Trunking??

[Jonathan Rennie]

I would appreciate members response on this issue.....

What is a TRUNK....because there is a LOT of confusion....

Is a trunk a single cable that carries lots of (tagged) vlans e.g."trunk on" on Ci$co platforms

or is it a group of cables connected between two switches, aggregated together to increase bandwidth and reliability e.g. LACP??

[Les Ligetfalvy]

Yes, and yes. It is both.

[Terhorst]

Hi Jonathan,

On the physical layer you have a bunch of cables between to switches (10, 100, 1000 mbps, glass, copper whatever). Logically, they are bundled together to get better performance or resilience. Protocols to enable trunking are 802.1Q, LAG, etherchannel etc. You dont care about what vlan the traffic belongs to on a trunk and therefore you need the tags. More confused?

Regards,
Alexander

[Les Ligetfalvy]

If I had known we would get points by the word and not the correctness of the answer, I might have said more. :(

Judging by the points, I have to assume these are not the answers you were looking for. Perhaps if you rephrased your question, we can provide a more worthy answer.

[Stuart Teo]

Jonathan,

When reading hp and cisco documentation, please note that the word "trunk" means something slightly different.

hp trunks are known as port-channels on cisco gear or aggregate links on servers.

[Les Ligetfalvy · Accepted solution]

Well... borrowing from the adv-traffic-mgmt.pdf, when discussing VLANs for a particular problem, it says:
"use only one cable or port trunk..."
and a little further down, it says:
"you can use a trunk of multiple physical links rather than a single physical link"

A port trunk is the aggregation of multiple ports, and a VLAN trunk is the aggregation of multiple VLANs. While HP does not appear to refer to them as "VLAN trunks", others like Cisco and Nortel do. Some people refer to any connection between core and edge switches as trunks also.

Basically, I see the term "trunk" as a form of aggregating something, whether it be ports, or VLANs, or traffic in general. In this multi-vendor world, the term "trunk" always needs to be supported with additional words.

I suppose this debate could go on forever. :(

[Jonathan Rennie]

If I had known we would get points by the word and not the correctness of the answer, I might have said more. :(


Isn't that the whole point?????

you get points for the correctness of the answer???

[Gonzo Granello]

Actually, I belive hp uses the term port-trunking as far as I remember. Indeed that means link aggregation compared to Cisco, they use the term trunk for multiple VLANs on a link.

LACP is an automatic form of port trunking, but has severe limitations from my point of view. Therefore, if possible, I would advise to disable LACP in general or at least on ports occupied by clients.

Some words on bandwith aggregation. 4 100MB links trunked will not really yield in 400MB bandwith. The reason for that is that switches have no algorhytm to split TCP traffic and handling traffic by established connection. Here is a quick example: assuming the above 400MB trunk, 2 active conversations, both utilizing the 1st link, #1 uses 25% and #2 65% bandwith, no problem. If now #1 requires 65% as well, one would assume that then another link for the 30% over 100MB usage would be used. That however is not the case, if fact you would see a packet drop most likley on both conversations. From this example (which is of course simplified) there are a couple of things to learn. In most scenarios, once a connection has been established, the next one will most likley use the same wire given that the mac table has not timed out.
For a lot of short, not bandwith intesive connections, trunking might be a great way to improve bandwith, for high bandwith apps i would be careful. Nevertheless, trunking is an excellent way to have redundancy between switches, given that different modules can be used for one trunk.

The short bottom line, LACP = off, Trunking for reliability = definitley, Trunks for more bandwith = maybe.

Hope that helps.

GG

[Les Ligetfalvy]

Well... since Gonzo started the digression, I can continue the topic creep. :D

The example I use to enlighten those that are not network engineers is that port trunking is like adding more train tracks between cities. Each port is like one set of tracks. Bandwidth is the speed of the train and utilization is the seats on the train.

Now it takes a certain length of time to go from one city to the other and unless there are no available seats, utilization does not factor, only the speed does. If the seats are full, you have to wait for the next train. This is where your imagination needs to work with me... assume there are as many trains as there are miles of tracks.

If you built more tracks, you can run more trains at the same time but they do not travel any faster so unless the seats are sold out, it still take the same time to get from A to B.

If your seats are not in a sold-out situation and you want to get from A to B faster, you need to build better tracks that can handle very high speed trains.

In other words, port aggregation will not speed up the train, only add more seats.

[Mark Castle]

But they will also enable the trains to keep running even if one or more of the rails breaks ;-)