Routing Protocols
Marcio Saito - Revision 2.0 March 1999

How does a router route packets?
   Routers are networking units that understand the Network protocol layer (specifically IP, the Internet Protocol). Their basic function is to connect different networks and allow exchange of packets of information between them.
   An IP packet is the unit of information that travels in an IP network. In addition to the User's data, an IP packet has a header that carries the network protocol information.
   An IP header includes, among other information, the source IP Address (the address of the station that originated the packet) and the destination IP address (the address of the station the packet should be routed to).
   IP packets are routed based on their destination IP address. Upon the receipt of an IP packet, the router lookup a database (routing table) and decides where to send the packet to.
   The routing table can be as simple as "if the packet is not to the LAN, send it to the WAN" (in the case of a single router connecting a small LAN to the external world through a single WAN interface). It can be as complex as a table with the addresses and corresponding interfaces to route to for each network in the Internet (in the case of a backbone router).
   To keep the routing table databases up to date the routers need to exchange information among them. That way, routers know about the existing paths to a specific destination, account for communication failures, decide how to route packets based on cost or priority. They exchange this information using routing protocols

Interior Routing Protocols
    Routing protocols are protocols that allow two or more routers to exchange routing information. If this protocol is established between two routers in the same "autonomous system", then it is called Interior Routing Protocol.
Loosely defined, an autonomous system is a set of routers and networks under the same administration. Usually it is a corporate network linking several local networks through the corporate backbone or a set of networks served by a single Internet Service Provider.
   The most common Interior Routing Protocol is the "Routing Information Protocol" (RIP). It is a very simple protocol based on "distance-vector" algorithms. It is very simple to deploy and configure and is appropriated for small networks.
   When the network gets more complex, another interior routing protocol can be used. "Open Short Path First" (OSPF) is a newer, more powerful, and much more complex protocol. It is based on "link state" algorithms and requires careful network design and configuration.

Exterior Routing Protocols
    If a routing protocol is established between different Autonomous System, it is called Exterior Routing Protocol.
   The first Internet exterior routing protocol was the "Exterior Gateway Protocol" (EGP). But, since the late 80's, EGP has been replaced by a newer, more powerful routing protocol called "Border Gateway Protocol" (BGP, now in the version 4, hence, BGP4).
   Because BGP4 routing tables frequently have to store information about a very large number of networks, they may require large amounts of router memory and the routing becomes very CPU intensive. BGP4, as the name indicates, need to be supported in the routers that interface an autonomous system to the external world.

CyROS support for Routing Protocols
    A router acting as an access router (a single router that connects a LAN to the WAN) usually does not need any routing protocol (its routing table has only "static routes", configured manually. Those static entries usually don't state anything more than "if the packet is not local, send to the WAN".
   A router in a central site or in a more complex network (usually with several routers in it) may need RIP or OSPF (only the most complex networks).
   A router that interfaces an Autonomous System (which is usually a large network) to the external world (Internet) needs to talk BGP4.
   CyROS currently (as of Mar/99) supports RIP and OSPF. Since Cyclades routers range from access to mid-range, that is usually more than enough for most applications.
   But, sometimes, we have people using the PR3000 as the backbone router of autonomous systems (especially in remote locations and/or in undeveloped countries) and, in that case, BGP4 becomes a requirement. There is work in progress to support BGP4 in the future.

Comments or suggestions to this document can be sent to Marcio Saito marcio@cyclades