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VLANs

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 .1.3.6.1.2.1.2.2.1.1, which is a index used to talk about interfaces .

The list of VLANs is available at the following OIDs:

• dot1qVlan
  .1.3.6.1.2.1.17.7.1.4.3.1.1.NNN
iso. org. dot. internet. mgmt. mib-2. dot1dBridge. qBridgeMIB. qBridgeMIBObjects. dot1qVlan. dot1qVlanStaticTable. dot1qVlanStaticEntry. dot1qVlanStaticName. NNN
NNN is a VLAN's number. The OID value is the VLAN's description.
• a3VlanGM
  .1.3.6.1.4.1.43.10.1.14.1.1.1.2.NNN
iso. org. dot. internet. private. enterprises. a3Com. generic. genExperimental. genVirtual. a3ComVlanGroup. a3ComVlanGlobalMappingTable. a3ComVlanGlobalMappingEntry. a3ComVlanGlobalMappingIfIndex. NNN
NNN is 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.

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:

• dot1qEgress
  .1.3.6.1.2.1.17.7.1.4.3.1.2.NNN
iso. org. dot. internet. mgmt. mib-2. dot1dBridge. qBridgeMIB. qBridgeMIBObjects. dot1qVlan. dot1qVlanStaticTable. dot1qVlanStaticEntry. dot1qStaticEgressPort. NNN
NNN is 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.
• ifStack
  .1.3.6.1.2.1.31.1.2.1.3.MMM.NNN
iso. org. dot. internet. mgmt. mib-2. ifMIB. ifMIBObjects. ifStackTable. ifStackEntry. ifStackStatus. MMM. NNN
MMM is a ifStackHigher value and NNN a 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               8
  

  A stack that contains a VLAN, encapsulation and a port interface, specifies:
  • For packets received through the given port that use the given encapsulation scheme and contain the given tag, those packets are members of the given VLAN.
  • For unencapsulated packets from the given VLAN that are to be transmitted out the given port, those packets must first be encapsulated using the given encapsulation algorithm and tag.
  This table is implemented by all 3Com network devices that support the encapsulation of multiple VLANs over a single interface.

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:

• dot1qPvid
  .1.3.6.1.2.1.17.7.1.4.5.1.1.NNN
iso. org. dot. internet. mgmt. mib-2. dot1dBridge. qBridgeMIB. qBridgeMIBObjects. dot1qVlan. dot1qPortVlanTable. dot1qPortVlanEntry. dot1qPvid. NNN
NNN is 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.
• a3VlanE
  .1.3.6.1.4.1.43.10.1.14.4.1.1.3.NNN
iso. org. dot. internet. private. enterprises. a3Com. generic. genExperimental. genVirtual. a3ComEncapsulationGroup. a3ComVlanEncapsIfTable. a3ComVlanEncapsIfEntry. a3ComVlanEncapsIfTag. NNN
NNN refers 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.

Compatibility table

• 3Com CoreBuilder 3500
  • List of VLANs: a3VlanGM
  • Interface/VLAN table: ifStack
  • Tagging configuration: a3VlanE
• 3Com SuperStack II
  • List of VLANs: a3VlanGM
  • Interface/VLAN table: ifStack
  • Tagging configuration: a3VlanE
• Cabletron Smartstack ELS100-S24TX2M (now called Vertical Horizon by Enterasys)
  • List of VLANs: dot1qVlan
  • Interface/VLAN table: dot1qEgress
  • Tagging configuration: dot1qPvid
• Enterasys Matrix E5
  • List of VLANs: dot1qVlan
  • Interface/VLAN table: dot1qEgress
  • Tagging configuration: dot1qPvid
• Bay Networks 2216T
  • List of VLANs: (dot1qVlan)
  • Interface/VLAN table: (dot1qEgress)
  • Tagging configuration: (dot1qPvid)

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:

• If B knows the VLANs on which A packets are tagged, the problem becomes rather simple. We assume then that A sees B on the same VLANs as B sees A.
• If not, we have two methods:
  • (very slow) we try to collect the ports on which B sees A, and to make the union of the VLAN lists for all these ports. We also get the VLAN list for the port on which A sees the MAC address of B. Finally, we do the intersection the VLAN lists for A and B.
  • (quicker) we try to collect the VLAN list for host B. We also get the VLAN list for the port on which A sees the MAC address of B. Finally, we do the intersection the VLAN lists for A and B.

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.