XORP is described as 'Internet Protocol routing software suite. It provides a fully featured platform that implements IPv4 and IPv6 routing protocols and a unified platform to configure them. It is the only open source platform to offer integrated multicast capability' and is an app in the Network & Admin category. There are four alternatives to XORP for Linux, BSD, Mac and Windows. The best alternative is BIRD, which is both free and Open Source. Other great apps like XORP are Quagga (Free, Open Source), FreeRouter (Free) and ExaBGP (Free).
BIRD is an open source implementation of a TCP/IP routing daemon for Unix like systems. It supports both IPv4 and IPv6, multiple routing tables, and a BGP, RIP and OSPF routing protocols, as well as statically defined routes.
Quagga is a network routing software suite providing implementations of Open Shortest Path First (OSPF), Routing Information Protocol (RIP), Border Gateway Protocol (BGP) and IS-IS for Unix-like platforms, particularly Linux, Solaris, FreeBSD and NetBSD.