lookback address

Unanswered Question
Jun 21st, 2007

Hi,

we would like to setup the loopback interface for all routers and we would like those loopback address can be routed. e.g.

router A

---

loopback0

192.168.254.1 255.255.255.0

router B

---

loopback0

192.168.254.2 255.255.255.0

router C

---

loopback0

192.168.254.3 255.255.255.0

router D

---

loopback0

192.168.254.4 255.255.255.0

router E

---

loopback0

192.168.254.5 255.255.255.0

how to make them routable and we can connect to each route using loopback address?

thanks

Best regards

I have this problem too.
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guruprasadr Thu, 06/21/2007 - 22:50

HI, [PLS Rate if HELPS]

Tell us for which kind of Protocol you are going to Route this Loopback address.

For Eg: BGP Means:

neighbor update-source Loopback0

ie., You can send the neighbor Updates via Loopback 0.

If GRE Tunnel Means:

This Loopback Address Can be used as Tunnel Source IP. So that the Tunnel will be always availaible without any Down-Time.

If you use the Physical Interface for Tunnel Source IP means there are possibilities where the Interface / Line Protocol of the Interface can go down.

Pls RATE if HELPS

Best Regards,

Guru Prasad R

mohammedmahmoud Thu, 06/21/2007 - 23:19

Hi,

You can route loopback interfaces normally like routing any other interface on a router, either via using static routes or using dynamic routing protocols.

Ex: 192.168.254.5 - routerE(10.10.10.1)<-->(10.10.10.2)routerD - 192.168.254.4

on router E you should have:

ip route 192.168.254.4 255.255.255.255 10.10.10.2

and on router D you should have:

ip route 192.168.254.5 255.255.255.255 10.10.10.1

And the same goes if using routing protocol.

Loopbacks are usually addressed as /32 as they are not connected physical to any other interfaces, then /32 is logical.

The important question is what did you mean with "how to make them routable and we can connect to each route using loopback address? "

HTH,

Mohammed Mahmoud.

leungcm Fri, 06/22/2007 - 00:09

Hi,

"routable" mean we can access the router using lookback from another network. It means that we set all static route for those loopback address. is it correct?

if so, there are a lot of static routes. Any approach to use samll amount of static route for it?

please advise

Best regards

mohammedmahmoud Fri, 06/22/2007 - 00:15

Hi,

Yes it is that exactly, and as you've said they'll be a lot of static route, thus you can run a routing protocol (RIP for simplicity) to do it for you.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

Richard Burts Fri, 06/22/2007 - 09:13

I agree with Mohammed that using a dynamic routing protocol is desirable rather than configuring lots of static routes. I do note one difficulty with the way this was described and using a dynamic routing protocol. Each of the loopback interfaces was described as being in 192.168.254.x with a 255.255.255.0 mask. That makes them all appear to be in the same network which is not subnetted. This will result in the routers receiving an advertisement for 192.168.254.0/24 which they will not put into the routing table because they already have 192.168.254.0/24 in their routing table as a connected network. It would work much better if each of the loopback interfaces was configured with a /32 mask.

HTH

Rick

mohammedmahmoud Sat, 06/23/2007 - 00:10

Hi Rick,

Totally agree with you on this point, and thats what i recommended to the original poster as a precaution on a previous post (using /32 for loopbacks), but you've perfectly described the problem that will occur when having a dynamic routing protocol (This will result in the routers receiving an advertisement for 192.168.254.0/24 which they will not put into the routing table because they already have 192.168.254.0/24 in their routing table as a connected network.).

BR,

Mohammed Mahmoud.

mahmoodmkl Mon, 06/25/2007 - 03:27

Hi Mohammed,

Can u plz explain why we need to use /32 mask with the loopback interfaces.

I understand that if we are going to use the normal mask as per to the current post it will cause the problems in routing updates.

I want to know how it will not cause the problems if we use /32 mask.My question might be silly but i m little confused on this.

Thanks

Mahmood

mohammedmahmoud Mon, 06/25/2007 - 04:17

Hi Mahmood,

You are very welcomed :), we are here to help the others, and also to interact with each other.

The whole idea is that if we use /24 allover the loopbacks (they are all now in a single subnet), each router will advertise this address as /24 to all the other routers (except when dealing with OSPF which advertise loopbacks as /32 by default), and thus each router receiving this advertisement as /24 and since it has the same route as connected with also /24 (from its local loopback), it will drop the route received from the other routers (both are /24 with one of them connected which will be preferred than the route received via the routing protocol).

Next, lets take the /32 case, each router will get a /32 route from the other routers (now each /32 is a separate subnet), and since each route is /32 then they are different routes in different subnets, it will be injected into the routing table of each router, and thus each router will have its local /32 (as connected), and the other routers /32 received from the routing protocol.

HTH,

Mohammed Mahmoud.

mahmoodmkl Mon, 06/25/2007 - 05:03

Hi Mohammed,

Thanks for u r explanation,now may this will be my last question as u said that each route will different subnet as we r using all the bits in the last octet.it means that we can use only 255 address per network,am i correct in thinking this.

Thanks

Mahmood

mohammedmahmoud Mon, 06/25/2007 - 05:11

Hi,

You are very very welcomed:) When using /32 subnet we would be able to use 254 address if we are subnetting a /24 address space, example if we are talking about 192.168.99.0/24, we can subnet it and have addresses from 192.168.99.1/32 to 192.168.99.254/32.

HTH,

Mohammed Mahmoud.

deveshkumar Fri, 06/22/2007 - 11:07

Best idea would be to use 32 bit netmask

like 192.168.0.1 255.255.255.255.

and then route it like any normal network

e.g.

router eigrp 1

network 192.168.0.1 0.0.0.0

ospf:

router ospf 1

etwork 192.168.0.1 0.0.0.0 area

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