OSPF Re-design Question

[terrygwazdosky]

I haven't done a ton of routing (mostly switching and firewalls) so I'm looking for a little advice. 

I've inherited an existing network and am looking at making some routing changes.  There are several branch office routers all connected in the same fashion as below.  Please take a look at the images of the current setup and my proposed changes and let me know if this will work or if there is a better design choice. 

I have labbed it in GNS3 and it is behaving, but I'd like some real world input.

Thank you.

[Giuseppe Larosa · Accepted solution]

Hello Terry,

it is fine to use a totally stub area to minimize routing information, but the area used should be the same on both WAN links because the IP subnets in remote office cannot be part of two different areas at the same time and you cannot redistribute connected in  a router internal to a stub area.

Another reason is that an OSPF area must be connected to area 0 directly and it cannot be connected via another area. So it is good design to use a single area for both WAN links and on the branch office with branch office router that is an internal node in this area.

You can play on OSPF cost on WAN link to aggregation services router in order to use it as a backup route you can use ip ospf cost for this.

You can also set an higher OSPF metric for the default route generated by ABR  on aggregation router.

area 10 default-metric

Hope to help

Giuseppe

[terrygwazdosky]

Giuseppe:

Thank you for your reply.

If I use area 10 on both links, wouldn't this make area 10 discontiguous (since the agg and core routers only link together in area 0)? 

I know this is problematic for the backbone area, but is it OK for stub networks?

Thanks.

[Giuseppe Larosa]

Hello Terry,

>> If I use area 10 on both links, wouldn't this make area 10 discontiguous (since the agg and core routers only link together in area 0)?

it is the opposite you are providing to area 10 two different paths to area 0 and this provides redundancy.

As explained in my previous post you can play with OSPF metric to make one path primary and one path secondary if desired.

This is the way to go for a proper design.

Hope to help

Giuseppe

[Hari Vishnu]

Hi Mate,

Don’t have much experience with routing.  Could you please elaborate below.

"Area used should be the same on both WAN links because the IP subnets in remote office cannot be part of two different areas at the same time and you cannot redistribute connected in  a router internal to a stub area"

I could not see any issue with remote office router being part of 2 different areas.

Thanks

Hari

[Giuseppe Larosa]

Hello Hari,

the objective of routing is to advertise the client facing IP subnets in the remote office.

Now, because the area is totally stub you cannot use redistribute connected.

Once you advertise an IP subnet using network .... area X command the routing information is propagated via the WAN link in area X to backbone area 0.

If the other WAN link is in different area Y that IP subnet that is part of area X is not advetised via area Y to area 0.

This is an OSPF specific concept.

Inter area routing and traffic forwarding goes via area 0. Area Y learns area X routes via area 0 and not directly from area X.

In more detail:  branch router would create one Router LSA for area X contaning routing information in area X only and one Router LSA for Area Y with links in area Y only.

For this reason the two WAN links have to be part of the same area X to provide redundancy at link level.

Hope to help

Giuseppe

[Hari Vishnu]

Got it. Thank you very much Giuseppe