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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
.188.8.131.52.184.108.40.206.1.1, which is a index used to talk about interfaces .
The list of VLANs is available at the following OIDs:
iso. org. dot. internet. mgmt. mib-2. dot1dBridge. qBridgeMIB. qBridgeMIBObjects. dot1qVlan. dot1qVlanStaticTable. dot1qVlanStaticEntry. dot1qVlanStaticName. NNN
NNNis a VLAN's number. The OID value is the VLAN's description.
iso. org. dot. internet. private. enterprises. a3Com. generic. genExperimental. genVirtual. a3ComVlanGroup. a3ComVlanGlobalMappingTable. a3ComVlanGlobalMappingEntry. a3ComVlanGlobalMappingIfIndex. NNN
NNNis a VLAN's number. The OID value is the number of an virtual interface; this number of an virtual interface represents a VLAN and refers to the ifIndex.
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:
iso. org. dot. internet. mgmt. mib-2. dot1dBridge. qBridgeMIB. qBridgeMIBObjects. dot1qVlan. dot1qVlanStaticTable. dot1qVlanStaticEntry. dot1qStaticEgressPort. NNN
NNNis a VLAN's number. The OID value is a bits' mask"; this mask (seen as a boolean list) coding if the VLAN is on a port (1) or not (0); the port 1 is the first bit, port 2 the second, etc.
iso. org. dot. internet. mgmt. mib-2. ifMIB. ifMIBObjects. ifStackTable. ifStackEntry. ifStackStatus. MMM. NNN
MMMis a ifStackHigher value and
NNNa ifStackLower value; they refer to the ifIndex; it's a coding of a relation father/son in trees of interfaces (port or virtual interfaces) where the interaces representing the VLANs are the roots, and ports belonging to this VLAN are the leaves. The OID value is 1(active).
An example of a VLAN configuration and all ifStackHigher/ifStackLower defined to code this configuration: +=======================+=======================+ | VLAN Xface 9 | VLAN Xface 11 | +=======================+=======================+ +=======================+ | ENCAPS Xface 10 | +=======================+ +=====+=====+=====+=====+=====+=====+=====+=====+ | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |<=== Ports +=====+=====+=====+=====+=====+=====+=====+=====+ ifStackTable Instances : ifStackHigher ifStackLower 0 9 0 11 1 0 2 0 3 0 4 0 9 10 10 1 10 2 10 3 10 4 11 5 11 6 11 7 11 8A stack that contains a VLAN, encapsulation and a port interface, specifies:
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:
iso. org. dot. internet. mgmt. mib-2. dot1dBridge. qBridgeMIB. qBridgeMIBObjects. dot1qVlan. dot1qPortVlanTable. dot1qPortVlanEntry. dot1qPvid. NNN
NNNis a interface's number and refers to the ifIndex. The OID value is a VLAN's number find in dot1qVlanStaticName and is the VLAN on which the interface tags.
iso. org. dot. internet. private. enterprises. a3Com. generic. genExperimental. genVirtual. a3ComEncapsulationGroup. a3ComVlanEncapsIfTable. a3ComVlanEncapsIfEntry. a3ComVlanEncapsIfTag. NNN
NNNrefers to the ifIndex and more precisely to an virtual interface (called encapsulation's interface) which is associated to a port in ifStack to configure the tagging behaviour of this port. It's the tag used when encapsulating packets transmitted, or de-encapsulating packets received through this interface. The OID value is the tag value (a VLAN's number).
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.
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.