[c-nsp] MPLS VPN with PE over GRE tunnels

[Christophe Fillot]

Gert Doering wrote:

Hello,
>
> There was something about the 6500 architecture that certain combinations
> of ingress and egress need packets to go through the forwarding plane twice,
> and you need to enable "packet recirculation" for it to do that.
>   
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/pfc3mpls.html

Especially:

In certain cases, the PFC3BXL or PFC3B provides the capability to 
recirculate the packets. Recirculation can be used to perform additional 
lookups in the ACL or QoS TCAMs, the NetFlow table, or the FIB TCAM 
table. Recirculation is necessary in these situations:

- To push more than three labels on imposition

- To pop more than two labels on disposition

- To pop an explicit null top label

- When the VPN Routing and Forwarding (VRF) number is more than 511

- For IP ACL on the egress interface (for nonaggregate (per-prefix) 
labels only)

Packet recirculation occurs only on a particular packet flow; other 
packet flows are not affected.The rewrite of the packet occurs on the 
modules; the packets are then forwarded back to the PFC3BXL or PFC3B for 
additional processing.