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802.1q trunk shows ARPA encapsulation. WHY?

Nov 29, 2011 7:50 AM

bearMedicine - CCNA 7 posts since
Aug 4, 2010

Hi

 

I have a weird situation where a trunk link is working in a lab environment. One end of the trunk is configured as an 802.1q trunk, yet it shows ARPA encapsulation in the 'show interface' command output. The other end of the link is not even configured as a trunk, yet it shows 802.1q encapsulation in its 'show interface' output. Can someone please explain this? Can ARPA and 802.1q interwork?

 

Config and command output is shown below:

 

"r1" (real names have been changed to protect the innocent   ) is a 7200.

"r2" is a 4000 series switch

 

r1#sh run int fa1/1

Building configuration...

 

 

Current configuration : 88 bytes

!

interface FastEthernet1/1

description r2 3/25

no ip address

duplex full

end

 

 

r1#sh int fa1/1

FastEthernet1/1 is up, line protocol is up

  Hardware is AmdFE, address is 000e.382d.f51d (bia 000e.382d.f51d)

  Description: r2 3/25

  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

     reliability 255/255, txload 7/255, rxload 6/255

  Encapsulation 802.1Q Virtual LAN, Vlan ID  1., loopback not set

  Keepalive set (10 sec)

  Full-duplex, 100Mb/s, 100BaseTX/FX

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:00, output 00:00:00, output hang never

  Last clearing of "show interface" counters never

 

 

r2#sh run int gi3/25

Building configuration...

 

 

Current configuration : 170 bytes

!

interface GigabitEthernet3/25

description Link to r1 fa1/1

switchport trunk encapsulation dot1q

switchport mode trunk

load-interval 30

duplex full

speed 100

end

 

 

r2#sh int gi3/25

GigabitEthernet3/25 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet Port, address is 0018.7306.2a88 (bia 0018.7306.2a88)

  Description: Link to r1 fa1/1

  MTU 1500 bytes, BW 100000 Kbit, DLY 10 usec,

     reliability 255/255, txload 7/255, rxload 8/255

Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Full-duplex, 100Mb/s

  input flow-control is off, output flow-control is off

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:27, output never, output hang never

  Last clearing of "show interface" counters never

 

Thanks, Bear

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