[c-nsp] 6500 + Policy Routing

[Anton Kapela]

> GRE tunnels are hardware switched".  One important caveat is 
> that every GRE tunnel MUST terminate on a DIFFERENT local IP 
> address - otherwise: software switching.

Emphasis on different local IP's is critical here. Getting the
double-pass-plus-gre-rewrite to work on PFC means packets need to be
caught by the control plane in such a way that permits the action of gre
encaps to happen. This is because the control plane hardware (which is
what ultimately directs/generates the rewrite information) cannot know
more about the packet than what is in the IP header of the gre packet.
To fully support hardware GRE processing AND terminate multiple gre
tunnels on the same IP, you'd (in theory) need to have line cards that
could create 'special' compact/truncated headers which contained more
than the usual five-tuple-plusplus; they'd need to include fields from
the GRE portion of the IP packet such as the tunnel "key" so as to
permit logical separation of traffic between tunnels. 

If you were to configure multiple 'tunnels' that 'landed' on the same
destination IP on a PFC-based system today, to the best of my knowledge,
all the gre packets would need to be punted to the MSFC for processing
because the PFC could not determine de-encaps/rewrite actions based on
data in truncated or compact headers. 

Favored ways to express this config on cat6/7600 would be multiple
loopback interfaces, such as:

Int lo10
Desc tunnel endpoint #1
Ip addr 10.0.0.1 255.255.255.255

Int lo11
Desc tunnel endpoint #2
Ip addr 10.0.0.2 255.255.255.255

Or something like this:

Int lo0
Desc tunnels term here, weeeee
Ip addr 10.0.0.1 255.255.255.255
Ip addr 10.0.0.2 255.255.255.255
Ip addr 10.0.0.3 255.255.255.255

I prefer the mass-secondary approach as it, again, saves IDB's, and you
don't get counters for packets which 'arrived' at the tunnel endpoint
IP's parenty interface. Each gre tun int does have working counters, so
nothing is gained by having multiple loopbacks. There is minimal utility
in having multiple dozes of loopbacks to 'hold up' gre endpoints. 

-Tk