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What is an em0 interface and what steps should be taken when the interface is down?

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Knowledge Base ID: KB13389
Last Updated: 17 Jul 2009
Version: 1.0

Summary:
On some of the later Routing Engines (RE) for M320, T320 and T640s (RE-1600, RE-3, RE-4) there exists an em0 interface that connects to the Control Board (CB) on the opposite plane. Here is what to look for when you see em0 interface is showing down in the logs.

Problem or Goal:
You may see the following logs in the messages file:

Feb 11 18:10:13 re0-abc /kernel: em0: Link is up 100 Mbps Half Duplex
Feb 11 18:10:13 re0-abc /kernel: em0: Link is Down
Feb 11 18:10:13 re0-abc mib2d[3387]: SNMP_TRAP_LINK_DOWN: ifIndex 17, ifAdminStatus up(1), ifOperStatus down(2), ifName em0
Feb 11 18:10:15 re0-abc /kernel: em0: Link is up 100 Mbps Half Duplex
Feb 11 18:10:16 re0-abc /kernel: em0: Link is Down
Feb 11 18:10:16 re0-abc mib2d[3387]: SNMP_TRAP_LINK_DOWN: ifIndex 17, ifAdminStatus up(1), ifOperStatus down(2), ifName em0
Feb 11 18:10:22 re0-abc /kernel: em0: Link is up 100 Mbps Half Duplex
Feb 11 18:10:22 re0-abc /kernel: em0: Link is Down
Feb 11 18:10:22 re0-abc mib2d[3387]: SNMP_TRAP_LINK_DOWN: ifIndex 17, ifAdminStatus up(1), ifOperStatus down(2), ifName em0
Feb 11 18:10:25 re0-abc /kernel: em0: Link is up 100 Mbps Half Duplex
Feb 11 18:10:25 re0-abc /kernel: em0: Link is Down

And the alarm could be triggered with communicating with the backup RE:

root@re0.abc01> show chassis alarms

1 alarms currently active
Alarm time Class Description
2009-02-11 17:50:09 PST Minor Loss of communication with Backup RE


Solution:
The master and the backup REs state information is exchanged via em0 interface.

The location of em0 interface on the CB l documented on the following link:

Redundancy Connection for a T320 Router and T640 Routing Node
Look at Figure 192

So when the log shows em0 link is down and loss of communication with the Backup RE under chassis alarms, then one of two hardware is faulty.
  • Backup CB (ethernet switch)
OR
  • Master RE (em0 interface)


Since it is quite unlikely both backup CB and master RE could be fault at the same time, it is suggested to:

1. Replace backup CB first with spare on site.
2. If it doesn't resolve this issue, replace the master RE.

Purpose:
Troubleshooting

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