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Loopback Interface??

abhi-adte
Level 1
Level 1

  Hi,

I want to know why do not have MAC address on loopback interface on Router or Switch??

If I see on useing #show int loopback 0

There is no MAC address.

If these are not support the MAC address then it's only depend on IP even on WAN area??

Please your helpful reply require..

3 Accepted Solutions

Accepted Solutions

Richard Burts
Hall of Fame
Hall of Fame

Let us start the explanation by reviewing a fundamental concept about MAC address. The purpose of a MAC address is to distinguish individual devices on the media (it is a link local layer 2 address).
Since the loopback interface is by definition not connected on any physical media there is no purpose in a link local address and MAC addresses are not supported on loopback interfaces.

HTH

Rick

HTH

Rick

View solution in original post

Hello,

Yes, the ping is via the IP addresses and IP routing. There is no

encapsulation required for that.

Regards,

NT

View solution in original post

Encapsulation is how you build a header for a particular physical media. Since there is no physical media associated with a loopback interface there is no encapsulation associated with a loopback interface.

HTH

Rick

HTH

Rick

View solution in original post

12 Replies 12

Nagaraja Thanthry
Cisco Employee
Cisco Employee

Hello,

Loopback interfaces are virtual interfaces created in software by the router/switch. Unlike other virtual interfaces, the loopback interface will not have any physical connectivity with another device. Whoever needs to communicate with the loopback interface need to go through another physical interface enabled with IP routing. So, communication between that physical interface and the loopback interface happens in software (no need of MAC). Hence you do not need a MAC address for the loopback interface.

Hope this helps.

Regards,

NT

Richard Burts
Hall of Fame
Hall of Fame

Let us start the explanation by reviewing a fundamental concept about MAC address. The purpose of a MAC address is to distinguish individual devices on the media (it is a link local layer 2 address).
Since the loopback interface is by definition not connected on any physical media there is no purpose in a link local address and MAC addresses are not supported on loopback interfaces.

HTH

Rick

HTH

Rick

If Router A have loopback and Router B is can ping to loopback int. of Router A, so only on basis of IP address ping to same??

any idea abour encapsulation loopback??

Hello,

Yes, the ping is via the IP addresses and IP routing. There is no

encapsulation required for that.

Regards,

NT

Encapsulation is how you build a header for a particular physical media. Since there is no physical media associated with a loopback interface there is no encapsulation associated with a loopback interface.

HTH

Rick

HTH

Rick

sundarji1999
Level 1
Level 1

Loopback Interface Will Use The MAC Address Of First Interface Of The Router/Switch

Just curious, you would see this where (being done)?

I'm sure its implementation dependent as Harold mentions but we had a thread earlier today that shed some light from a different question.

 

https://community.cisco.com/t5/ipv6/loopback-interface-eui-64/m-p/4896288#M4866

 

Ah, interesting, but might there also be different behavior between IPv4 and v6?

I believe so. I would have to lab up later but possibly uses the exit interface MAC when encapsulating the packet if it has one (not serial interface)

Hi @David Ruess ,

To put this in context, the MAC address is used to derived the EUI-64 host identifier when the loopback interface is configured with an ipv6 address using the eui-64 keyword. This has nothing to do with the MAC address that traffic originating from the loopback will use. 

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Gotcha. Thanks for the clarification. I never questioned it or ever even noticed a Loopback didn't have a MAC except for IPv6 configuration. I definitely made some assumptions about the question asked that I will go back and check in my lab to help me understand.

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