HP2920-24 - max distance between switches when stacking

[frankbyrne]

Hi - we want to create a stack of 4 x 2920's across two locations - 2 switches in each room. The rooms are connected with OM4 fiber and are about 150m apart. Can we do this? What is the max distance we can have between the rooms to enable stacking?

Thanks.

[parnassus]

As far as I know, No...you can't do that.

With IRF or VSF you can (using 40G, 10G or 1G Ethernet ports which became logically tied to IRF/VSF usage...clearly depending on the switch/modules used)...and you can do because IRF/VSF ports (that can be aggregated for link redudancy too!) are exactly 1G/10G/40G Ethernet ports with their typical ethernet restrictions (Fiber Optic transceivers for Long Haul can sustain up to 70 km).

So why you can't do that with Aruba 2920?

Because the Aruba 2920 implements backplane stacking through specific stacking module (Aruba 2920 2-ports Stacking Module J9733A) and related special stacking cables (Aruba 2920 0.5/1/3 m long Stacking Cable, respectively J9734A, J9735A and J9736A)...so the stacking deployment (up to four Aruba 2920 units) is physically limited by the adoption of the longest stacking cable available (3 meters) connected between near units in a stacking chain topology (Unit 1 -- 3 m -- Unit 2 -- 3 m -- Unit 3 -- 3 m -- Unit 4 for a grand total of just about 9 meters between the Unit 1 and Unit 4...you can do that staying within the same rack or just between very near racks...not between far DC's racks).

With Aruba 3800 Switch series...since you can stack up to ten units...the distance limit increases to just about 3x9=27 meters...still too short.

A side note about the special case of only two Aruba 2920 units "dual chained" together: two 0.5/1 or 3 m long stacking cables placed between them will reach up to 40Gbps full-duplex, 20Gbps on each single stacking cable's port...20Gbps is the admitted chain/ring topology speed between two adjacent units, that's clear, when only one stacking cable can be deployed...so when you see 40Gbps is not the same as QSFP+ exactly as when you see 20Gbps doesn't mean that those stacking ports are two 10Gbps Ethernet ports aggregated togheter (which can use typical Cat. 6A or Cat. 7 Ethernet cables).

HPE recommends 0.5 meters long stacking cables and a ring topology (so last unit reconnects back to first unit of the chain).

The Aruba 2920 backplane stacking is not like Comware based IRF (frontplane) stacking technology or the other, more recent, ArubaOS-Switch based (frontplane) VSF stacking technology recently introduced on Aruba 5400R zl2 (in v3 mode and with v3 modules only) and, lately, also on Aruba 2930F.

In the end...if you must do stacking splitted between far DCs you will be forced to shift for a Comware 5/7 based units choice (probably better for the typical DC job IMHO) or to Aruba 2930F/5400R zl2 if you like the ArubaOS-Switch based units (Formerly ProVision of the HP ProCurve line).