Hi
If you are receiving a default route and advertising your networks on each side, it should work like a charm.
The convergence time might be slower depending of the cause of the breakdown. If your routeur is directly connected to those of your ISP, it should be very quick (interface down then flush of session and routes). If not, you might need bfd to be more fast to use the other link.
You can check it for each peer :
* check received prefixes : sh ip bgp neigh <IP_PEER> received
* check advertised prefixes : sh ip bgp neigh <IP_PEER> adverti
If you want to select wich default route will be priorised then you can add metric on the received route with a route map.
Your ISP might have an online looking glass tool to verify the state of your networks.
Edit 08/10/2018 : Sorry I forgot to mention that your network prefix need to be "Provider Independant". If it is "Provider Aggregatable", it will not be possible to transit through two differents ISPs or there will need to adapt their routing policy to process your prefix.
So, it might be necessary to get two PA networks and to use NAT to convert each other depending on wich ISP you traffic is occuring.
Regards
Jérôme
