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Port link settings mismatch can cause latency issues on the EX-series Switch if configured incorrectly

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Article ID: KB14241 KB Last Updated: 24 Jun 2010Version: 2.0
Summary:
High latency or low throughput may be seen when the interface ether-options speed/duplex are incorrectly set on the EX or remote device.  This KB describes how to verify if the high latency or low throughput is caused by port link settings.


Symptoms:
The EX Swtich interface ether-options settings are mismatched compared to remote device settings or autonegotiation failure.
Solution:
Latency can be seen on ports when the local or remote device settings are misconfigured or autonegotiation fails and or possible duplex conflict.  For example if autonegotiation is enabled on the EX interface and the remote link partner is not able to negotiate on link mode setting when speed is negotiated at 100Mbps.

Commands from EX:

Example of Successful Negotiation:

"show interfaces ge-0/0/13 extensive | match "Link|neg" "

root@EX3200# run show interfaces ge-0/0/13 extensive | match "link|neg"
Physical interface: ge-0/0/13, Enabled, Physical link is Up
Link-level type: Ethernet, MTU: 1514, Speed: Auto, Duplex: Auto,
Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled,
Link flags : None
Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0,
Autonegotiation information:
Negotiation status: Complete
Link partner:
Link mode: Full-duplex
, Flow control: Symmetric/Asymmetric,
Remote fault: OK, Link partner Speed: 100 Mbps
Flow control: Symmetric, Remote fault: Link OK

As you can see in above output Auto-negotiation is enabled on EX Switch and the Negotiated Status is complete.This shows that EX has complete link partner inforation which is link Mode as full duplex and Speed as 100mbps.

Example of Failed Negotiation:

With other equipment vendors it is possible that Link Mode settings are not negotiated correctly, or misconfigured, and the Link Partner information on the EX Switch shows link mode as UNKNOWN and InComplete as shown below.

root@EX3200# run show interfaces ge-0/0/13 extensive | match "link|neg"
Physical interface: ge-0/0/13, Enabled, Physical link is Up
Link-level type: Ethernet, MTU: 1514, Speed: Auto, Duplex: Auto,
Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled,
Link flags : None
Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0,
Autonegotiation information:
Negotiation status: InComplete
Link partner:
Link mode: UNKNOWN, Flow control: Symmetric/Asymmetric,
Remote fault: OK, Link partner Speed: 100 Mbps
Flow control: Symmetric, Remote fault: Link OK

As shown above the EX Switch has auto-negotiation enabled and speed is successfully negotiated to 100mbps but due to various reasons (misconfigured link settings, failed negotiation) the link mode could not be negotiated.  Because of this the EX Switch port is showing parter link mode as UNKNOWN and negotiation status as InComplete.

In this case, when speed is set to 100mbps and Link Partner's link mode is UNKNOWN, the EX link mode is defaulted to HALF Duplex.

If you see Link Mode of partner is showing UNKNOWN try setting the link mode on EX to full-duplex manually or disable auto-negotiation and set the speed and link mode manually to match with remote side.


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