Multihoming

Multihomed: A network with more than one transit provider.
Multi-Attached: More than one L3 connection to the same ISP.
PI Addresses: IP addresses not owned by the ISP.
PA Addresses: Provider Aggregatable. You might have permission to use a /24, but it comes from a much larger /18.
Asymmetric Flow: Egress traffic and Ingress traffic are via different ISPs.

BGP Multihoming

Best Scenario: Announce PI prefixes to both provider via BGP.

To have BGP multi-homing, a v4 site with PI addressing needs at least a /24 prefix for v4 and a /48 for v6.

Filter outbound prefixes to match networks you have. Do not re-advertise the full table, back out.

I. van Beijnum, BGP: Building Reliable Networks with the Border Gateway Protocol. Sebastopol, CA: O’Reilly Media, 2002, ISBN: 978-0-596-00254-1.