Network Outage: %ETHCNTR-3-LOOP_BACK_DETECTED

The network is performing well, unfortunately we received a call that the few of the directly connected to each other are not working and the links attached to the switches were also not working. After logging on to the switches, we saw a weird error message "%ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/1". Fa0/1 was uplink to another switch and the port state was showing error disabled. The switch port goes in error disable mode if it detects any error situation on the port. The port is shut down and need to enable manually or automatically if configured. By default, all the cisco switches and routers send loopback frames (ethertype 0x9000) with SA=DA=It’s own mac-address. If the Keepalive packet is received on the same port it was sent from, then a loop exists which even cannot be detected by spanning tree protocol. This is likely the same as of split horizon technique in layer three devices. The problem most likely arises because of unmanaged switches or hubs connected to the switches.
To overcome the problem, we need to add “no keepalive” command under interface which prevents the port from being disabled. This issue is documented in Cisco bug ID CSCea46385.

Disable "no errdisable detect cause loopback" command is another way to handle the problem. By doing this when the switch detects the error disable message of loopback, it will not take any action.

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Auto Negotiation Myth

The debate on auto negotiation is still on. I have faced the lot of outages because of auto negotiation. Sometimes you find that the working link with auto negotiation configured come up with half duplex consequences packet drops which may lead to the various outages. According to the IEEE 802.3u clause number 28, 802.3z clause number 37 standards the auto negotiation should be enabled on devices. This is optional but always recommended. The best is to read the SUN’s Ethernet Auto Negotiation Best Practises.
But I used to prefer to disable the auto negotiation and forcefully set the all capabilities. But after reading this paper, I come to know that after disabling auto negotiation we are not able to detect the following problems:-
1. Bad Cable Problem
2. Link failures
3. Link Partner Capabilities
4. Performance Issues
5. Pause Frames or Flow Control
If we are facing problems by enabling auto negotiation, In that case we need to talk to the vendors rather than disabling it.

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One Port Of Switch Is Up and Another Port Of Another Switch Is Down

One port is up and another port is down, the issue seems to be weird one but it happens when the two switches are connected over fiber. This happens because one side of the switch is not receiving the signal. In the given diagram, see the connectivity of the ports.

The failure of one fiber because of any reason make that port down. The port which is receiving the signal is showing up and the port which is not receiving the signal is showing down. This type of problem leads to the spanning tree loop in the metro ethernet or ring topology. To overcome this problem, UDLD (Uni Direction Link Detection) is required. This protocol forces the both end ports to make down.
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