Optimise OSPF using dual routers and links

[markthaysen]

Hi,

I'm trying to optimise my current network layout using ospf and gre tunnels. I have two DC's and multiple office that connect to each of the DC's via dual routers. Primary routers at each branch connect to the primary router at each DC. Same goes for the secondary router at each branch. GRE tunnels are used for the link to the DC with ospf over the tunnel, basically a dual hub and spoke topology. DC are connected to each other using primary and secondary router with tunnels same as branch connectivity .

What I'm trying to achieve is a seamless failover if either a primary or secondary circuit drops. At the moment all of my ospf metrics are redistributed as E2 so default metric AD of 20. We control the preferred path by using ip ospf cost command but I want to see if there is a more elegant way in which I can have dual routes to two different DC's, but depending on the branch have a preferred path to a particular DC.

The problem I'm having at the moment is that if I make adjustments to the primary router for a branch to use DC A then the lower cost for DC B is used instead of the secondary routers path to DC A, and vice versa.

This also means I am unable to properly use equal path costing on either of the routers as they are superseded by the other tunnel on the router.

I am happy to redesign the entire layout and metric , however I have to maintain the GRE tunnels in the topology and each router must have a link to both DC's from either router.

thoughts?

[Nagendra Kumar Nainar]

Hi,

Can you explain more about your topology?. Do you have 4 GRE tunnels from each branch (2 each router)?. Where are you redistributing this OSPF routes to?. 

are you using "ip ospf cost" under GRE tunnel towards your DC?.

[markthaysen]

Hi Nagendra,

Yes thats correct 2 tunnels per router.

Our toplogy is pretty small so we only uise a single area, no stubs .

Primary router from branch has two tunnel to DC A and DC B primary, the cost on these are at deafult while the Secondary router has two tunnels to DC A and DC B which are set to a slightly higher cost making them less preferred.

this means my ospf routes learnt only has one route in the routing table to either DC A or B , this problem I'm having is that when that circuit drops the ospf reconvergance casue a blip on the network and blackholes some of my traffic while it learns the new router through the secondary router.

What I ideally want is a way to set the ospf routing table to have both paths from priamary and secondary to  DC A and for DC B so that if I lose a circuit there is a smoother transistion of traffic .

Depending on the branch DC a might be more preferrable to DC B but I still need trafic destined for DC B to use the correct tunnel. If I do equal costs across the two routers then it means that now the tunnnel on the primary router to DC B would never get used as I have to set it's cost higher than the cost to DC  A in order to have both paths equal cost.

[paul driver]

Hello

If applicable make the secondarys links over a non backbone area.

Ospf will always prefer to choose the backbone area links over non backone area links to the same destination

res

Paul