VLAN Discovery
We start with VLAN discovery. This is executed by calling _get_vlan_data()
Switching capabilities are described by what is called the MIB-2 Q-Bridge MIB. The Q-Bridge defines the switch ports and vlans on a device. Note that in this context, a port is NOT the same as an interface as described above. A (switch) port is typically a physical port on the device, and as such has a port-id. Interfaces can be virtual (eg. “interface Vlan 100”). Virtual interfaces have interface-id’s but not port-id’s.
The often confusing part is that on many (but not all) devices, the interface-id and port-id are the same number. However, this is not a given, so we need to discover the mapping of (switch) port-id to interface-id, and then can discover vlan(s) on those ports.
This mapping is done with the “dot1D-Bridge” entry dot1dBasePortIfIndex (.1.3.6.1.2.1.17.1.4.1.2), which maps a physical port-id to an ifIndex.
self.get_snmp_branch(‘dot1dBasePortIfIndex’)
Note: if we cannot read this, we cannot continue, and will error out! (These MIB entries are required!)
Next we read existing vlan id’s from dot1qVlanStaticTable (.1.3.6.1.2.1.17.7.1.4.3),
We read dot1qVlanStaticRowStatus (.1.3.6.1.2.1.17.7.1.4.3.1.5) to get all vlans:
self.get_snmp_branch(‘dot1qVlanStaticRowStatus’)
and if any vlans are found, we try to get the names from dot1qVlanStaticName (.1.3.6.1.2.1.17.7.1.4.3.1.1)
self.get_snmp_branch(‘dot1qVlanStaticName’)
We then read the vlan status (dynamic, static, etc.) from dot1qVlanStatus (.1.3.6.1.2.1.17.7.1.4.2.1.6)
self.get_snmp_branch(‘dot1qVlanStatus’)
Note
The process of discovering vlans, interfaces, (switch) ports, ethernet addresses and IP address is explained well in this StackExchange posting:
Once we know existing vlans, we move on to finding the interfaces via snmp.
Note
Some devices implement the IEEE 802.1Q Q-Bridge MIB (IEEE8021-Q-BRIDGE-MIB), instead of MIB-2 Q-Bridge MIB. Drivers for these devices will override the function _get_vlan_data() and read the data from that MIB. See eg. the Aruba AOS-CX over SNMP driver in snmp/aruba_cx/connector.py