[c-nsp] Question for LACP/LAG gurus

[Dmitry Kiselev]

Hello!

While building several new LAGs on IOS XR I found strange behaviour of
Cisco IOSes for "system priority" LACP parameter. Most of classic IOSes
allow to set value from 0 to 65535, but IOS XR does not:

12.2(55)SE2 Switch(config)#lacp system-priority ?
  <0-65535>  Priority value

12.2(54)SG Switch(config)#lacp system-priority ?
  <0-65535>  Priority value

12.2(33)SRE Router(config)#lacp system-priority ?
  <0-65535>  Priority value

IOS XE 12.2(33)XNF2 Router(config)#lacp system-priority ?
  <0-65535>  Priority value

IOS XR 4.0.1 Router(config)#lacp system priority ?
  <1-65535>  Priority for this system. Lower value is higher priority.

Moreover, IOS XR does not form the LAG if received partner system priority is zero.
In this case each bundle port shows the error message "Partner System ID/Key
do not match that of the Selected links" and remain "configured" state.

It would remain a theoretical nuance, but Extreme Networks switches advertise
priority=0 by default on all LAGs cousing some troubles in setup. Interesting
fact thats in the same time zero is invalid value in Extreme switch configuration  :) :)

Extreme# configure sharing 7 lacp system-priority ?
  <system_priority>  System Priority (1..65535)

I take a short look inside IEEE 802.1AX-2008 standart and IEEE 802.3-2005 clause 43
and does not see any special case for priority=0.  Does anybody in the list familar
enough with LACP to explain me why Cisco IOS XR does not like zero here?

Thanks

-- 
Dmitry Kiselev