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Cisco-SB SNMP VLAN Overview

CBS switches use primarily the CISCOSB-vlan-MIB for VLAN definition and interface vlan management.

There are some things to note:

Switch ports have several modes:

  • Access - ie single untagged frames

  • Trunk - dot1q encapsulated

  • General - access both untagged and tagged frames. We are not sure what the difference is with Trunk, which also allows both untagged and tagged frames.

The port mode is read from and written to the vlanPortModeState mib (.1.3.6.1.4.1.9.6.1.101.48.22.1.1.<ifindex>), where:

  • 10 = General

  • 11 = Access

  • 12 = Trunk

General mode

The untagged vlan is read from and written to the Q-Bridge standard “dot1qVlan” MIB id (dot1qVlan.<ifIndex> = <vlan-id>)

It appears that the general mode allowed tagged vlans are represented by the standard Q-Bridge “egress port” vlan fields. As we don’t support this mode at time of writing, this data is ignored.

Access mode

The untagged vlan is read from and written to “vlanAccessPortModeVlanId” MIB id (vlanAccessPortModeVlanId.<ifIndex> = <vlan-id>)

Trunk mode

The untagged vlan on a trunk port is read from and written to vlanTrunkPortModeNativeVlanId.<ifIndex> = <vlan-id>

The vlanTrunkPortModeEntry branch has all vlan memberships. This is a bitmap that is indexed by interface, and grouped by 1024 vlan numbers.

E.g.

vlanTrunkModeList1025to2048.14 = <bitmap>

For the interface with ifIndex 14 (likely “GigabitEthernet14”), the bitmap has a 1 for every vlan from 1025 -> 2048 that is enabled on the trunk configuration.

Likewise for vlanTrunkModeList1to1024.<ifIndex>, vlanTrunkModeList2049to3072.<ifIndex> and vlanTrunkModeList3073to4094.<ifIndex>

Note

This is the opposite of the Q-Bridge VLAN bitmaps, where the index is the Vlan ID, and bitmap represents the switch ports that are active on that vlan!