I need to tag outgoing packets with a VLAN ID of 0 and a 802.1p tag of 3. The GUI or CLI won't let me create a VLAN with ID of 0. When I create a bridge filter rule to "set priority" on outgoing packets, I see packets hitting the rule but no priority tag is applied, I am assuming because there is no VLAN tag. Any ideas would be greatly appreciated.

VLAN ID 0 is reserved under the 802.1q standard which is probably why the RouterOS implementation doesn't permit it.

Thank you for your response, that makes sense why I cannot directly use VLAN 0. The packets that I sniffed from the router I need to emulate indeed has the VLAN tag set to 0 and a priority tag set. This is a blurb I found elsewhere on the internet:

Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames

VLAN 0 packets should carry an 802.1p priority but otherwise should be handled as untagged packets

So I suppose my question is, am I doing something wrong with my bridge filter to apply a priority tag to an untagged packet, or is the Mikrotik router not applying the priority tag because there is no vlan tag?

There is no place to put the priority value unless there is a tag. This is why tag ID 0 is allowed -- for layer-2 priority communications of packets belonging to no Vlan. In my experience there are many types of networking devices which get this wrong and will not allow a tag with a vlan ID of zero. I also find a similar problem with equipment not allowing a tag value of 4095 which is the throw-away vlan.

Because of this common implementation problem of not allowing a tag value of zero, I always use vlan tagged packets with ID>0 on all networks. That works better than trying to get all the vendors to fix their implementations.