We already have BGP implemented (ISP redundancy) at the main office building. It's got its ARIN approved ASN and a /24 prefix announced to the upstream ISPs.
My question is that if we were going to implement BGP at a branch office, do we need to apply a new AS number? Will it be a brand new setup or we can somehow use the same ASN? The branch office has its own /24 prefix and Internet connection. It talks to the main office through a L2L VPN tunnel.
If we can use the same ASN, what are the special configurations we need to be aware of? Thanks in advance.
Use the same AS number and run IBGP between your branch and main office. There is no special configuration, but you will be able to have loadsharing and redundancy between both ISPs.