Complex, redundant networks, such as the Internet, require different routing policies from those typically found on a LAN. Ideally, the routers will know all the paths that lead to the target, but configuring them manually can quickly become confusing and lead to mistakes.

Quagga: Zebra for Admins

On the Internet, any attempt to do this manually would simply be impossible. The solution is to distribute dynamically changing route information automatically – including optional redundancies – if multiple paths lead to the goal. The web world has developed special routing protocols for this, usually only found on routers by Cisco or Juniper. However, Quagga [1] gives IT administrators the option of participating in the world's largest group of routers – with a Linux computer. The Quagga project originated with the Zebra Routing Daemon by Japanese developer Kunihiro Ishiguro [2]. The software is included in all popular Linux distributions and also runs on Unix derivatives like Solaris and Free/Net/OpenBSD.

Quagga doesn't handle the routing (that is still the domain of the underlying operating system kernel); however, it does provide a number of routing protocols – Routing Information Protocol (RIP) [3], RIPng [4], Open Shortest Path First (OSPF) [5] [6], Border Gateway Protocol (BGP) [7], and Intermediate System to Intermediate System (IS-IS) [8] – and it modifies the kernel routing table on the routes it learns.

Scouting 10

The Internet consists of many, sometimes multiply, connected subnets between which packets need to find the optimum path. A network of routers managed by a team of system administrators is known as an "autonomous system" (AS). Routing protocols are divided into protocols that distribute the routes within an autonomous system (which can also be subdivided, so that many routers need to manage many routes) and protocols that distribute the routes between the autonomous systems.

The protocols within an AS are known as Interior Gateway Protocols (IGP), and those for outside an AS are known as Exterior Gateway Protocols (EGP). RIP, OSPF, and IS-IS are IGPs; the only EGP in use is BGP. Within an enterprise, an IGP will thus typically be used for routing, whereas a BGP handles routing between providers and, in some cases, between a provider and an enterprise.

RIP is the oldest IGP – the first version is from 1988. The current version is version 2, which also supports IPv6 via RIPng. Despite this, RIP is considered outdated and is thus not used frequently. In contrast, OSPF is one of the most commonly used IGPs. It introduces a hierarchy of areas to divide the network, where one area can contain multiple networks. The single-stage hierarchy starts with area 0, which is the backbone area. Each further area must be connected via a router, even if it does not need to exist physically in a production environment. Customized configurations assign interfaces to individual areas.

The routers then use multicast to exchange data via their LAN interfaces (on the multicast IP address of 224.0.0.5 ); this can optionally be authenticated via MD5. The routers first send HELLO messages to discover their neighbors. These messages are followed by Link State Announcements (LSAs), which contain information about the routers' own routing table, areas, and the interface bandwidths. This information is then mutually used to update the routing tables.

When two paths lead to a network, but one router is connected to a gigabit network and the other to a 100Mb network, the path through the faster router wins. Where the paths are equivalent, the administrator can manually add a weighting value to the configuration, which is then reflected in the LSA information. If a route fails, the router on the alternative path handles traffic from that point on.

Dynamic Routing

Admins are not typically confronted with BGP until they redundantly connect their own networks to the Internet as an autonomous system or work for an ISP. The Internet would not work without BGP: The routers on the Internet backbone do not know any default routes; they only know routes to the networks of all other autonomous systems. Thus, the routing tables are correspondingly large (some 400,000 entries in April 2012.)

In contrast to OSPF, admins must configure the connection between two routers that exchange routes via BGP explicitly at both ends. The details include the partner's IP, authentication information, and the number of one's own AS. If multiple paths lead to the target, the goal is initially determined by the weighting specified by the administrator. Link speeds, as used by OSPF, do not influence the path selection.

BGP does not just attempt to find the shortest path; it also tries to implement routing policies that reflect contractual agreements and thus provider-dependent costs. This is exactly what the Quagga routing daemon on Linux does.