[c-nsp] Drawbacks of Redistributing Default Route from BGP intoIGP(ISIS)

[Chris Cappuccio]

Osama I Dosary [oid at saudi.net.sa] wrote:
> Thank you Oliver. But is there a good reliable  alternative to 
> redistribution of the default route?
> Is this what most ISPs are doing?
> Or do they just have BGP everywhere?
> 

I have a simple solution.  Most of my routers do carry full route tables,
but some are not capable (3550s for this example).

All routers talk BGP and OSPF.  Connected routes are distributed in both
BGP and OSPF, and static routes are only distributed through BGP.  I use
a route-map on each BGP speaking router to set a specific community on all
internal routes, static or connected. 

For the 3550s, they talk BGP to everything else, and only take BGP routes
with my internal community.  (Actually, everything else is also configured
to only send the 3550s the BGP routes as well) I have two internal peer groups,
one for routers that take full routes, and one for routers that only take
internal routes.

This takes care of BGP.  I would consider sending peer routes to the second
peer group, but with a limit of 16,000 routes on the 3550s, it's not worth the
bother, since I probably have about that many peer routes today.

On to OSPF... In my main datacenter, there are currently two routers that
are capable of talking to the outside world.  On every router, under router
ospf, I set a router-id that is the same as my loopback0 IP.  Next, on
the two routers that see the outside world (And also carry full BGP tables)
I use 'default-information originate' under router ospf.

Finally, I set 'distance 200 x.x.x.x 0.0.0.0' on the 3550s, where x.x.x.x is
the router-id of one of the boxes that talks to the outside world but doesn't
normally carry traffic (it announces a community which specifies lower
localpref to one of my upstreams, who charges hefty prices but happens to be
an alternate fiber route out of town).  This way, traceroutes from the 3550s
3550s usually look like they do from a full BGP speaker.

I haven't used default-information originate much, I think it also required
that I do something like ip route 0.0.0.0 0.0.0.0 loopback0...Naturally, any
router with this statement must carry full BGP tables!

There's probably a better way to do this, but I haven't spent the time to
figure that out yet...I'll send you config snippets if that helps

--
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