[c-nsp] Cisco CRS, ASR9K & 7600 MPLS hashing/load-balance: are EXP bits included?

adamv0025 at netconsultings.com adamv0025 at netconsultings.com
Thu May 18 09:58:22 EDT 2017

> Irina Arsenieva
> Sent: Thursday, May 18, 2017 12:19 PM
> Hello all,
> A quick question - are EXP bits included in ECMP hashing algo on Cisco
> ASR9K and 7600 acting as LSR (_NOT_ headend/tailend/LER)?
> I have a particular case where FAT PWE with static flow labels are not
> balanced well, so looking to fix that with EXP bit rewrite.
> Many thanks
Very good question I actually don't know answer to.  
It looks like in Junos it can be enabled/disabled, but IOS and XR are
different, different hash inputs in different situations, can't see any
config option to enable/disable EXP/TOS/802.1p. 
Although personally I would not want the hashing to change depending on the
QOS markings on the packet.
See, if you look at all the usual hash inputs they are time invariant (well
u can argue the ingress interface -but that's not usually on by default). 
In case you allow for down/up marking in the core, then introducing QOS
markings into the hash algo may result in a case where if you down mark some
packets due to congestion it changes hashing resulting in even more


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