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New Member

OSPF design guide

Need some suggestion for OSPF design,

I have (3) sites connected via Opteman layer 2 link (Ethernet Hand off).  The Opteman routers are managed by Telco.

Site one has 11 Vlan Interfaces (Layer III)                


ip address

vlan 2

ip address

vlan 3

ip address 255.255.0

Site two has about 5 Vlans interfaces.

Currently, we are just using static routes between those sites, and now would like to migrate to OSPF.  Here is what I think the optimal configuration.

I can have Opteman routers at both end configured as a ABR and the WAN interface at both ends can be assigned area 0.  Site one has 4503 layer III switch connected to Optman Router, and Site two has 3550 switch connected to Opteman Router.  For Site one, should I assign the interface between the router and switch and the vlan interfaces which are eleven are in one area such as AREA1 and for site two assign the interface between the router and and 3550 switch and five vlan interfaces in AREA2 or somone would recommend to assign each vlan interface in its own area.

Is it okay to not have a dedicated Border Router since our network is not very large.  It is mid size environment.

What is the best way to migrate from static routes to OSPF with minimal downtime.  I am thinking of enabling OSPF first and when  all the routes at both ends learned via OSPF the I start removing static routes gradually.

Any feedback will be appreciated.


  • LAN Switching and Routing
VIP Super Bronze

Re: OSPF design guide

If you don't have a large network, you can simply put all your routers (4503, 3550 and telecom devices) in one area (area 0).

The other option would be to put each office's LAN in a different area and the connection between the 2 offices in area 0.

If you deploy OSPF on all your devices, since the AD for OSPF is 110 and for static route is 1, the static routes would be the primary until you delete them.  You definitely need an outage window to migrate from one to another.


New Member

Re: OSPF design guide

Thanks Reza,

What is the rule of thumb regarding deciding the size of OSPF to determine whether all the interfaces should be put in area 0 or put each LAN interface in its own area. If I go with putting each interface in its own area then I will just use the vlan ID associated with each LAN interface as an OSPF area.  Do you agree with this suggestion?

We also have MPLS and IPSEC and once the inital OSPF in place.  We will enable BGP over MPLS circuit and redistirbute OSPF as a backup and use IPSEC VPN as a last resort if both Opteman and MPLS fail at the same time highly unlikely, but never know.

Your inputs will be appreciated!

VIP Super Bronze

Re: OSPF design guide

Hi Abbas,

There isn't really a rule of thumb for the number of routers in an area.  Years ago that the routers were a lot slower with less memory and CPU, I heard that you could put up to 50 routers in one area.

The thing that is important to look at is the size of the routing tables, because recalculation of too many routes become some what difficult.  So, if you don't have a large routing table, it make since to go with one of the options I posted above.  It is not really necessary to go that far and put each vlan in its own OSPF area. To hard to admin and too much work that is not needed it. Keep it simple if you can.


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