While studying for Cisco and Microsoft certifications I read a lot about NIC Teaming, Etherchannels and similar stuff.
I know you can do static teaming in MS Windows Server or creating a port-channel in Cisco or use Dynamic protocols such as LACP and PAgP. But they all offer load-balancing using source or destination MAC address or IP address, a combination XORed together or similar things, right? And it seems there's no way to increase your actual throughput if you have a long long file transfer from one server to another (ex. having 4 GigE bonded together to increase it to 4 Gbps).
So I wonder if it is true? Is there a way to use all the bandwidth of bonded interfaces? I heard from few sources that there's something like SLB (Switch-Assisted Load Balancing w/ Fault Tolerance) which supports full load balancing and allows to use full bandwidth (like 4 Gbps from my example between two servers in one transaction).
I've heard so many contradictory things about that, and I am totally confused. Help me guys!
Best Answer
At layer 2, all load balancing is, at best, done by an XOR or hash of the source and destination MAC, and if you're lucky, it may even read into layer 3 and hash that data too.
At layer 3, however, where we're basically talking about multiple gateways (so, effectively, two physical links with a unique next-hop across each) you can max out the bandwidth across the links IF you're prepared to do per-packet balancing.
Before I go on, per-packet balancing is generally a bad thing due to the fact that it can result in out-of-order packet delivery, this can be especially terrible with TCP connections, but that of course comes down to the implementation and most modern stacks can tolerate this relatively well.
In order to do per-packet balancing, obviously one requirement is that the source and destination IP addresses are not at all on-link to the devices that have the multiple paths since they need to be routed in order for balancing to be possible. Redundancy can be achieved via a routing protocol such as BGP, OSPF, ISIS, RIP, or alternatively, BFD or simple link-state detection.
Finally, there is of course a transport layer solution - protocols like SCTP support connecting to multiple endpoints, and TCP already has drafts in the making that will add options to do similar things. Or... you can just make your application open multiple sockets.