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k6ccc
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LAG (LACP) or RSTP or both???

Wed Aug 15, 2018 7:36 pm

This is an area where have no experience so looking for suggestions. My situation at home is as follows. In my Family room I have a CSS324-24G-2S with various computers, WiFi, lighting controls, and monitoring devices connected. Both my DSL and cable internet connections also come into the Family room switch. In my garage data cabinet is a second CSS324-24G-2S switch that has both RB750 routers, three unattended use computers, environmental monitor, IP controlled power switches, and several WiFi APs. There is currently a 1Gb/s ethernet connection connection between the switches. Because the routers are in the garage and both internet services come into the Family room, obviously the link between the two switches is critical. There are about a dozen VLANs on the link between the two switches. I plan shortly on adding a second link between the two switches that will be 1Gb/s multimode fiber that will take a completely diverse route between the house and garage.

That leads to my question. My objective for having the second link is reliability in the event that one path fails. As I understand it, I can do this with either RSTP or LAG or a combination of both. I am sure there are advantages and disadvantages to each. I have done a fair amount of reading about both, but as I said, I have no experience along these lines, so looking for opinions, suggestions, etc on the best way to set up these two paths.
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Re: LAG (LACP) or RSTP or both???

Fri Aug 24, 2018 1:42 am

Nobody have anything?
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Re: LAG (LACP) or RSTP or both???

Sun Sep 02, 2018 10:07 am

You can`t do it with RSTP.
You can do it with:
LAG LACP
LAG Static teaming

LACP seems to be a little buggy at least for me in SwOS.
 
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Re: LAG (LACP) or RSTP or both???

Sun Sep 02, 2018 9:36 pm

Thanks for the reply. I'm curious why I can't use RSTP. I've read the Wiki about a million times (OK, not really a million) and don't see why I could not use RSTP, and in a different thread where this topic came up, it was recommended that I should always use RSTP. From my own research, I had assumed that I would want to implement LAG LACP.

As for LAG, in SwitchOS there seems to be VERY little configuration. I have never heard the term "LAG Static teaming" before. I may know the concept, but never seen that term. What is it?
This connection will be between two CSS326-24G-2S switches currently running SwOS 2.8

What do you find buggy about LACP in SwOS?
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
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Re: LAG (LACP) or RSTP or both???

Sun Sep 02, 2018 10:08 pm

You can do and should do lacp
This is exactly what you need, 2 links combined for reliability and redundance.
Rstp has nothing to do with this.

Lacp well there is not much to configure, it is missing long and short timeouts but thats it, there is very little configuration. Just make sure that one of sides is in active mode, or set both to active.

Static team or static lag is same as lacp, but you have to define ports, specific ports on which team is made.

For me, lacp and static teaming are not working at all on my server 2016 box with 3 nics...
 
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Re: LAG (LACP) or RSTP or both???

Sun Sep 02, 2018 10:26 pm

Thanks again. You pretty well told me what I thought I already understood from reading, but I had been confused when some other guy in a different thread told me that I should always be using RSTP. It will be a while before I have time to bury another conduit between the house and garage, so I'm in no hurry to implement this - although I may for test lay the fiber on the ground for a quick test.
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 2:41 pm

Hello,

you should do both - the purpose of each technology is different, (m/r)stp is there to prevent loops, link aggregation to provide higher aggregate bandwidth.

1) Link aggregation is different from link aggregation control protocol (LACP). LACP is the protocol which will "setup" link aggregation on your two ports (that is if you select LACP as active). LACP will send some frames on the wire and if the other side responds corrently, then link aggregation will be formed. You can do link aggregation without LACP, this is what is reffered to in the post above as "manual" link aggregation. This is potentially dangerous because it can, under specific cicumstances, create loops. Best practice is to always use LACP for link aggregation (both side can be active, or one active and the other passive).

2) STP is preventing loops by "shuting down" interfaces which would under normal conditions create loops. So if you have two links to the same switch only one of them will be active.

You absolutely can and should have STP active at the same time as LACP. This way you have some form of loop protection and higher aggregate bandwith.
 
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 6:54 pm

Well, now I'm back to confused. First of all, I fully understand how bad a loop can be. Last year at work on a microwave network that supports a large public safety radio 2-way radio system, we had a loop protection failure that resulted in a broadcast storm that took down the entire network. Really bad!
First gregecslo said you can't do what I want to do with RSTP and then hnevbramborovychlidi said I should be using RSTP. Reading the latter's narrative, I get the feeling that RSTP would functionally override what the LACP is doing. The disadvantage of RSTP is it is far slower than LACP in detecting a link failure.
Now if I was running three or more switches in a ring, that's what RSTP is for, whereas on a link between two switches, LACP would work better. And yes, as shown in the bottom of the drawing below if I had a ring from A to B to C and back to A, I would want to use RSTP for the ring between the three switches, and if I had multiple links between A and B, I could run LACP between them. Does that make sense?
Image
Or am I full of something bad?
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 7:19 pm

LACP is for link aggregation.
2 ports on switch A and 2 ports on switch B.
2GB link between them with LACP.

