ppp0 connected but cannot route traffic

I have below ppp0 config under peers directory

pty "pptp vpnserveraddress.com --nolaunchpppd"
name vpnusername
password vpnpassword
remotename PPTP
require-mppe-128
defaultroute
usepeerdns
debug

after connecting the route does not get added with error not replacing existing default route via eth0_default_gw_route_ip.

Also ping -I ppp0 google.comgives nothing.

What am I missing here in the config?

I am doing this on centos.