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

James Bensley jwbensley at gmail.com
Fri May 19 07:30:45 EDT 2017

On 18 May 2017 at 12:19, Irina Arsenieva <ecralar at hotmail.com> wrote:
> Hello all,
> A quick question – are EXP bits included in ECMP hashing algo on Cisco CRS, 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
> Alex

Hi Alex,

For the 7600s the EXP bit is not used in the hasing algo. The below
info is lifted directly from the Cisco Docs so a bit vague, you
probably need to lab it and opena TAC case to get exact beahviour
verified. I'm pretty sure like 99% sure the ASR9000 also doesn't
support using the EXP bit. I have no idea about the CRS.


The maximum number of load balancing paths is 8. The PFC forwards MPLS
labeled packets without explicit configuration. If the packet has
three labels or less and the underlying packet is IPv4, then the PFC
uses the source and destination IPv4 address. If the underlying packet
is not IPv4 or more than three labels are present, the PFC parses down
as deep as the fifth or lowest label and uses it for hashing.

MPLS Layer 2 VPN Load Balancing: Load balancing is based on the VC
label in the MPLS core if the first nibble of the MAC address in the
customer Ethernet frame is not 4.
Note: Load balancing is not supported at the ingress PE for Layer 2
VPNs. Load balancing is done based on the VC label and it is

MPLS Layer 3 VPN Load Balancing: MPLS Layer 3 VPN load balancing is
similar to basic MPLS load balancing.

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