Wired Intelligent Edge

Hi All,

Just wanted to know I have trunked configured by previous engineer between aruba 3810 switch and avaya 4850 switch. When creating the trunk the previous network engineer didnt added the untagged vlan.

Now I'm replacing those avaya switches with aruba 2930f model. I know untagged vlan is important but as stated above can a trunk be configured without untagged vlan can all vlan be tagged. Please suggest me some best practice or guide in this case.

Thanks,

Rish

Hi Rish,

Untag VLAN we use in edge ports to assign that VLAN to incoming traffic (without VLAN tag)  from the end device. if packet incoming with VLAN tag that VLAn should be tag in the port otherwise it will be discarded.

It is not necessary to untag vlan between the switch to switch links.

more info: https://community.hpe.com/t5/aruba-provision-based/tagged-untagged-trunks-etc/td-p/6722082#.XtheLjozbDc

I think best practice is to untagged network management vlan and tagged others.

The untagged "pvid" vlan is important because switches use it to transport switch management frames like LLDP, spanning-tree and BPDU messages.

For ANY switch to switch (switch interlink connections) its important that ALL VLANS are TAGGED. With the exception of 1 Dummy VLAN that is used for nothing else then switch management frames, no other devices are allowed on this "Dummy" vlan.

For example:

vlan 1 disable

vlan 2 untagged/pvid (dummy vlan only for switch management frames)

vlan 10 tagged (management switch login, airwace monitoring, etc.)

vlan 11 tagged (normal traffic)

vlan 12 tagged (normal traffic)

etc.

interface 1/1
name "Uplink to switch..."
no untagged vlan 1
untagged vlan 2
tagged vlan 10-12

Most vendors uses default vlan 1 as pvid vlan and can mostly changed in the configuration, others are using vlan 1 fixed as pvid vlan. Check that in the vendor documentation. 

Hi Marcel and other members,

Thanks for replying to this post, before we marked this solution I want to know if in trunking/lacp is it important that both side of trunk have same vlans tagged so lets say between a trunk on core switch we have vlan 115 tagged while on access switch we dont have that vlan tagged so does that going to affect in anyway other than dropping the frames I guess.

Thanks,

Rish

Hi Rish,

Thats correct! Vlans must be the same tagged/untagged on both ends. When there is a mismatch, that specific vlan dont come through.

When a vlan is add to a interface at one site all Traffic for that vlan will be uplinked on that interface, if the vlan is missing on the second switch, the second switch will drop the traffic as “dropped rx frames” because the switch decide the traffic is not for him.

Hi! it will sound like a joke but, for one moment, think about your scenario as having a bridge over a river connecting two banks (left and right) and, on top of that bridge, at each side, there are check points permitting/denying cars to pass by verifying if particular cars colors are permitted or not: say only red and blue cars can egress/ingress the left bank at one left check point while traversing the bridge...what will happen if the other checkpoint at right bank will refuse incoming/outgoing red (or blue, if you like) cars while the other one is actually permitting outgoing/incoming red cars to pass?

Here you have the "mismatch" and cars are "dropped" (red cars don't reach the other bank because they are refused by the checkpoint).

Will this denial affect the blue (or red cars) flow? No it will not because blue (or red cars) are indeed permitted to flow in both directions through both checkpoints.