what is loopback interface and loopback address in ospf

[Ranjith KR]

Hi, can anyone help me to find the answer for the following question.

what is the loopback interface and loopback address in ospf?

[Latchum Naidu]

Hi Ranjith,

Vlan interfaces are generally used for management on switches and Loop-backs are often used for management on routers. That's the primary difference.

It has long been recommended that a loop-back address be used for management on routers. The thinking behind that is that if there is any way to your router you can get to the loop-back - if you used a real interface address if that was the interface that was down, you would lose access.

The main reason for using a loop-back on a switch as the management address is that it would fall in line with a simple policy "we always use lo0 as management, lo1 - 9 as tunnel end points" sort of thing, and there is no need to modify the policy for routers or switches, and standard security configs may be a little easier.

Coming to Loop-back address in OSPF "The RID will be the highest physical interface on the router ... unless a loopback interface has been configured. If a loop is configured, the RID will be the highest loop address."

Hope this clear you.

If this answered your question please click on correct answer.
Regards,
Naidu.

[Ranjith KR]

Hi Naidu,

Thank you for your valuable information.I have gone through some non-cisco forums and found some answers and I provided below.

"Unlike a physical interface, it never goes down and hence has many uses. In BGP it is used to establish peering between neighbors, so that if even if the physical interface went down for a short time the session does not go down. can use any network address on the loopback interface. If the loopback address is included in the routing process, the route gets advertised. Choose the address carefully. Use an address that is not used elsewhere on your network. Some engineers used the loopback for managing routers and reserve a network segment for the loopbacks. For example, router 1 has the loopback 192.168.0.1/32, router 2 has the loopback 192.168.0.2/32 and so on".

Please let me know if the above answer is correct or not.

Regards

Ranjith. K.R

[Latchum Naidu]

Hi Ranjith,

Yes, the above statement is appsolutely correct and BGP peering with Loopback address is a best idea.

Assuming the router has multiple physical interfaces or paths to which the loopback can be reached.  If the router only has one physical interface or one path to it's neighbor(s), using a loopback doesn't provide any further stability.

The following are best practices for BGP and addressing:

• When configuring an iBGP session, use/configure the other router's loopback address.
• When configuring an eBGP session, and there is only one link to the other router, use the other router's physical interface address (usually the address on the other side of the /30 or /31 link).
• When configuring an eBGP session, and there are multiple links to the other router, use eBGP multihop and the other router's loopback address.

Because a router can have both iBGP and eBGP sessions, the above rules use the "other router" terminology.

All routers should be configured with a loopback address because IGPs (OSPF, IS-IS) will use that address for the router-id, and you almost always run BGP with an IGP.

Please rate the helpfull posts.
Regards,
Naidu.

[Latchum Naidu · Accepted solution]

Hi Ranjith,

Seems you received correct answer.

But the compensation give just 3.1...can you please justify...

If something is still remain please raise.

Regards,

Naidu.

[binoyingen]

Hi ,

can anyone help me to understand what happens in OSPF with the Router has 2 loop back interface configured and one loop back is configured with public IP for DMVPN purpose. My question is whether the Loopback interface with Public IP also advertise in OSPF rooting table if so how to eliminate.