Outbound and Inbound traffic in BGP

[misaleh]

I'm confused about something and I'd like your help in understanding. Where I work, we have a router that's connected to 2 ISPs via BGP. Weight is set to 100 to ISP-A and 50 to ISP-B so outbound traffic is going through ISP-A and AS-Path prepend is modified so inbound traffic is going through ISP-B. What I'm confused about is, what kind of traffic goes through ISP-A and what kind of traffic through ISP-B? Like if a client is downloading, uploading, or browsing, traffic goes through A or B? Thanks in advance.

[Richard Burts]

Joseph makes a very interesting point about the possibility of NAT. Most of our discussion has been based on basic behavior of packet forwarding logic and using weight and prepend to direct outbound and inbound traffic. And based on those attributes it is likely that asymmetric traffic would be the result. But this is BGP with 2 ISP. In that case there is likely to be a unique NAT for traffic using ISP-A and a different unique NAT for traffic using ISP-B. If there are unique NATs then anything originated inside would go out a particular path and the response/ack would come back on the same path. So traffic path would be symmetric. 

[Joseph W. Doherty]

Possibly one point of confusion is how up and down might be used.  From the perspective of a particular host, up and down might be used as synymons for out and in.  However, for a network topology, again, up would be toward the core or center of a hierarchial topology and down toward the edge.

Another possible point of confusion is having multiple paths.  From your OP, the two paths behave like a single duplex path.  However, if both paths were being used equally in both directions, up or down traffic might use either ISP, and a flow's opposite direction traffic might use the same ISP or the other ISP.

[MHM Cisco World]

The traffic is TCP, I make lab and capture the traffic and show total length of each packet send receive 

This is Upload

the 21.0.0.4 upload the data to 120.0.0.1 you can see the length of packet is high up to 500 Bytes and it take the path through the Edge1, 
you can see that TCP ACK use Edge2 but the packet is small around 40 bytes

This is Download 
same Lab and I reverse the TCP connation Now the 21.0.0.4 download from 120.0.0.1
you can see the download traffic pass through Edge2 it around 500 bytes and TCP ACK send via Edge1 but it small 40 Bytes

So as I mention before Upload and Download is effect by which device is Server and Client in TCP, 
if the 21.0.0.4 is server then it Upload if the 21.0.0.4 is client then it download 

note:- check my previous lab for more how I config the as-prepend and weight for prefer path