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[EX] Behavior of Cisco and EX series switches when Link Layer Discovery Protocol (LLDP) is enabled

  [KB23996] Show Article Properties


Summary:
This article provides information about the behavior of EX and Cisco switches, when LLDP is enabled.
Symptoms:
In the following topology, Cisco and EX switches are connected with each other with a trunk link and the Cisco switch can see the EX switch with LLDP; but  the EX switch is unable to see the Cisco switch LLDP.

Topology
:

Cause:

Solution:
Cisco Behavior for LLDP:

The Cisco IOS always sends LLDP on VLAN 1; whether trunked or not. If the native VLAN ID is not 1, the frames are sent as 802.1Q tagged with VLAN ID 1; even if the VLAN is not configured on the trunk.

EX behavior for LLDP
:

The EX switch, as it has only LLDP (IEEE standard), sends LLDP packets without any tagging. Junos will not accept any packets on an 802.1Q trunk port, unless it is untagged and there is a defined native VLAN or the frame is tagged with a VLAN ID that is allowed on the trunk.

Due to the above mentioned behaviors, the Cisco device can see the EX switch; but EX switch is unable to see the Cisco device.

The workarounds are as follows:

  • Set the native VLAN 1 on both sides of the trunk (Cisco switch and EX switch).

  • Cisco sends LLDP packets on VLAN 1. In EX switches, the native VLAN id is not 1; but 506. So the LLDP frames are sent with the tagging of VLAN 1; but on the EX side, VLAN 1 is not allowed. So, the packets are being dropped. To resolve this issue VLAN 1 should be added in the allowed VLAN list on the trunk from the EX switch, so that it can can accept the LLDP packets from VLAN 1 on the Cisco switch. The configuration on the EX switch is as follows:
    user@switch# set interfaces ge-0/0/X unit 0 family ethernet-switching port-mode trunk vlan members [1 10 20 30]
    user@switch# set interfaces ge-0/0/X unit 0 family ethernet-switching port-mode trunk native-vlan-id 506
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