We have dual service entrances to our building, connecting to GigEth 0/2 and 0/3 (differing Public IPs on both) on our NetVanta3140. We have BGP configured between those two connections so that traffic will auto re-route should one of the connection go down. GigEth0/1 connects to our local LAN (Private IP). I have a PBX on private side which has a specific secondary public IP configured on it needing to communicate with carrier SBX on public side. This public IP on my PBX needs to be defined as a secondary Public IP within the NetVanta. I can't define same secondary IP on both GigEth 0/2 and 0/3, how do I configure this so that PBX will communicate properly, no matter which service entrance happens to be the functioning circuit at the time?
If your two public IP blocks are from different ISPs and you do not have your own IP space advertised to both, then BGP won't help in this situation. ISP A won't route public space belonging to ISP B. You could try to see if your carrier can assign two public IP addresses to your PBX in a failover scenario. Or use a hostname for the PBX with SRV records.