
For example, RIP uses hop count, EIGRP uses a combination of bandwidth and delay, and Cisco's implementation of OSPF uses bandwidth. 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. For R3 in the figure, network 172.16.3.0 is two hops, or two routers away.

The Evolution of Dynamic Routing Protocols

Dynamic routing protocols have been used in networks since the early 1980s. The first version of RIP was released in 1982, but some of the basic algorithms within the protocol were used on the ARPANET as early as 1969.

One of the earliest routing protocols was Routing Information Protocol (RIP). RIP has evolved into a newer version RIPv2. However, the newer version of RIP still does not scale to larger network implementations. To address the needs of larger networks, two advanced routing protocols were developed: Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS). Cisco developed Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP), which also scales well in larger network implementations.