How we discover Vlan Memship

We already should have existing vlan information at this time.

We call SnmpConnector()._get_port_vlan_membership()

We use the Q-Bridge MIB for port vlan information. See https://mibs.observium.org/mib/Q-BRIDGE-MIB/

Untagged vlan

The Q-Bridge “dot1qPvid” entry maps every switch port (i.e. port-id) to the untagged vlan on that port.

self.get_snmp_branch('dot1qPvid')

So here is the first time we need the mapping from port-id to interface-id (see Vlan discovery) to read or set the untagged vlan on an interface.

Some devices may not implement the above dot1qPvid mib,

Tagged Vlans

We look at the “Current” and “Static” vlan branch of the Q-Bridge MIB.

# THIS IS LIKELY NOT PROPERLY HANDLED !!!
# read the current vlan untagged port mappings
# retval = self.get_snmp_branch(dot1qVlanCurrentUntaggedPorts)
# if retval < 0:
#    self.add_warning(f"Error getting 'Q-Bridge-Vlan-Untagged-Interfaces' ({dot1qVlanCurrentUntaggedPorts})")
#    return retval

# read the current vlan egress port mappings, tagged and untagged
retval = self.get_snmp_branch('dot1qVlanCurrentEgressPorts')

# read the 'static' vlan egress port mappings, tagged and untagged
# some devices do not add ports to "dot1qVlanCurrentEgressPorts" until the interface is in the "up" state
# reading this will show the static vlan configs for these ports...
retval = self.get_snmp_branch('dot1qVlanStaticEgressPorts')

dot1qVlanCurrentEgressPorts.<vlan-id> has a sub-oid for each vlan, where the returned value is a bitmap of all ports active on this vlan, in either tagged on untagged mode. The port-id is the offset of the bit in the bitmap.

E.g. bit 1 in byte 1 is port-id 1. Bit 3 in byte 2 is port-id 11. Etc.

We also read dot1qVlanStaticEgressPorts, similarly a bitmap, but for ports that are ‘statically defined’. As the comments show, some devices do not add ports to “Current” bitmap until the interface is in the “up” state. Reading this will show the static vlan configs for these ports…

Thus the key difference between dot1qVlanStaticEgressPorts and dot1qVlanCurrentEgressPorts in the Q-BRIDGE-MIB lies in whether they represent the administratively configured VLAN membership or the actively operational VLAN membership.

In summary:

dot1qVlanStaticEgressPorts - This shows which ports are assigned to a VLAN by the device configuration and survives a reboot.

dot1qVlanCurrentEgressPorts - This shows which ports are currently, when read, transmitting traffic for the VLAN, either tagged or untagged. This includes ports dynamically learned (e.g., via GVRP), even if not explicitly configured. The same warning about interface not in “up” state as above goes here as well. (Thus the reading of dot1qVlanStaticEgressPorts as well)

Ie. If there is NO dynamic learning and all ports are “up”, the two entries should be the same!

Note: at present, we don’t read dot1qVlanCurrentUntaggedPorts.<vlan-id>. This only returns the untagged ports, again indexed by vlan id, and the return value is the bitmap of ports active. We should have already gotten this data from dot1qPvid

IEEE Q-Bridge MIB

This MIB defines a similar structure to the Q-Bridge MIB. Some devices implement this MIB (or both). See https://mibs.observium.org/mib/IEEE8021-Q-BRIDGE-MIB/

As mentioned in the vlan discovery section, you can learn Vlans, static or dynamic, from ieee8021QBridgeVlanStatus.<vlan-id>

Then you can get names from ieee8021QBridgeVlanStaticName.<vlan-id>

Like Q-Bridge, you can read tagged and untaggeded ports in vlan from ieee8021QBridgeVlanCurrentEgressPorts.<vlan-id> and ieee8021QBridgeVlanStaticEgressPorts

And get the list of untagged ports from ieee8021QBridgeVlanCurrentUntaggedPorts.<vlan-id> and ieee8021QBridgeVlanStaticUntaggedPorts

Note that all these entries also return bitmap values that have the bit set for the corresponding Port-ID, i.e. not the ifIndex! You have map the port-id back to the proper interface ifIndex!