OSPF, 2 routers, 2 areas, one link

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Dec 18th, 2009

Hello! Please help: we have 2 routers, 2 areas (0 and 10) and only one 10G link between them. Right now this link belong to area 0, so area 10 is discontigous.

What is the right solution for this situation?

I have this problem too.
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Jon Marshall Fri, 12/18/2009 - 03:57

m.sobolev wrote:

Hello! Please help: we have 2 routers, 2 areas (0 and 10) and only one 10G link between them. Right now this link belong to area 0, so area 10 is discontigous.

What is the right solution for this situation?

Sorry, it's not clear what you mean, could you draw a topology diagram to show exactly what you mean.

Jon

Jon Marshall Fri, 12/18/2009 - 05:00

m.sobolev wrote:

Drawing1.jpg

Thanks for the diagram, makes it a lot clearer.

As to the solution, a bit difficult to say without knowing why it is like this in the first place.

why is the link in area 0 ?

why are the vlan 10/20 interfaces in area 10 ?

do you need 2 areas or could you just place vlan 10/20 in area 0 ?

How much of your network does the diagram represent ?

Jon

Ganesh Hariharan Fri, 12/18/2009 - 04:59

Hi,

I will suggests to make separate area for area 20 in Router B and it will work in ospf world without any problem.

hope this helps

Regards

Ganesh.H

m.sobolev Fri, 12/18/2009 - 05:28

Link between switches it L2 trunk, and OSPF runs on SVI, subnet /30, for example 10.32.0.0/30

Ganesh Hariharan Fri, 12/18/2009 - 05:38

It will work but you need to configure ip unnumbered for wan interfaces.

hope this will work !!

Regards

Ganesh.H

Jon Marshall Fri, 12/18/2009 - 05:37

m.sobolev wrote:

To make things little bit more interresting:

http://img12.imageshack.us/img12/9008/drawing2q.jpg

Actually this is valid ie. you don't have a problem with the above topology because area 10 is not discontiguous, unlike your earlier diagram.

Jon

m.sobolev Fri, 12/18/2009 - 05:44

I dont get it

I think switch A and switch B each run its own ospf process and both of them serves area 0 and area 10, but between them there is only area 0, so area 10 seems to be discontigous?

Jon Marshall Fri, 12/18/2009 - 05:49

m.sobolev wrote:

I dont get it

I think switch A and switch B each run its own ospf process and both of them serves area 0 and area 10, but between them there is only area 0, so area 10 seems to be discontigous?

Don't worry, it's me not getting it For some reason i though the switches were routers and you have a L2 switch connecting the vlan 10 interfaces. Apologies for being a bit dense !!

Could you not run the link between the 2 switches as a L2 trunk ?

Jon

m.sobolev Fri, 12/18/2009 - 05:55

L2 trunk is key design feature, you can imagine switches are datacenters connected by dark fiber and we need shared L2 domain for server farms

Marwan ALshawi Mon, 12/21/2009 - 18:04

although the idea is not very clear but lets try to narrow it down

first

in each of your L3 switches you see area 10 route of the other end as inter area route O IA ?

if this is the cause because of area 0 separating area 10 between the two switches

now the question is are you looking to see these routes as intra area route appears as only O in the routing table ?

if yes you could try creating a tunnel interface in each of your switches and put that tunnel interface under area 10 in both of your switches

lets see if this will help or not because i am not sure if this is what you looking for or not

Good luck

m.sobolev Mon, 12/21/2009 - 21:26

You're right, on both switches I see area 10 routes as inter area routes (IA).

I think your proposal to make a tunnel will work, but something tells me that there is a better, maybe clever design.

Thank you!

Marwan ALshawi Tue, 12/22/2009 - 00:23

if you are thinking about better design in this case you need to start think about redesign the OSPF areas

because what we trying to do here is fixing the design issue, that's way all the suggested solutions here are work around

now in term of design:

do you have any backdoor link ( backup link) between area 10 ?

if not and the only link you have is the one between the switches in area 0 then what's the problem if you see the route as O or O IA ?

i mean if there is no other links intra or inter area route will make No deference because either way the traffic has one path , however if you have other links in this case you need to tell us because the picture in this case will be differnt 

good luck

please rate the helpful post

Marwan ALshawi Tue, 12/22/2009 - 01:54

ok

now i can see we have more than one topology you gave us

i will stick with the last one

however if we compare it with the first one

in the first one you have vlan 10 and vlan 20

now based on this topology

as you mentioned switch A and switch B located in differnt locations and as it shown they configured as HSRP

now if you have a host using switch A as the primary HSRP and switch A gose down how this host will use switch B as the gateway ?

i am thinking about the HSRP and vlan 10 in this topology

do you have any link L2 or L3 in Area 10 ?

the link to switch C still has no effect because it is in area 0 and you can make switch A - switch B preferred by changing interface cost - easy one -

i only need to under stand area 10 how this is designed physically and logically

then we can make a decision

m.sobolev Tue, 12/22/2009 - 02:00

All switches are at different locations, so if switch A goes down, all hosts on this DC are unaccessible, and traffic flows must be rerouted to backup DC (B)

Nagendra Kumar ... Tue, 12/22/2009 - 02:42

Hi,

From what I understood from diagram and other discussion is, you have 2 switches (A and B) connected through L2 trunk and have Vlan 10 in area 10 and some vlan x in area 0. With this, you will have 2 OSPF neighborship between these two switches. (One via vlan 10 on area 10 and other via vlan x on area 0).

So if you have any router/network connected to vlan 10 on any switch, it will appear as intra route on the other switch. Now can you explain, what you are trying to acheive?.

Regards,

Nagendra

m.sobolev Tue, 12/22/2009 - 04:31

You are correct except one moment - vlan 10 is ospf passive interface and switches are making neighborship only using single interface belonging to area 0.

m.sobolev Tue, 12/22/2009 - 04:52

Lests start again:

We have to sites, A and B, we need layer 2 between them because server A and server B must be in single L2 domain, so we use L2 trunk and HSRP to reserve gateway.

Site C have L3 connections with A and B, and we use OSPF for dynamic routing in this circle. Area 0 serves all cross sites links. Area 10 is servers farm (both A and B), lets say area 20 is for site C.

The question is: what is the correct OSPF design between site A and B?
Marwan ALshawi Tue, 12/22/2009 - 02:59

ok then HSRP in the diagramnot valid option it has no effect

as long as this is the case i would say just leave as it is becuase if you changed the route of area 10 to be as intra area route using the tunnel method or any other method no deffrence you will have

as i mentioned either way the traffic will go through one path

good luck

if helpful Rate

m.sobolev Tue, 12/22/2009 - 04:26

HSRP is needed by servers that must be placed in single L2 domain

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