Loopback Interface??

[abhi-adte]

  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..

[Nagaraja Thanthry]

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 · Accepted solution]

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

[abhi-adte]

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??

[Nagaraja Thanthry · Accepted solution]

Hello,

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

encapsulation required for that.

Regards,

NT

[Richard Burts · Accepted solution]

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

[sundarji1999]

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

[Joseph W. Doherty]

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

[David Ruess]

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

[Joseph W. Doherty]

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

[David Ruess]

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)

[Harold Ritter]

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,

[David Ruess]

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.