Problem:  Duplex mismatch on Juniper EX Switches.  Juniper Gigabit switch-ports run at half duplex when connecting to 100Mb interfaces (e.g., EX2200 uplink to SSG).

Troubleshoot:  Confirm the interface state from the switch port:  (a) Check for outbound collisions and (b) Autonegotiation status.

root@EX2200> show interfaces ge-0/0/0 extensive
  Solution:  Disable autonegotiation on the switch port.  Manually configure interface link settings:

root@EX2200# delete interfaces ge-0/0/0 ether-options auto-negotiation
root@EX2200# set interfaces ge-0/0/0 ether-options speed 100
root@EX2200# set interfaces ge-0/0/0 ether-options full-duplex
root@EX2200# set interfaces ge-0/0/0 ether-options no-auto-negotiation MDI Consideration:  Disabling auto-negotiation also disables auto-mdix.  This has no impact for crossover cables.  However, no data will pass with straight-through cables.

Straight-through cables can be used by disabling mdi-x detection and setting the mdi mode.

• mdi mode sets the interface to straigh-through mode.
• mdix mode sets the interface to crossover mode.

root@EX2200# set interfaces ge-0/0/0 gigether-options no-auto-mdix
root@EX2200# set interfaces ge-0/0/0 ether-options mdix
Troubleshoot Example:
root@EX2200> show interfaces ge-0/0/0 extensive
Physical interface: ge-0/0/0, Enabled, Physical link is Up
 Interface index: 129, SNMP ifIndex: 501, Generation: 132
 Description: Uplink-To-SRX Gateway
 Link-level type: Ethernet, MTU: 1514, Speed: Auto, Duplex: Auto, BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled,
 Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online, Media type: Copper Device flags : Present Running
 Interface flags: SNMP-Traps Internal: 0x4000
 Link flags : None
 CoS queues : 8 supported, 8 maximum usable queues
 Hold-times : Up 0 ms, Down 0 ms
 Current address: 54:e0:32:0a:63:03, Hardware address: 54:e0:32:0a:63:03
 Last flapped : 2015-11-05 18:50:43 UTC (04:53:31 ago)
 Statistics last cleared: Never
 Traffic statistics:
 Input bytes : 1256633066638 82512 bps
 Output bytes : 1392350385008 1076248 bps
 Input packets: 1770081587 101 pps
 Output packets: 1603112698 117 pps
 IPv6 transit statistics:
Input bytes : 0
Output bytes : 0
 Input packets: 0
 Output packets: 0
 Input errors:
Errors: 0,
Drops: 0, Framing errors: 0, Runts: 0, Policed discards: 0, L3 incompletes: 0, L2 channel errors: 0,L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0 Output errors:
 Carrier transitions: 3, Errors: 0, Drops: 0, Collisions: 247932333, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0
 Egress queues: 8 supported, 4 in use
 Queue counters: Queued packets Transmitted packets Dropped packets
 0 best-effort
 0 1702583017 366
 1 assured-forw 0 0 0
 5 expedited-fo 0 0 0
 7 network-cont 0 1795806 0
 Queue number: Mapped forwarding classes
 0 best-effort
 1 assured-forwarding
 5 expedited-forwarding
 7 network-control
 Active alarms : None
 Active defects : None
 MAC statistics: Receive Transmit
  Total octets 1256864002527 1392689372675
  Total packets 1770488775 1603506730
  Unicast packets 1769692120 1579600991
  Broadcast packets 796655 15971137
  Multicast packets 0 7934602
  CRC/Align errors 0 0
  FIFO errors 0 0
  MAC control frames 0 0
  MAC pause frames 0 0
  Oversized frames 0
  Jabber frames 0
  Fragment frames 0
  Code violations 0
Autonegotiation information:
  Negotiation status: Incomplete
  Local resolution:
   Local link Speed: 100 Mbps, Link mode: Half-duplex
  Packet Forwarding Engine configuration:
   Destination slot: 0 (0x00)
  CoS information:
   Direction : Output
   CoS transmit queue
References:

http://www.juniper.net/documentation/en_US/junos14.2/topics/reference/configuration-statement/no-auto-mdix-edit-interfaces.html

https://www.juniper.net/documentation/en_US/junos15.1/topics/reference/configuration-statement/mdi-mode-edit-interfaces.html

http://www.juniper.net/documentation/en_US/junos14.1/topics/task/configuration/ex-series-gigabit-interfaces-cli.html

Juniper Network Administration Switching