Nexus 7706 Override Port Groups

[noisey_uk]

Hi

I'm working on a migration to a Nexus 7706 with N77-M348XP-23L line cards. The Cisco Nexus 7000 Series Virtual Device Context Configuration Guide (and some other documentation such as datasheets) tell me that this line card has 2 x 24-interface port groups. My understanding of Nexus switching (for about a decade) is that we must allocate an entire port group to a particular VDC. However, there's a statement in this document that suggests it's possible to override this behaviour (with caveats):

"When interfaces in different VDCs share the same port ASIC, reloading the VDC (with the reload vdc command) or provisioning interfaces to the VDC (with the allocate interface command) might cause short traffic disruptions (of 1 to 2 seconds) for these interfaces. If such behavior is undesirable, make sure to allocate all interfaces on the same port ASIC to the same VDC."

Am I misreading this? Is it possible to assign more than one VDC per ASIC? If so, how?

This other post: https://community.cisco.com/t5/switching/cisco-7710-vdc-port-allocation-n77-m348xp-23l/td-p/3177385 suggests that it won't permit allocation other than in the 2 port groups.

Thanks!

[Alex.M · Accepted solution]

Hi noisey_uk,

Excellent question.

If I'm understanding correctly, your question revolves around how an ASIC can belong to multiple VDCs if it is at all possible. The short answer to your question is yes, a single ASIC can belong to at most two VDCs but only on F-series line cards.  There are some licensing requirements to allow this feature to be enabled on F-series cards but keep in mind that out of those two VDCs that 'share' an ASIC, one of those must be the storage VDC. 

This sharing of an ASIC is possible due to the ethertype headers used in FCoE (0x8906 and 0x8914) identifying traffic destined to the storage VDC.  Please note, that N77-M348XP-23L lines cards are 'hardware ready' for FCoE and therefore, cannot have multiple VDCs applied to the same ASIC, at least for now.

A guide to setup a storage VDC can be found at:

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus7000/sw/fcoe/config/cisco_nexus7000_fcoe_config_guide/storage_vdc_on_cisco_nexus_7000_series_switches.pdf

Regards,

Alex