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List of VLANs

Let's refer to RFCs for further information about OIDs' descriptions, and also MIBs' descriptions (*.mib) given by constructors for each equipment.

In the following descriptions, ifIndex refers to the OID ., which is a index used to talk about interfaces .

The list of VLANs is available at the following OIDs:

Interface/VLAN table

The table of interfaces and VLANs assocations (i.e. for each port, the list of VLANs it is part of) is available at the following OIDs:

Tagging configuration

The behaviour for non tagged packets (actually, we want to know on which VLAN number the untagged packets are tagged for each port) is described in the following OIDs:

Encapsulation is the process of adding control information to a datagram so that it can be transmitted by a protocol or networking technology other than that for which it was originally intended.

Compatibility table

Heuristic for forwarding lists dispatching by VLAN

For 3Com switches, it rules because we get the forwarding lists by VLAN using special community strings, suffixing the standard community name with @ followed by the VLAN number.

For other switches, it is much more tricky. We have to guess on which VLANs mac addresses of the forwarding lists are seen by hosts. Suppose you have a host A that sees on one of its ports the MAC address of host B, without knowing on which VLANs B packets are tagged.

If at least one of the hosts A and B is VLAN-aware:

Otherwise, both A and B are not VLAN-aware:

We look for the host seeing B on one of its ports but with a minimum number of entries in the corresponding forwarding list. Then we fetch the tagging configuration for this port, and we assume this MAC is always seen on this VLAN.