Migrating a non-backbone normal OSPF Area to a NSSA area with zero downtime

[jahed.miah.uddin]

Hello,

I'm attempting to migrate a non-backbone normal OSPF area to a NSSA area which is simple enough. However, the challenge is to perform this with zero downtime. Since changing the OSPF stub flags will cause a drop in neighborships and hence a drop in connectivity, I wanted advise on how to best approach this. Please see the diagram attached. The purple routers are connected via a GRE tunnel as they are service provider ASBR routers injecting external routes, and for all other routes the Area 120 routers simply route to the core (Area 0). Hence, area 120 would be more efficient as an NSSA. Is there any way of doing this without losing connectivity?

Kind Regards,

J

[Bilal Nawaz]

If you can get another interface on the links like a SVI, or sub-interface, or GRE tunnel, then you could create the NSSA neighborships along with the existing, once all is up, take away the old OSPF links. There is also an option to physically cable new links in if you have capacity to do so with new IP addressing. A miss-match of area types will tear down an existing ospf neighborship as you have rightly pointed out.

[jahed.miah.uddin]

Hey,

Many thanks for your reply, much appreciated. So all those edge switches are all configured with SVI's and Port-channels - these all participate in OSPF:

router ospf 1
 router-id x.x.x.x
 log-adjacency-changes detail
 auto-cost reference-bandwidth 1000000
 passive-interface default
 no passive-interface Vlan1
 no passive-interface Vlan322
 no passive-interface Port-channel1
 network 10.1.40.0 0.0.0.255 area 12
 network 10.2.222.0 0.0.0.63 area 12
 network 10.3.208.0 0.0.15.255 area 12
 network 10.4.239.0 0.0.0.255 area 12
 network 10.5.240.0 0.0.0.255 area 12
 network 10.6.255.0 0.0.0.255 area 12

So then should I create new SVI's for each corresponding existing SVI on each router. These new SVI's should have IP addresses in the network ranges as in the above network statements but configured  under the interface "ip ospf area 13 nssa". 

[Bilal Nawaz]

Create a new SVI, (new VLAN) - trunk them on the port-channels and trunks, use a different address blocks because you will have conflicting ip address if you assign SVI with IP in same subnet, parser will not allow you to do this by default.

[jahed.miah.uddin]

Those edge switches are not L2 port-channels but L3 port-channels, hence will I require to provision a new L3 port-channel to each uplink switch with these new interfaces configured in the new area?

interface Port-channel1
 description L3 Etherchannel to 6509
 no switchport
 ip address 10.1.1.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf hello-interval 2
 ip ospf dead-interval 6
 logging event trunk-status
 logging event bundle-status
 end

[Bilal Nawaz · Accepted solution]

Yes the new links are to be in the new area. Once you cut your old links, the new links should become operational - fairly seamlessly.