The Network monitoring topology sometimes shows one link between switches, though it has two physical connections.

In the above diagram, we can see that only the Access1 switch has two links. Other switches have 2 physical connections. However, it shows only one.

To identify the issue, we need to collect the following output from the switches:

DIST1:

root@> show lldp neighbors
Local Interface    Parent Interface    Chassis Id          Port info          System Name
xe-0/0/2           ae2                 40:9e:a4:0b:41:00   xe-0/2/0           Access3
xe-1/0/2           ae2                 40:9e:a4:0b:41:00   xe-0/2/1           Access3
xe-0/0/7           ae5                 40:9e:a4:0b:47:00   xe-0/2/0           Access1
xe-1/0/7           ae5                 40:9e:a4:0b:47:00   xe-0/2/1           Access1
xe-0/0/6           ae4                 40:9e:a4:0b:48:80   xe-0/2/0           Access4
xe-1/0/6           ae4                 40:9e:a4:0b:48:80   xe-0/2/1           Access4

Non-working access layer switches:

Access4:

root@> show lldp neighbors
Local Interface    Parent Interface    Chassis Id          Port info          System Name
xe-0/2/0           ae0                 ac:a0:9d:98:3f:80   xe-0/0/6           DIST1
xe-0/2/1           ae0                 ac:a0:9d:98:40:00   xe-1/0/6           DIST1

Access3:

root@> show lldp neighbors
Local Interface    Parent Interface    Chassis Id          Port info          System Name
xe-0/2/0           ae0                 ac:a0:9d:98:3f:80   xe-0/0/2           DIST1
xe-0/2/1           ae0                 ac:a0:9d:98:40:00   xe-1/0/2           DIST1

Working access layer switch:

Access1:

root@> show lldp neighbors
Local Interface    Parent Interface    Chassis Id          Port info          System Name
xe-0/2/0           ae0                 ac:a0:9d:98:3f:80   xe-0/0/7           DIST1
xe-0/2/1           ae0                 ac:a0:9d:98:3f:80   xe-1/0/7           DIST1

OpenNMS will check the "show lldp neighbor" output from the devices to build the topology. There are 2 links connected to the distribution switch via the port channel for all access switches except 2. For one link, we can see access switch is pointed to a different Mac address ac:a0:9d:98:40:00 (Chassis ID), and for that, the link is not displaying in the OpenNMS topology.

Please see the below output from Pgsql database

opennms=> select * from lldpelement where lldpsysname='DIST1';

id | nodeid | lldpchassisid | lldpchassisidsubtype | lldpsysname |    lldpnodecreatetime    |  lldpnodelastpolltime    

-----+--------+---------------+----------------------+--------------+----------------------------+---------------------------

927 |    30 | aca09d983f80 |                   4 | Dist_Block_A | 2024-09-13 12:30:05.729+00 | 2025-03-27 06:41:14.16+00

(1 row)

As per the design, one device can only have one mac address. If the output from switch displays different mac address apart from ac:a0:9d:98:3f:80, it will not display the link. Because one switch can only have one Chassis ID as per the Pgsql/Opennms.

If you need to see 2 links between the switches, please reach out to the EX TAC for help.