Distance Vector Protocols

Distance-vector routing tells neighbors which destinations are reachable and a metric describing the distance. A router combines a neighbor's advertisement with the cost to that neighbor to derive its own candidate path.

Learning Through Neighbors

If Router A advertises a distance of three to a prefix and Router B reaches A with cost one, B can derive distance four through A. The information describes a direction and metric, not a complete topology map, which is why the approach is sometimes called routing by rumor.

CheckIf a neighbor advertises metric 3 and the link cost is 1, what metric is derived through it?

2 31 4

Loops and Count to Infinity

After a failure, neighbors can mistakenly advertise a route back to each other, gradually increasing its metric. Protocols mitigate this with finite infinity values, split horizon, route poisoning, poison reverse, triggered updates, and timers. These mechanisms reduce but do not turn every topology change into instantaneous convergence.

CheckWhat is split horizon intended to reduce?

The number of bits in every IPv4 address. Encryption overhead in application payloads. Advertising a learned route back toward the neighbor it came from.

RIP Metrics and Limits

RIP uses hop count. A route with metric 16 is unreachable, so the largest usable metric is 15. That bounds loop escalation but also limits network diameter. Fewer hops do not necessarily mean lower latency or more bandwidth.

RIPv2 uses periodic and triggered updates and supports CIDR information. It commonly multicasts updates rather than broadcasting an entire table in every circumstance. Authentication and filtering still require deliberate configuration.

CheckWhat does RIP metric 16 represent?

The fastest path with sixteen parallel links. Infinity, meaning the destination is unreachable. A route learned from BGP.

Evaluating a Learned Route

Check neighbor state, received and advertised prefixes, metric, next hop, route installation, and data-plane reachability. A route can be valid within RIP but lose to another route source under local preference policy.

CheckWhy can RIP's lowest-hop route perform poorly?

Hop count does not encode link bandwidth, latency, loss, or congestion. RIP always chooses the route with the most hops. Hop count is measured in bytes of disk space.

Summary

You can now explain both the simplicity and limitations of distance-vector routing.

1. Derive candidate distance from a neighbor's advertisement.
2. Recognize loop and count-to-infinity behavior.
3. Explain RIP's 15-hop usable limit and metric 16.
4. Verify route installation and data-plane outcome separately.