BROCADE IP PRIMER

49F Link 50FPowerConsole1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48C o n s l o eP w rC P 2 P U 3C PU 1T x A c tR x A c tA c t i v e1 2 3 4 5 6 7 8 9 1 0 1 1 1J-F16GCJ-F16GCServerIron 850J-F16GCJ -F16GCJ-F16GCJ-F16GCJ -F16GCLoad Balancing MethodsSource NATSLB is all well and good, but what if you have so many servers that you don'thave enough ports to house them in your load balancer? What if the serversneed to be physically attached elsewhere in the infrastructure? What if it's notpractical to route all traffic to and from your real servers through the load balancer?If you still used SLB, the load balancer could still alter the incomingpacket, and it would reach one of the servers. The problem would be in thereply. The reply would come from, not the VIP address as the client was expecting,but from the actual IP address of the server that received the request. Thiswould confuse the client, and it would drop the packet.?sourceclient IPINTERNETdestinationserverIPsourceclient IPdestinationVIP2 1 3 1 4 1 5 1 62 M M S W1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 4 1 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 2 11 3 4 1 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 2 11 3 4 1 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 4 1 1 5 1 6destinationclient IPsourceserver IPRemember that for SLB to function, all traffic had to travel through the loadbalancer. While the incoming traffic would definitely be traveling through theload balancer, the return traffic would not. How do we make the return traffictravel back through the load balancer? We implement Source NAT.Source NAT takes the incoming packet, and substitutes the destinationaddresses (IP and MAC) accordingly to the load-balanced server it has chosen.But then, the process also substitutes the source addresses (IP and MAC) withits own address. To the server, it will look like the packet originated from theload balancer. The server will process the request and send its reply back tothe source, which it believes to be the load balancer. The load balancer willthen take the reply, substitute the VIP address as the source, and the real client'sIP address as the destination, and forward the packet back to the client.Brocade IP Primer 101