IP ROUTING
Many protocol models have existed over the years, but today the TCP/IP model dominates. And at the network layer of TCP/IP, two options exist for the main protocol around which all other network layer functions revolve: IP version 4 (IPv4) and IP version 6 (IPv6).
Internet Protocol (IP) focuses on the job of routing data, in the form of IP packets, from the source host to the destination host. IP does not concern itself with the physical transmission of data, instead relying on the lower TCP/IP layers to do the physical transmission of the data. Instead, IP concerns itself with the logical details, rather than physical details, of delivering data. In particular, the network layer specifies how packets travel end to end over a TCP/IP network, even when the packet crosses many different types of LAN and WAN links.
Forwarding The Packet
Routers and end-user computers (called hosts in a TCP/IP network) work together to perform IP routing. The host operating system (OS) has TCP/IP software, including the software that implements the network layer. Hosts use that software to choose where to send IP packets, often to a nearby router. Those routers make choices of where to send the IP packet next. Together, the hosts and routers deliver the IP packet to the correct destination.
How Network Layer Routing Uses LANs and WANs
While the network layer routing logic ignores the physical transmission details, the bits still have to be transmitted. To do that work, the network layer logic in a host or router must hand off the packet to the data-link layer protocols, which, in turn, ask the physical layer to actually send the data. The data-link layer adds the appropriate header and trailer to the packet, creating a frame, before sending the frames over each physical network.
The routing process forwards the network layer packet from end to end through the network, while each data-link frame only takes a smaller part of the trip. Each successive datalink layer frame moves the packet to the next device that thinks about network layer logic. In short, the network layer thinks about the bigger view of the goal, like “Send this packet to the specified next router or host…,” while the data-link layer thinks about the specifics, like “Encapsulate the packet in a data-link frame and transmit it.”
The following list summarizes the major steps in a router’s internal network layer routing for each packet
- Use the data-link Frame Check Sequence (FCS) field to ensure that the frame had no errors. If errors occurred, discard the frame.
- If the frame was discarded, discard the old data-link header and trailer, leaving the IP packet.
- Compare the IP packet’s destination IP address to the routing table, and find the route that best matches the destination address. This identifies the next-hop router IP address.
- Encapsulate the IP packet inside a new data-link header and trailer, appropriate for the next-hop, and forward the frame.
IP Header
The routing process also makes use of the IPv4 header. The header lists a 32-bit source IP address, as well as a 32-bit destination IP address.
IP Routing Protocols
For routing logic to work on both hosts and routers, each host and router needs to know something about the TCP/IP internetwork. Hosts need to know the IP address of their default router so that hosts can send packets to remote destinations. Routers, however, need to know routes so they forward packets to each and every reachable IP network and IP subnet.
The best method for routers to know all the useful routes is to configure the routers to use the same IP routing protocol. Alternately, a network engineer could configure (type) all the required routes, on every router. However, if you enable the same routing protocol on all the routers in a TCP/IP internetwork, with the correct settings, the routers will send routing protocol messages to each other. As a result, all the routers will learn routes for all the IP
networks and subnets in the TCP/IP internetwork.
Many routing protocols use the same general steps for learning routes:
- Each router, independent of the routing protocol, adds a route to its routing table for each subnet directly connected to the router.
- Each router’s routing protocol tells its neighbors about the routes in its routing table, including the directly connected routes and routes learned from other routers.
- After learning a new route from a neighbor, the router’s routing protocol adds a route to its IP routing table, with the next-hop router of that route typically being the neighbor from which the route was learned.
