Overview of IP routing protocols, packet forwarding and proxy ARP.
The principle of IP routing proved to be very flexible and scalable in the growth of the Internet and TCP/IP based networks.
IP routing denotes protocols for exchanging IP address range reachability like RIP, BGP and OSPF.
In contrast to IP routing, IP packet forwarding collectively means all functions performed when an IP router receives a packet and forwards it over the output interface indicated by an IP route in the routing table.
When an IP router performs a route lookup, it calculates a route decision based on different properties like prefix (mask) length, route precedence and metrics.
Routing protocols for exchanging route information can be coarsely classified as distance vector and link state protocols. Distance vector protocols like RIP (Routing Information Protocol) exchange information about the path cost to specific targets (IP address ranges). Routers that talk distance vector protocols receive reachability information about all sub-networks indirectly from neighboring routers.
In contrast to distance vector protocols, link state protocols like OSPF disseminate information about the link state of each router link in a network to all routers in the network. Thus link state protocols tend to converge faster to topology changes since all routers have firsthand information of the topology of the network.
Proxy ARP may be a convenient solution when it comes to add additional subnets without having to add routes to routers and hosts. A proxy ARP enabled router would answer ARP requests on behalf of the targeted hosts mimicking a local network access to the requesting host.