Estoy con el compañero de arriba, las rutas están bien pero posiblemente no sean necesarias...
Para tener esa "alta disponibilidad" entre dos gateways, tendrías que apoyarte en los routers/switches de tu red y en otros protocolos (RIP, por ejem).
Como solución provisional (si tus routers/switches no soportan protocolos dinamicos), podrías estudiar el crear un script.
A mi, a bote pronto, se me ocurre monitorizar la salida por un gateway cada x tiempo (ping a www.google.es, por ejem), detectar perdida de paquetes (con ping o con otro método) y con el comando route cambiar el default gateway. Con un script se podría hacer...
Evidentemente, ese cambio supondría resetear todas las conexiones activas en ese momento.
Esto me lo he encontrado por la red:
ECMP (Equal Cost Multi-Path) Routing
This routing mechanism enables packet routing along multiple paths with equal cost and ensures load balancing. With ECMP routing, you can use more than one gateway for one destination network (Note! This approach does not provide failover). With ECMP, a router potentially has several available next hops towards a given destination. A new gateway is chosen for each new source/destination IP pair. It means that, for example, one FTP connection will use only one link, but new connection to a different server will use another link. ECMP routing has another good feature - single connection packets do not get reordered and therefore do not kill TCP performance.
The ECMP routes can be created by routing protocols (RIP or OSPF), or by adding a static route with multiple gateways, separated by a comma (e.g., /ip route add gateway=192.168.0.1,192.168.1.1). The routing protocols may create multipath dynamic routes with equal cost automatically, if the cost of the interfaces is adjusted properly. For more information on using routing protocols, please read the corresponding Manual.
