ospf query

Unanswered Question
Mar 24th, 2009

Hi,

There are 3 sites A,B,C. Site A aggregration device will be configured with ospf & connected

to 2 isp uplinks.Site A needs to access servers in DC which is located off C.Even internet site for A

goes thru B.A has a default tunnel running to B.B is then used to link A with the servers off C.

All devices in B are running on static as of now.C has devices running ospf.

A to B & B to C are connected thru ISP links.

1)My query is would it work the way it is working now, if i enable ospf only for local segments in Site A

& not end to end from A to C.( Note C is already running ospf).2)Would the isp have to do anything if

i intend to run ospf end to end.

Would this cause any disruption in flow.

Thanks in advance!

I have this problem too.
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Edison Ortiz Tue, 03/24/2009 - 06:52

Hi Sunny,

In order to establish OSPF adjacency, OSPF speaking routers must be directly connected. Based on your explanation, they aren't so you must create a GRE tunnel between A<->B and B<->C.

Once you do that, you can enable OSPF on the GRE tunnel interface and any other link that needs to be advertised into the OSPF topology. For external routes or static routes, you will need to redistribute them.

The ISP won't know that you are running any dynamic routing protocol as the packet will be encapsulated in the tunnel.

HTH,

__

Edison.

suthomas1 Tue, 03/24/2009 - 08:55

So for A & C to have adjacency & exchange routes..i need to have ospf enabled with GRE. Apart from this,what if we enable ospf on B,which would mean all three are ospf enabled.

In case i just enable ospf on A..would it still be able to communicate via B to go across C as its doing now using static routes & GRE.

Also, am bit confused as to what is the best criteria to determine which should be designated as area 0 in an ospf design & what mandates...which routers/network would belong to which area..viz area 0 or area 20 et al.

Thanks & sorry for any confusion!

Edison Ortiz Tue, 03/24/2009 - 10:38

In case i just enable ospf on A..would it still be able to communicate via B to go across C as its doing now using static routes & GRE.

Yes, you can potentially create a GRE tunnel from A to C and enable OSPF between these 2 locations without touching B. My understanding from your original thread is that you wanted OSPF between the 3 sites.

Also, am bit confused as to what is the best criteria to determine which should be designated as area 0 in an ospf design & what mandates...which routers/network would belong to which area..viz area 0 or area 20 et al.

Depends on the number of links|routes in your network. If your network is quite small (less than 100 links|routes), I recommend going with a single Area 0.

HTH,

__

Edison.

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