Multiple Aggregated Links Between Devices possible or not ?

[Nobleman]

Hello, everybody.
According to 'The All-New Switch Book', only a single 802.3 aggregated link is possible between a pair of devices. Can someone please shed light on why this is? My reading of 802.3 suggests that so long as devices choose different operational keys for each aggregation, then there should be no reason multiple links don't work. The relevant sections seem to be 43.3.6.1 and 43.6.1.Maybe there's another way.I'm using Cisco products, and have the following goals:

- 2 aggregated links, each one is tagged, and they're carrying different VLANs so that unrelated applications can't impact each other's available bandwidth.

- I want to use LACP because PAGP doesn't support standby links.A standby link is important to me so that loss of a single link doesn't diminish the available throughput, and also doesn't cause unpredictable redistribution of traffic when the link count changes.

If I had to, I could run one aggregation with PAGP and one with LACP, because only one of the aggregations really requires a consistent link count.

[Nobleman]

Here's what Mr Seifert and Mr Edwards had to say on the matter, in section 9.5.1: Here More.

"IEEE 802.3ad specifies a method for vendor-interoperable link aggregation under the following constraints:

- "Only one multiple-link aggregation is supported between a pair of devices."

Additionally, there's a footnote:

"There can be any number of non-aggregated individual links between a pair of devices."

Curiously, Cisco assures me that creating multiple LACP aggregations between a pair of Cisco switches is a supported configuration. While I'm relieved that I don't need to redesign this aspect of the network, I remain puzzled by the contradiction.

[Marco-D]

Here's what Mr Seifert and Mr Edwards had to say on the matter, in section 9.5.1: Here More.

"IEEE 802.3ad specifies a method for vendor-interoperable link aggregation under the following constraints:

- "Only one multiple-link aggregation is supported between a pair of devices."

Additionally, there's a footnote:

"There can be any number of non-aggregated individual links between a pair of devices."

Curiously, Cisco assures me that creating multiple LACP aggregations between a pair of Cisco switches is a supported configuration. While I'm relieved that I don't need to redesign this aspect of the network, I remain puzzled by the contradiction.

"Vendor interoperable" means between devices from different vendors.The standard doesn't say it will work between Cisco and any other companies switch, nor does Cisco.

[Nobleman]

You snipped out the relevant part of the quote from Rich's book. Vendor interoperable isn't important to me. I'm only using one vendor. The key phrase was "Only one multiple-link aggregation is supported" So, Rich says I can only have one link per the standard, but Cisco says I can have two standards-compliant links. This still looks like a contradiction to me. Am I misunderstanding something? Perhaps you're arguing that many 802.3ad aggregations are permissible between Cisco devices, but only one 802.3ad aggregation is possible if the devices are from different vendors?

That doesn't make much sense to me.It seems to suggest that Cisco's technique for bringing up multiple aggregations is NOT 802.3ad compliant. Afterall, if something is standards-complaint, then vendor- interoperability should be a foregone conclusion.

Thanks for your input.The discussion is pretty much academic at this point because Cisco has blessed the configuration, but I still hope to clear up my misunderstanding.

[Marco-D]

You snipped out the relevant part of the quote from Rich's book. Vendor interoperable isn't important to me. I'm only using one vendor. The key phrase was "Only one multiple-link aggregation is supported" So, Rich says I can only have one link per the standard, but Cisco says I can have two standards-compliant links. This still looks like a contradiction to me. Am I misunderstanding something? Perhaps you're arguing that many 802.3ad aggregations are permissible between Cisco devices, but only one 802.3ad aggregation is possible if the devices are from different vendors?

That doesn't make much sense to me.It seems to suggest that Cisco's technique for bringing up multiple aggregations is NOT 802.3ad compliant. Afterall, if something is standards-complaint, then vendor- interoperability should be a foregone conclusion.

Thanks for your input.The discussion is pretty much academic at this point because Cisco has blessed the configuration, but I still hope to clear up my misunderstanding.

In general when using an extension to the standard, you should be sure to document it such that others who follow you will know what you did. Someone later might decide to replace one Cisco device with one from another vendor, not knowing that an extension is being used. There used to be discussion here about using 10baseT at 150m. The standard requires 100m on Cat3 cable, and the numbers pretty much allow for 150m on Cat5 cable. That is, as an extension to the standard.Doing that will cause problems later on if someone wants to run, for example, 100baseTX through the cable.

The standard tells you what will work.It doesn't guarantee that not following the standard will result in a system that won't work.