How can I implement a backup 6500 that broadcasts the same OSPF networks?

[Benjamin Crites]

I feel as though the answer for this is extremely simple, but my routing experience is very minimal. We have a 6500 switch that shares about 10 OSPF networks, which is behind out 7200 router; and we have a spare 6500 we would like to put into place at our second location as a failover. Can I add the same OSPF networks to this second 6500, or will this cause issues since two switches will be broadcasting the same network. 

In the case that our core 6500 goes down, i would need the other 6500 to continue broadcasting these networks. 

[Benjamin Crites]

Maybe to answer my own question, can i broadcast the same networks on both, but make the core the DR with priority levels? 

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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Posting

Yes, it's possible for two locations to advertize the same networks, but this implies that both locations can (normally) reach all the hosts on those networks (at either location).  Also, if there's some kind of failure, some hosts (like one location's hosts) often can no longer be reached.  That's normally ok, but what's normally not okay is for hosts on the same network to be partitioned.

Also, when you start to have networks across locations, sometimes there's requirements for hosts to logically migrate to the site that still is advertizing the host's network.

It can all get rather complicated depending on what exactly you're trying to accomplish.

[Benjamin Crites]

This is what I was curious about, and i thank you for takign the time to answer. So just a little more information, basically we have 2 border routers that run BGP on the WAN side, and OSPF on the LAN side.  Each of those will go down into a 6500 (which i want to broadcast the same networks.) These two 6500's will be connected physically with a fiber we have. And both sites will have the same servers (one is the main farm, the other will take over with the same server IP if they lose network connectivity to each other) I know there are some technicalities, but is this a logical thing? is it possible? I plan to do HSRP for all the network gateways on the 6500s.

[Benjamin Crites]

Here is a quick topology i put together so you can see what i'm doing.

[john lewis]

where are the 10 networks defined in the OSPF statements? the 6500 or 7200?

[Benjamin Crites]

Currently on a 6500. The second 6500 doesn't exist yet. Both routers from different isps go to ththa single 6500

[john lewis]

ok, thanks, that clears up some...so,

in your HSRP setup, you could have 3 ip addresses, the .1 for the shared gateway and the .2 & .3 for the actual gwy interface on each 6500, each of the 6500's have an OSPF instance to advertise the 10 networks, only the primary should be answering as the other is in a standby state, this would work in a hot standby environment where the other site is meant to be a fail over.

I have also seen it work as a way of keeping the gateways closer (physically) to the end points as sort of a load dividing (not balancing) method.  I.E. if you have production servers in both locations, all of them on a, b, c, networks are closer to core then that 6500 has the active gwy, and x, y, z is closer to the backup site then that 6500 has the active gwy for those nets....

I hope this helps some....i really wish i had visio up and running, would help visualize my intent better.  I am a little rusty but utilized this hsrp setup extensively.

I'll be out for the rest of the day, but will check back in tomorrow to see how this goes....

[Benjamin Crites]

Thanks for your input and help sir! Were going to be putting it in some time in the next two weeks or so; should be a fun learning experience ha.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, I believe that's doable.  The tricky part is dealing with certain failures since you have servers covering for each other.  Consider if the internal OSPF link fails between the two sites.  The backup servers "think" they need to take over.  This issue might be addressed by also having a L2 tunnel between the sites, across the "outside".