Main difference between link-state and distance vector routing protocols

Many books declare difference between distance vector and link-state protocols mainly in these things:

  • link-state protocols respond quickly to network change
  • also they send triggered updates when network change occurs
  • these updates only contains necessary information and not whole routing table
  • etc…

Hmmm now I am little bit confused, also EIGRP can do that things and its distance vector protocol. EIGRP just implement some enhancements, so its not so bad as RIP for example.

So what is the biggest difference? We know that OSPF builds 3 types of tables:

  1. Neighbour table
  2. Topology table
  3. Routing table

Still no difference as EIGRP builds the same tables! But the difference is how each of the protocol builds these tables. Mainly speaking about topology table. EIGRP build the topology table from the ROUTING table advertisements of its EIGRP neighbours. OSPF builds it from TOPOLOGY advertisements form its OSPF neighbours. Here as an example: Lets say that EIGRP router heard about one particular network from 3 sources. The router put the best path to the routing table and topology table and also put the paths which are compatible with the feasible condition to the topology table so they act as a backup routes and can be immediately installed  to routing table when the best path will be unavailable. But… here comes it… then the router send only the best route to reach the network to other routers. The router wont send all the 3 paths, only the best one. What would the OSPF router do in opposite? It would sent all the information it contains including the 3 paths in topology table. In reality, this is what actualy OSPF is doing with its LSA (link-state advertisements). The link state database must be the same within a area. So OSPF is exchanging its topology table, not only routing table as is in case of EIGRP.

SUMMARY:

  • EIGRP protocol exchanges only best path information from routing table between other EIGRP routers to reach the destination = just one best path
  • OSPF exchanges topology table information from all the neighbours in same area = all possible paths to reach the destination
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