[c-nsp] "next-table" Equivalent for IOS XR - Default Route into Global Routing Table

[Gert Doering]

Hi,

On Tue, Aug 29, 2023 at 02:28:53PM +0200, Mark Tinka via cisco-nsp wrote:
> So yes, our default routes point to Null0. I changed that to something
> useful and it still didn't work. It's almost as if the traffic exiting the
> VRF toward the global table wanted to follow a label switched path, and not
> an IP-based path. Not sure whether "label mode per-vrf" would have helped to
> obfuscate the fact that the global table default routes pointed to Null0,
> but it's too late to test now. The box has been swapped out.

My guess after staring long and hard at IOS XR and VRF leaking is that
the CEF structures are getting in the way here - on ingress forward lookup,
as far as I understand, the system expects to find complete egress
information, as in "output line card, output interface, encapsulation,
destination MAC".

When you create a route to another VRF "with an egress interface", this
information can then be populated properly.  Asking for "go to the other
VRF and do a routing table lookup over there" needs packet recirculation,
and (again, guessing from how I understand the architecture) this is just
not possible.

... unless you add a loop cable somewhere.

... maybe they could have made a virtual loop cable (LT-), but maybe not...


So, yes, I would be interested what exactly happens inside the box, and
why it does not work / how hard it would be with existing ASR9k NPUs to
make it work (technically) but I expect there will be no answer on this.

gert

-- 
"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             gert at greenie.muc.de
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