HSRP with OSPF routing

My 2 cores Catalyst 3750 have hsrp configured to provide gateway redundancy for the access switches. I'd like to implement OSPF routing between the cores and the WAN router see pic). But I am not sure which subnet I should use between the cores and the WAN router. Also, I'd like to know if there are any sample config for HSRP with OSPF because I have never done it. Thanks

scenario
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leblancAccountingAsked:
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Henk van AchterbergSr. Technical ConsultantCommented:
you can choose any 2 /30 networks you like for the link between the router and the switches.

you just set up three ospf links, router-switch (2x) and switch-switch and it will failover.
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Don JohnstonInstructorCommented:
But I am not sure which subnet I should use between the cores and the WAN router.
I think that you're over thinking this.  As Henk stated, use any (unused) network between the cores and WAN router.  HSRP and OSPF are irrelevant at this point. First, just get connectivity established.  Then configure OSPF.

Also, I'd like to know if there are any sample config for HSRP with OSPF because I have never done it.
Running OSPF with HSRP is a non-issue.  The two protocols are independent of each other. The OSPF config that you use on a switch that is not running HSRP is exactly the same config if the switch is running HSRP.
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leblancAccountingAuthor Commented:
From the access switch to the core, it will just be layer 2 so there is no need for OSPF there. Not to reinvent the wheel, I am just wondering if you know any site with a sample config of HSRP and OSPF. Thanks
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Don JohnstonInstructorCommented:
Layer 2 between the cores and the WAN router? What are the IP addresses of the WAN router interfaces?
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leblancAccountingAuthor Commented:
Layer 3 between the cores and the WAN router.
Layer 2 between the cores and the access switch
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Don JohnstonInstructorCommented:
once again, what are the IP addresses assigned to the WAN router interfaces?
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leblancAccountingAuthor Commented:
That was one of my questions. I was thinking just a /30, like 10.10.30.1/30 (f0/0) and 10.10.30.5/30 (f1/0) on the router.
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Don JohnstonInstructorCommented:
Okay. That will work. But I would go with 10.10.0.1/30 (f0/0) and 10.10.0.5/30 (f1/0). That way you preserve your VLAN-to-subnet scheme (VLAN 30 = 10.10.30.0/24, VLAN 40 = 10.10.40.0/24, etc).

Then just enable OSPF on the cores and WAN router like you normally would.
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leblancAccountingAuthor Commented:
Great. Now I am a beginner when it comes to OSPF, so I have a quick question. When I enable ospf on f0/1 of core 1 and core 2, it will advertise 10.10.10.0 & 10.10.20.0. Correct? Or do I have to enable OSPF under vlan 10 and vlan20 as well?
What I am trying to understand is when you enable OSPF on an interface or use the command network 10.10.0.2 0.0.0.0 (under ROUTER OSPF 1 on core1 for example), it will just allow the interface to send the hello packets. How does it advertise the subnets that it knows?
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Don JohnstonInstructorCommented:
I don't know what switches you have, but on most Cisco platforms, you don't actively enable OSPF on an interface (unless you've got a Nexus switch).  The "network" command defines which interfaces are classified as OSPF interfaces. Which, in a way, enables OSPF on the interface (how's that for talking out both sides of my mouth?).

When an interface is classified as an OSPF interface, it will attempt to form adjacencies out that interface (send hellos, LSA, etc.) and the network connected to that interface will be advertised to any OSPF neighbors.

So on your core switch, if you use the OSPF network command "network 10.0.0.0 0.255.255.255 area 0", the switch will send hellos out all interfaces with an IP address starting with 10. And any network that begins with 10 will be advertised.
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leblancAccountingAuthor Commented:
So what is the pros and cons to use the below instead of network 10.0.0.0 0.255.255.255 area 0, other than extra typing?

router ospf 1
network 10.10.0.2 0.0.0.0 area 0
network 10.10.10.1 0.0.0.0 area 0
network 10.10.20.1 0.0.0.0 area
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Don JohnstonInstructorCommented:
Well, that's how I do it so obviously that's the best and correct way.  ;-)

Seriously, the primary benefit is it allows you to make small changes later. For example, if you want to change the 10.10.10.0 network to area 5, all you have to is delete the statement and then re-enter it with the new area number.  

But if you had used a network statement with 10.0.0.0, now you've got to remove OSPF from all the interfaces and recreate it.  Much more disruptive.
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leblancAccountingAuthor Commented:
Make sense Don. I will go ahead and implement this to see how it goes. Thx
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