Distance Vector Routing Protocols
- examples: RIP, IGRP, EIGRP
- a router using a DV protocol knows 2 things:
- Distance to final destination
- Vector (or direction) traffic should sent
- used in smaller environments where routes do not change often, such as a LAN’s
- do not scale well, so not suitable for WAN’s
- each DV router sends some or all of it’s routing table in updates, but only to it’s neighbours
- DV routers only know about their neighbours
- built in features for preventing routing loops:
- Split Horizon – a routing protocol will never advertise a route back out of the interface that the route was learnt on
- Poison Reverse – if a network becomes unavailable then a router will advertise that network with a metric of 16 (max is 15) , which allows other routers to quickly learn that a network is unavailable
Distance Vector Routing Protocol Comparison Table:
| algorithm? | Bellman-Ford | Bellman-Ford | DUAL |
| update method? | broadcast | multicast to 224.0.0.9 | multicast to 224.0.0.10 |
| update frequency? | every 30 Seconds | every 30 Seconds | when there’s a change |
| update contains? | whole routing table | whole routing table | only route changes |
| max routes per update? | 25 | 25 | n/a |
| equal cost load balancing? | yes, by default | yes, by default | yes, by default |
| un-equal cost load balancing? | no | no | yes, with “variance” command |
| max hop count? | 15 | 15 | 255 (100 is default) |
| authentication? | no | clear-text & MD5 | MD5 |
*note – EIGRP is not a true DV protocol but has some features of DV and is included here for comparison.
How does a Router decide which route to use?
When a router knows of multiple routes to get packets to a particular destination network, it has to decide which is the best route to use.
Here is the logic that the outer uses to make the forwarding decision:
- multiple routes = route with longest Prefix wins
- multiple routes + same prefix length = route with lowest Administrative Distance wins
- multiple routes + same prefix length + same AD = route lowest Metric wins
- multiple routes + same prefix length + same AD + same Metric = router will Load Balance
Administrative Distances
| TYPE OF ROUTE | ADMINISTRATIVE DISTANCE |
|---|
| Static (with exit interface) | 0 |
| Static (with next hop IP) | 1 |