Automated OSPF Redundancy on a layer 3 switch

[Tom2000]

A layer 3 switch has access to two routers over an ethernet link directly connected. Each router has its own OSPF area and therefore clients hanging off of vlans within the switch cannot see both routers at one time. at no time will both routers be connected to the switch at the same time. they are regularly chopped and changed. So in order to maintain OSPF adjacencies on the switch, the following lines must be changed on all connected interfaces and router ospf configurations utilising the particilar OSPF area on the connected router:
Router OSPF 1
area 50 stub

interface GigabitEthernet0/1
ip ospf 1 area 50
interface Vlan100
ip ospf 1 area 50
interface Vlan200
ip ospf 1 area 50

Vice versa, changed accordingly depending on which router with a different OSPF area has been connected.
So in an effort to mitigate this tedious configuration change every time the router with a different ospf area is utilised, is there a way to configure the layer 3 switch with both/multiple ospf areas applied on it so that there is an instant recognition of what OSPF area is being utilised, and the switch will know how and where to connect with the correct ospf area?
Hopefully my explanation of what I want to achieve makes sense. Regards.

[paul driver]

Hello
If the L3 switch has ip addressing assigned to each of its vlan SVI's and has access ports associated to those vlans then the rtrs can be attached to the access ports, based on what addressing they are running, thus they should form a adjacency with the L3 switch  as/when they are connected.

kind regards
Paul

[Giuseppe Larosa]

Hello Tom2000,

you can use two different OSPF processes because the area-id are different.

router ospf 10

area 50 stub

router ospf 20

area 60 stub

on each L3 interface you can then apply

interface Vlan X

ip ospf 10 area 50

ip ospf 20 area 60

This configuration should be accepted by the device and this should allow you to connect to each router one at a time.

Warning if both routers are connected the results are not deterministic the two OSPF processes will act as "ships in the night" without sharing any DB info.

So each of them will present a candidate prefix to the IP routing table daemon and AD metric will be used to decide regardless of OSPF route type.

But if only one router is connected at a time you have no problems and you can save on switch configuration.

Hope to help

Giuseppe

[Tom2000]

Hi Giuseppe, thanks for the reply.

I acknowledge that the area ID's are different, however applying them both onto any physical interface does not seem to work. I can apply one area to one interface and anything extra will cause it to overwrite the current ospf area applied to any interface, including Vlans.

Is there a way to apply ospf areas within the switch outside of applying them onto physical interfaces? as in, can they be applied somewhere else within the configuration so that the interface will be able to recognise multiple OSPF areas? for situational awareness I am using a Cisco Catalyst 3560.

[Giuseppe Larosa]

Hello @Tom2000 

the old way to configure OSPF should still be available

router ospf 10

router-id 10.10.10.10

area 50 stub

network 10.0.0.0 0.255.255.255 area 50

router ospf 20

router-id 20.20.20.20

area 60 stub

network 10.0.0.0 0.255.255.255 area 60

try to see if this router ospf based config works. Please note provide different OSPF RIDs for the the two processes

Hope to help

Giuseppe

[MHM Cisco World]

I don't get what you want 100% BUT
config the Area under the interface 
Area 1 for interface 1
Area 2 for interface 2

don't not config Network of interface with Area under the OSPF.
"when you config Network of interface with Area under the OSPF it will override the Area under interface"

[Tom2000]

That works fine, and I can gain OSPF routes through having two OSPF areas on different interfaces. however I cannot apply both areas to the VLAN interfaces. not being able to do this means that the clients and servers hanging off of these particular VLANs cannot be part of both OSPF areas at once. I want to be able to have the VLANs recognise both of these areas and be able to create adjacencies without manually having to change the config of the VLAN interfaces to the respective OSPF area whenever the router is changed.

[MHM Cisco World]

Ok, vlan for area 0 this make vlan subnet send to both area and sw connect to area 0 and work as abr.

Or 

"This is not so optimal"

Config loopback with area 0 and config vlan with any other area, this make sw abr and advertise the vlan subnet to both routers area.

[MHM Cisco World]

Simply if the ABR connect two area but not connect to Area 0 then it never exchange the LSA inter-Area it only exchange the LSA intra-Area. 
so we want ABR to connect to Area 0 to exchange the LSA inter-Area.