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Configuring the Transmission of Maximum VLAN Name TLVs in LLDP

This topic discusses the support of transmitting maximum number of VLAN Name TLVs in Link Layer Discovery Protocol (LLDP) protocol data units (PDUs).

Generally, an LLDP PDU can incorporate a maximum of 5 VLAN Name TLVs. To advertise more than 5 VLAN Name TLVs, run the set protocols lldp interface <name> transmit-max-vlan-name-tlv command. This command is a one-time switch which enables the support of transmitting maximum number of VLAN Name TLVs. However, to disable the support and reset the system to limit up to 5 VLAN Name TLVs, run the delete protocols lldp interface <name> transmit-max-vlan-name-tlv command.

Note:

The <name> in the above commands indicates the interface name.

The maximum number of VLAN Name TLVs that can be carried is calculated dynamically. The number is based on the maximum transmission unit (MTU) capacity of the PDU while considering the other necessary TLVs that need to be carried in the PDU.

Considerations for Enabling Transmission of Maximum VLAN Name TLVs in LLDP PDUs

Consider the following conditions before enabling the transmission of maximum VLAN Name TLVs in LLDP PDUs:

  • In a default scenario, wherein VLANs configured on the interface have IDs 2 to 1000, with a default MTU value of 1500, 68 VLAN Name TLVs are transmitted.

  • If MTU of the interface is increased to 9192 which is the maximum, then 523 VLAN Name TLVs are transmitted. The number of VLAN Name TLVs may vary based on the size of VLAN ID. That is, if the VLAN IDs are more than a single digit, then the maximum number of VLAN Name TLVs, which are transmitted decreases.

  • If the vlan-name-tlv-option name is selected to be transmitted in the VLAN Name TLV, the number of transmitted TLVs changes based on the size of the names of the VLANs. By default, VLAN Name is transmitted as “vlan-ID#”. Therefore, the number of VLAN Name TLVs may increase, or decrease depending on the VLAN names being larger or smaller compared to the default name.

  • The VLANs that are transmitted is decided by the VLAN Name TLV key. By default, it is decided by the VLAN ID. For example, if VLANs are configured from 101 to 200, the first 68 VLANs are transmitted.

  • Each LLDP PDU transmits the same VLAN Name TLVs until new VLANs are added or deleted, or if the MTU is changed. If new VLANs are added or deleted, then the next set of TLVs are formed based on the conditions mentioned above.

  • The maximum VLAN Name length which can be accommodated in the VLAN Name TLV is 34 bytes. Therefore, if the VLAN Name length is more than 32 bytes, that vlan is eliminated form the list.

  • As an example, the table below contains the maximum number of VLAN Name TLVs which can be sent in a given scenario. In the table, the configured VLAN IDs start from 101 and go till 200, which means that the default VLAN Name has a length of 10 bytes, and the maximum VLAN Name that is configured is with the length of 32 bytes. The MTU is set to the default value of 1500.

    Table 1: Maximum Number of VLAN Name TLVs
    Variations TLVs Data Size Maximum Number of VLAN Name TLVs
    Having vlan-name-tlv-option set as vlan-id (each of length - 15 bytes) Having vlan-name-tlv-option set as Name and VLAN size of 32 bytes (each VLAN Name TLV of length - 39 bytes)

    Example 1

    Mandatory TLVs:

    • Chassis id- 7 bytes

    • Port Id- 4 bytes

    • TTL- 2 bytes

    • End tlv- 0 bytes

    13 bytes

    65

    27

    Port Description TLV

    8 bytes

    System Name TLV

    25 bytes

    System Description TLV

    171 bytes

    System Capabilities TLV

    7 bytes

    Management Address TLV

    8 bytes

    Mac-PHY status TLV

    9 bytes

    Link Aggregation subtype 3 and Subtype 7

    9 bytes + 9 bytes

    Max Frame size TLV

    6 bytes

    2 Juniper Specific TLVs

    16 bytes each

    Port Vlan-ID TLV

    6 bytes

    Example 2 -Includes TLVs in Example 1 with the addition of...

    3 DCBX TLVs

    25 bytes + 25 bytes + 6 bytes

    60

    25

    LLDP MED Capabilities TLV

    7 bytes

    Example 3 -Includes TLVs in Example 2 with the addition of...

    5 Management Address TLVs of type IPV6

    36 bytes

    50

    20

    Power via MDI

    12 bytes

    Extended power via MDI

    7 bytes

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