L2 Disjoint Network impact for vlan-1

[bruno.fernandes]

Hello Guys,

In a scenario with two different switching domains and I need to connected a UCS to both of them, normally I would use vlan-manager to trunk only specific vlans to each uplink.... Nevertheless I have the following questions:

1. what happens to "vlan 1" ?

2. I know that vlan-1 is mapped to both uplinks, but what uplink gets elected to process broadcasts ?

Thanks in advance,

Bruno Fernandes

[Evan Mickel]

The link below should help to clarify this:

https://www.cisco.com/c/en/us/td/docs/unified_computing/ucs/ucs-manager/GUI-User-Guides/Network-Mgmt/3-1/b_UCSM_Network_Mgmt_Guide_3_1/b_UCSM_Network_Mgmt_Guide_3_1_chapter_01000.html#concept_4E7BF4F5FA754183B5FE3298755800A1

So the answers in order would be:

1) VLAN 1 is implicitly present on all uplinks.

2) VLAN 1 will be assigned a designated receiver (DR) in an identical manner to all other VLANs present.  The DR is the port that will handle all broad/multicast traffic.  If you wish to view the DR for any given VLAN the command below is what you'll need to run from with NXOS of each FI.

show platform software enm internal info vlandb id <uint>

Thanks!

[Walter Dey]

That's why we preach since decades not to use Vlan 1 (nor Vsan 1)  for production.

[bruno.fernandes]

Yes I 100% agree but I do see this from time to time in customers and try fix things, normally when there is a migration

Regards,
BF

[bruno.fernandes]

Hi Evan,

1. But but how is the DR elected in the case of vlan-1 for a disjointed l2 when you are connected to both switching domains ?

Thanks
Regards
BF

[LukaszTJB · Accepted solution]

Since Vlan 1 is automatically assign on all Uplinks it is not usable for productive traffic in a disjoint Layer 2 configuration. The DR election is random and there is no preemption in case of a link failure. 

[bruno.fernandes]

Hi Lukas,

That's what I was suspecting, that fact is not very clear on documentation at least for vlan-1

Thanks,

Regards,

Bruno Fernandes