Dynamic Routing

Network Route: a route to a network/subnet. Also, the one with a prefix lower than /32

Host Route: route to a specific host → 2/32

A router form “adjacencies / neighbor relatioships / neighborships” with adjacent routers

If multiple routes to a destination are learned: if it has the same administrative distance, the lower metric will decide which one to use.

There are two types of dynamic routing protocols:

• IGP (Interior Gateway Protocol): This type is used inside a single autonomous system (AS),which is a single organization. For example: A company or a ISP.
  • It is further divided into two Algorithm types (process it uses to share routes and choose the best one):
    • Distance Vector:
      • They were invented first. RIPv1 and Cisco’s proprietary IGRP (Later updated to EIGRP).
      • In this algorithm, the router only have information that their neighbor provided. It is called this way because each router advertises the metric (Distance) and the next-hop router or direction (Vector).
      • It is called “routing by rumour”
      • Each router will advertise part or their whole routing table.
      • Protocols:
        • RIP (Routing Information Protocol)
        • EIGRP (Enhanced Interior Gateway Routing Protocol)
    • Link State:
      • This algorith however, provides enough information for the router to build a “connectivity map”, that will be the same for all routers, that tells how the internetwork is arranged. Therefore, it doesn’t only know which neighbor to forward to a route, it knows how the entire path leads to the destination on the route.
      • However, each router will process the “connectivity map” differently.
      • This happens by routers sharing information about their interfaces (connected routes) to other routers.
      • Altough CPU-heavy, they tend to reac faster to network changes.
      • Protocols:
        • OSPF (Open Shortest Path First)
        • IS - IS (Intermediate System to Intermediate System)

• EGP (Exterior Gateway Protocol): It is used between autonomous systems.
  • Path Vector:
    • BGP (Border Gatway Protocol)

If a router learns two routes to the same destination with the same administrative distance and the same metric, both will be added to the routing table. Therefore, the traffic will be load-balanced between the two routes. → ECMP (Equal Cost Multi-Path)