Help me understand LACP

[saurabhwahile]

Hey guys,

 

So I am planning to use LACP on my servers, but I'm a bit confused as to how LACP or ethernet bonding works. 

 

1. Are the connections bonded, between two machines, or between a server and a switch?

2. What if a non-LACP machine wants to connect to a LACP bonded connection? Will it have reduced speed, or no connection at all?

3. So if I have a LACP bonded link with 2 connections bonded together(1 Gigabit each) and if 2 separate machines(Also 1 Gb each) try to download data from the server, will the bandwidth be shared between the two separate clients? (So, basically, its a 2:1 connection) (Also assuming the server can saturate the effectively 2Gbit link)

4. If anyone has used this, let me know any other nuances or /things/ that I need to know before jumping on LACP. My use case is for performance improvement(No redundancy etc)

 

Thanks!

 

[mynameisjuan]

2 hours ago, saurabhwahile said:

1. Are the connections bonded, between two machines, or between a server and a switch?

They are bonded between the server and the switch. So if you want to use port channel with 2 gig ports, you can provide 2 gigs of traffic from the server to the switch. 

 

2 hours ago, saurabhwahile said:

2. What if a non-LACP machine wants to connect to a LACP bonded connection? Will it have reduced speed, or no connection at all?

LACP just lies on the NIC of the server and the interface of the switch. LACP is just a bonding protocol for the ports. Machines themselves are not "LACP machines"

 

2 hours ago, saurabhwahile said:

. So if I have a LACP bonded link with 2 connections bonded together(1 Gigabit each) and if 2 separate machines(Also 1 Gb each) try to download data from the server, will the bandwidth be shared between the two separate clients? (So, basically, its a 2:1 connection) (Also assuming the server can saturate the effectively 2Gbit link)

In that situation each machines will cap at 1 gig. If you have 4 machines trying to download LACP will try to load balance but it is not perfect but will try to provide 500mbps per device (its all dependent on timing)

 

2 hours ago, saurabhwahile said:

4. If anyone has used this, let me know any other nuances or /things/ that I need to know before jumping on LACP. My use case is for performance improvement(No redundancy etc)

Today its much easier and cheaper to pick up a switch with 2 SFP 10gig ports and let the switch just switch as normal. You can find them for <$200. LACP can be finicky on some systems (servers). You can do it no problem and its really its a good protocol. 

 

I hope this helps.

[saurabhwahile]

Hey! Thanks for the reply! It definitely helps!

 

I am however concerned about what you said about servers, that it can be finicky. I am planning to use LACP on a mission critical small business server. We cannot afford to have a downtime. Is LACP a good choice? In that case?

[mynameisjuan]

6 hours ago, saurabhwahile said:

Hey! Thanks for the reply! It definitely helps!

 

I am however concerned about what you said about servers, that it can be finicky. I am planning to use LACP on a mission critical small business server. We cannot afford to have a downtime. Is LACP a good choice? In that case?

If you have a 10gbe port on your switch then I would go the route of getting a 10gbe card on your server and connecting it to the switch then that switch to all devices. 

 

LACP will work but if the server has a hiccup with the NIC card for whatever reason it could drop and you might need to reboot the server or disable/enable the NIC. But this is a MIGHT happen scenario!

 

Another reason is LACP balances traffic based on a few different protocols but default is destination mac address. So say you have a ton of devices and they pull a ton of data often from the server. The way traffic randomly is balanced between the 2 1gb ports means at times you might see 5 devices trying to pull a ton of data through one of those gig ports while the other port is not even being utilized. This is why etherchannels=/=bonding. You will see odd performance issues at times. (note this is only noticeable with lots of devices and heavy data transfer)

 

So go 10gbe if you can but if you cannot LACP will still work well, just know its quirks.

[Alex Atkin UK]

I have been fiddling with LAG (not LACP as the cheap Smart switch I could afford doesn't support it) and must concur that its awfully fiddly.  Maybe LACP is more reliable, but it took a few tries to get it working and I'm not convinced its stable.  I see occasional dropped packets on the interface.

I'm planning to switch to 10Gig as soon as I can afford to as it just doesn't seem worth the hassle.

If its mission critical, I honestly wouldn't cheap out.