If not using LACP or static team 2 ports connected between switches would create LOOP. This is what RSTP is for. To prevent loops and lear about topology changes: https://networklessons.com/spanning-tre ... tree-rstp/

1. So you can`t have just 2 cables between switches and expect to have 2 GB if LACP or static teaming is not used
2. 2 cables between switches with no RSTP (STP) and no lacp or static team will result in a loop
3. 2 cables between switches with RSTP (STP) and NO lacp or static teaming will result in only 1 link up and second blocked to prevent loop
4. 2 cables between switches WITH RSTP and WITH team will result in 2GB link and failover.
 
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 8:07 pm

LACP is for link aggregation.
2 ports on switch A and 2 ports on switch B.
2GB link between them with LACP.

If not using LACP or static team 2 ports connected between switches would create LOOP. This is what RSTP is for. To prevent loops and lear about topology changes:
Right. Got that part.
1. So you can`t have just 2 cables between switches and expect to have 2 GB if LACP or static teaming is not used
2. 2 cables between switches with no RSTP (STP) and no lacp or static team will result in a loop
3. 2 cables between switches with RSTP (STP) and NO lacp or static teaming will result in only 1 link up and second blocked to prevent loop
Got that part too. With #3, you get loop protection, but no additional usable bandwidth between the switches, and RSTP while faster than STP is still pretty slow.
4. 2 cables between switches WITH RSTP and WITH team will result in 2GB link and failover.
This is the statement that confuses me. Although maybe I'm reading more into that than is there. If running RSTP on top of LACP, I would think that the STP is going to block one port so you are NOT going to get 2GB of throughput. If it's not blocking one port, than what is it doing, or to put it another way, why should it be there?

As I said in the first post, in general terms, I understand what (R)STP does, and what LACP does, what I am lacking on is how or if they work together on a two connection link between two switches, and should I be using one vs the other vs both.

I know I can't just plug in two cables between the two switches as that would create a loop. As I understand it (R)STP will prevent a loop by blocking one of the links. In the event of a failure of one link, the (R)STP will simply use only the one working link. LACP will aggregate the two links functionally into one link of higher bandwidth which will prevent a loop and if one fails, the aggregated link will simply have less bandwidth. Further, that the link failure detection in LACP is far faster than RSTP or even more so with STP..

For my purposes, my primary objective of this is to have redundancy in the event of a failure of one link between the switches. I don't really need the additional bandwidth (although It can't hurt), but I do want a failure detected quickly so traffic is impacted for as short of amount of time as practical.
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 8:23 pm

As I understand it (R)STP will prevent a loop by blocking one of the links. In the event of a failure of one link, the (R)STP will simply use only the one working link.
Yes, when configured properly (bridge priority etc...)

LACP can handle 2 cables/ports because protocol supports it and it`s designed to do it. It doesn`t need rstp at all.

BUT:
If it's not blocking one port, than what is it doing, or to put it another way, why should it be there?
Imagine sys admin that:
a) plugs in dumb switch and connect 2 cables = disaster if no rstp is there
b) plugs in managed switch with wrongly configured lacp and plugs in 2 cables = disaster if no rstp
c) ...

So rstp is here to prevent issues.
But for your question, when LACP or static team active rstp doesn`t play any role as it simply puts port in edge mode.
 
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 8:32 pm

But for your question, when LACP or static team active rstp doesn`t play any role as it simply puts port in edge mode.
So in a summary, RSTP in this case is only there to prevent you from doing something stupid (or cover your backsides if or when you do). Do I have that right?
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 8:58 pm

I would say so.
If you have many switches use of rstp becomes obvious...
 
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 9:07 pm

I would say so.
If you have many switches use of rstp becomes obvious...
Yes, makes sense.

Thanks for your help. Sounds like I largely had it figured out, but fully admit that I only knew enough to be dangerous!
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 9:59 pm

You really need (R/M)STP to run on top of LACP bonding if you add the "Switch C" for the whole system to work (your lower picture).
The LACP bonding itself will be treated like one physical port by RSTP - it can't disable only part of it.
But if there is no other potential loops, and the LACP bonding is the only link connecting two parts of the network nothing will change if you won't run STP on it.
 
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 10:25 pm

Like I said, many switches different story 😁
 
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Re: LAG (LACP) or RSTP or both???

Thu Sep 06, 2018 10:45 pm

You really need (R/M)STP to run on top of LACP bonding if you add the "Switch C" for the whole system to work (your lower picture).
The LACP bonding itself will be treated like one physical port by RSTP - it can't disable only part of it.
But if there is no other potential loops, and the LACP bonding is the only link connecting two parts of the network nothing will change if you won't run STP on it.

Nope. Two switches only and never any more. I completely understand needing RSTP if I added the third switch to create a ring, but that is not gonna happen.

Thanks again to all who chimed in. have a better understanding of a few parts here. Now to implement it (although that will take adding about 30 feet of buried conduit and there are higher priorities to deal with first)...
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
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Re: LAG (LACP) or RSTP or both???

Sat Jul 27, 2019 12:22 am

Hi!

I know this thread is already a bit dated, but the issue I'm having seems to be related to using LACP and RSTP at the same time.
I have a non-MikroTik switch where I can disable RSTP on each port forming the LAG, but enable it for the LAG itself. I don't know what that switch is doing with RSTP when one l port fails and the LAG becomes a single port, but that's a different story.

On the CSS326 I have no such choice. How do I tell the switch that it should look at the LAG when doing RSTP and not at the individual ports? Or is it smart enough to figure what the admin wants?

Cheers
Alex

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