[c-nsp] OSPF - Prefer inter-area over intra-area

[Ben Steele]

Can you not just summarise the redundant routes at each site with  
static's over the wireless link with a higher AD and redistribute  
those static's into OSPF?


On 15/03/2008, at 1:22 PM, nachocheeze at gmail.com wrote:

> We have a client with a network that's got a main hub site and two
> 'remote' sites.  By remote, I mean they're far enough away to have to
> be connected by leased metro 1 Gb Ethernet circuits.  However, one of
> the "this is just how it is" items is that the two remotes are
> actually only a couple of blocks away from each other.  The client has
> purchased and installed a wireless radio link (also 1Gb) between the
> two sites.
>
> So what we have right now is pretty much this:
>
>           Area 0
>         Main HUB
>              /   \
>             /     \
>            /       \
> Circuit   /         \   Circuit
>          /           \
>         /             \
>        /               \
>       /                 \
> Site A------------------Site B
>           Wireless
>
> The Main Hub connects to the local enterprise OSPF area 0.  Site A was
> rolled out as a non-zero stub area (let's say 'area 666'), with one of
> the routers in the Hub acting as it's area border router (sending it a
> default).  Site B is different legacy install, put in a long time ago,
> and currently connects back to site A as a non-routed vlan trunking
> interface.  The client wants to remove the trunking bits and make site
> B a routed connection, and use the wireless link between A & B to back
> up each remote site if either of the metro circuits break.
>
> I've considered making site B the same OSPF stub area as site A so
> that the OSPF learned default from the hub can route traffic from both
> sites back to the hub if either leased circuit goes down.  That works,
> but the problem with that is that since site A and site B would be the
> same OSPF area, traffic between site A and site B would always take
> the wireless path (intra-area routes are always preferred to
> inter-area routes).  The client doesn't want that; since the leased
> lines are considered "better" than the el-cheapo wireless, they want
> all traffic from the remotes to come back to the hub (even traffic
> that is from A-to-B) unless they can't due to a carrier circuit
> failure.  Seems like what they want is something like an "on-demand
> OSPF dial backup", but instead of old school ISDN, they want it to be
> the wireless link.
>
> I suppose I could redo OSPF and make both sites A and B part of the
> OSPF area 0; then set a low metric on the wireless link between the
> two.  But then I lose the OSPF learned default route from the hub site
> that I get from the stub area implementation, and have to use static
> defaults or else originate a default from somewhere else upstream
> (which I'm loathe to do at this point for various reasons).
>
> What kind of options would I be looking at?
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