Failover routing - ISP switch

a small shell script that ping a test ip and if it is
unreachable switch to the other gateway.
Schedule this script to run every minute or so from cron.

#!/bin/bash
GW1="192.168.10.254"
GW2="192.168.55.254"
TESTIP="192.71.220.10" # Any reliable Internet ip that responds to ping.
CURGW=`/sbin/route -n |awk '/^0.0.0.0/ {print $2 }'`

if ping -w2 -c3 $TESTIP >/dev/null 2>&1; then
echo "Active ISP is Ok."
else
if [ "$CURGW" = "$GW1" ]; then
 NEWGW="$GW2"
else
 NEWGW="$GW1"
fi
/sbin/route del default
/sbin/route add default gw $NEWGW
fi  

This script requires that both your ISP's are pre-configured and are
not messing with your routing table when connected/disconnected,
esp. PPP is know to be quite unpolite when dealing with routes.
You probably want some sort of test to see if the new path is
working as well. Other concerns may be iptables/tc reloading
and stuff like that.