lookback address

Unanswered Question
Jun 21st, 2007
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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



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guruprasadr Thu, 06/21/2007 - 22:50
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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