Loopback Interface

[mainodin87]

Hi Friends,

What is the exact use of loop back interface, why it was introduced ? I've gone through theory notes, but not getting clear concept.

Regards,

Mainodin

[Mark Malone]

Hi The loopback is an always up/up logical interface and is widely used for monitoring/mgmt

Think of it this way if you have couple of wan links your monitoring, 1 link  drops and your taking the source of the monitoring from that link what if it goes down , how will you know if its the link that's dropped or the actual router is not gone as you have lost monitoring access and cant see it anymore

When you use a loopback and use that as a source of monitoring you will know when the router is down as its a logical interface which is always up/up so the source of monitoring should always be loopback where possible

Loopbacks are also used to simulate networks , there also use in ospf for the rid and participate in dr/bdr elections , in EIGRP the rid is used to prevent external routing  loops

These are just some of the reasons its important to have loopbacks in your network , it has many uses depending on your setup

[mainodin87]

Hey Mark,

Thanks for the reply, but am still confused can you please explain this with an example.

"Link is drop or router is down" .. what does this mean ?

Mainodin

[Mark Malone · Accepted solution]

Hi if your polling a router remotely but using the actual serial interface or Ethernet as the source of the mgmt traffic if that link/circuit goes down how will you know the router has not lost power or if its just the circuit that has failed that's why you use the loopback as a source of mgmt. traffic as its always up/up so if that's gone down then you know its not just the circuit/link that has failed but the whole router has gone down , hope that makes sense not really sure how else to put it

[mainodin87]

Hey Mark,

That was real quick :) now its clear thank you so much.

Regards,

Mainodin