Multiple ASBRs: summarizing prefixes from BGP into OSPF

Ondrej Zajicek santiago at
Fri Jul 28 12:15:15 CEST 2017

On Wed, Jul 26, 2017 at 12:20:29PM +0100, Israel G. Lugo wrote:
> Hello,

> What I need, unless someone has a better suggestion, is some way to
> summarize externals in OSPF, such that the summary prefix is only
> announced if I've got more specific routes inside it.


>   area {
>     nssa;
>     /* transit VLAN to the tennant routers */
>     interface "eth0.22" { stub; check link yes; };
>     external {
>     };
> }
> >From my understanding, I believe the external keyword on an NSSA would
> be to aggregate in the opposite direction (backbone -> NSSA area).

No, it is used for aggregation in NSSA->backbone direction. But it works
only for NSSA-ext routes received from the NSSA area. If you export
external route to OSPF on ABR, it is always exported directly to
backbone (regardless of its nexthop).

> Is there any way to do what I want? I.e., summarize externals into OSPF,
> if and only if I have specific routes inside the summary prefix? That,
> or a conditional route, that only gets inserted if another certain route
> exists, or if the BGP peering is up.

Not directly, you can use the trick with with recursive route:

Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3,
"To err is human -- to blame it on a computer is even more so."
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