I have a telstra provided IP WAN network running BGP.
I advertise 0.0.0.0 from my main site by the network 0.0.0.0 command under router bgp 61111, at the Disaster recovery(DR) site, I have the network 0.0.0.0 backdoor under BGP 62222 because there is internal eigrp running over a gigabit link between the two sites. This all works ok. Now if the main Router IP WAN link goes down, is there anyway that the DR can start advertising the default route automatically
How are the sites interconnected to main site and DR site?
I will suppose that you have an MPLS L3 VPN service where each site peers with SP PE node using eBGP sesssions.
IF this is the case it should be enough for the DR site CE node that you manage to advertise a default route with appropriate AS path prepending
ip prefix-list only-default permit 0.0.0.0/0
route-map selective_prepend permit 10
match ip address prefix only-default
set as-path prepend 62222 62222
route-map selective_prepend permit 20
router bgp 62222
neigh ISP-peer remote-as ISP-ASN
neigh ISP-peer route-map selective_prepend out
This should provide a default route that is used only when primary route at HQ disappears
if your network scenario is different the above solution does not apply to it.
Hope to help