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Distance vector

As the name implies, 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. A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network.
Distance vector routing protocols include RIP, IGRP, and EIGRP.

RIP
Over the years, RIP has evolved from a classful routing protocol (RIPv1) to a classless routing protocol (RIPv2). RIPv2 is a standardized routing protocol that works in a mixed vendor router environment. Routers made by different companies can communicate using RIP.
Features of RIP:
- Supports split horizon and split horizon with poison reverse to prevents loops.
- Is capable of load balancing up to six equal cost paths . The default is four equal cost paths.
RIPv2 introduced the following improvements to RIPv1:
- Includes the subnet mask in the routing updates, making it a classless routing protocol.
- Has authentication mechanism to secure routing table updates.
- Supports variable length subnet mask (VLSM).
- Uses multicast addresses instead of broadcast.
- Supports manual route summarization.

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) was developed from IGRP, another distance vector protocol. EIGRP is a classless, distance vector routing protocol with features found in link-state routing protocols.
EIGRP features include:
- Triggered updates (EIGRP has no periodic updates).
- Use of a topology table to maintain all the routes received from neighbors (not only the best paths).
- Establishment of adjacencies with neighboring routers using the EIGRP hello protocol.
- Support for VLSM and manual route summarization. These allow EIGRP to create hierarchically structured large networks.
Advantages of EIGRP:
- Although routes are propagated in a distance vector manner, the metric is based on minimum bandwidth and cumulative delay of the path rather than hop count.
- Fast convergence due to Diffusing Update Algorithm (DUAL) route calculation. DUAL allows the insertion of backup routes into the EIGRP topology table, which are used in case the primary route fails. Because it is a local procedure, the switchover to the backup route is immediate and does not involve the action in any other routers.
- Bounded updates mean that EIGRP uses less bandwidth, especially in large networks with many routes.