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When we have multiple ISPs terminating on the router with full BGP routing tables, is there a guarantee that an incoming packet takes the same ISP for out going? How the BGP route selection process works when we have multiple ISPs? When we have multiple ISPs, it is still 1 routing and 1 forwarding tables? How the routes aer maintained in routing and forwarding table separately when we have multiple ISPs? Any pointers to BGP route selection process with multihoming would be helpful...

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    IMO, getting the full routing table is highly overrated, and requires lots of memory on your router (~750k routes per ISP). Ask each ISP for a default route and their connected customers (or filter routes that way). The path difference beyond 1 or 2 ASes is nil.
    – Ron Trunk
    Aug 27, 2016 at 14:25
  • Getting the full table is important when dealing with transit-free upstreams because if you don't then a single peering dispute can cause you to lose connectivity to parts of the Internet.
    – Peter Green
    Aug 30, 2016 at 20:50
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it.
    – Ron Maupin ♦
    Aug 6, 2017 at 20:07

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Routing is not gauranteed to be symetric. Indeed it often isn't.

The RIB will contain information about routes from both ISPs, the "best" route to each destination will then be selected and installed in the FIB. What exactly "Best" means will depend on configuration.

More-specific (longer prefix) routes always win over less-specific (shorter prefix) ones.

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