10.5. Dynamic Routing

The reference tool for dynamic routing is currently quagga, from the similarly-named package; it used to be zebra until development of the latter stopped. However, quagga kept the names of the programs for compatibility reasons which explains the zebra commands below.

BACK TO BASICS Dynamic routing

Dynamic routing allows routers to adjust, in real time, the paths used for transmitting IP packets. Each protocol involves its own method of defining routes (shortest path, use routes advertised by peers, and so on).

In the Linux kernel, a route links a network device to a set of machines that can be reached through this device. The ip command, when route is used as the first argument, defines new routes and displays existing ones. The route command was used for that purpose, but it is deprecated in favor of ip.

Quagga is a set of daemons cooperating to define the routing tables to be used by the Linux kernel; each routing protocol (most notably BGP, OSPF and RIP) provides its own daemon. The zebra daemon collects information from other daemons and handles static routing tables accordingly. The other daemons are known as bgpd, ospfd, ospf6d, ripd, ripngd, and isisd.

Daemons are enabled by creating the /etc/quagga/*daemon*.conf config file, daemon being the name of the daemon to use; this file must belong to the quagga user and group in order for the /etc/init.d/zebra script to invoke the daemon. The package quagga-core provides configuration examples under /usr/share/doc/quagga-core/examples/

The configuration of each of these daemons requires knowledge of the routing protocol in question. These protocols cannot be described in detail here, but quagga-doc provides ample explanation in the form of an info file. The same contents may be more easily browsed as HTML on the Quagga website:

→ http://www.nongnu.org/quagga/docs/docs-info.html

In addition, the syntax is very close to a standard router’s configuration interface, and network administrators will adapt quickly to quagga.

IN PRACTICE OSPF, BGP or RIP?

OSPF (Open Shortest Path First) is generally the best protocol to use for dynamic routing on private networks, but BGP (Border Gateway Protocol) is more common for Internet-wide routing. RIP (Routing Information Protocol) is rather ancient, and hardly used anymore.