PE to P TE Tunnel
Load mpls.te.pe.to.p.tunneling.init.cfg
The basic IP addresses, L3VPN between XRv14 and XRv13, OSPF, RSVP, and MPLS-TE has been pre-configured.
There is currently a bidirectional TE tunnel between CSR8 and XRv12. XRv11 has been modified via OSPF metric to use XR12 as the bestpath to CSR8.
Make the necessary configuration changes so that XRv14 and XRv13 can ping each other in their L3VPN. Do not add any additional TE tunnels. Do not enable mpls ldp on the te tunnel interface of XRv12.
Answer
First LDP must be enabled between XRv12 and XRv11:
At the moment, XRv12 will receive VPN traffic destined for 11.11.11.11 with only the VPN label exposed. This is because CSR6 will pop the top label as it is the penultimate hop no the TE tunnel. CSR8 must add an additional label - XRv12’s LDP label for XRv11. We are restricted from running LDP directly on the TE interface of XRv12, so instead we enable the acceptance of targetted sessions.
It is interesting to note that right now traffic from XRv13 to XRv14 does actually work. This is because the tunnel terminates on the PE (CSR8). However, the reverse direction does not, as the tunnel terminates on a P router (XR12).
We should see the targetted session (initiated by CSR8) come up. Traffic between XRv13 and XRv14 is now working. We should see a three label stack at CSR6 consisting of:
{ <TE tunnel label> <XRv12’s LDP label for XRv11> <VPN label>}
Theory
Note that doing a targetted session on CSR8 doesn’t solve the problem. mpls ip is specifically needed under the tunnel interface of the headend of the tunnel which terminates at a P router. This is because the tunnel must know to push two labels instead of just one.
We can see the difference in the LFIB of CSR8. Before the change, the LFIB entry for XRv11’s loopback points to a LSP tunnel (the [T] indication) but is not labeled with an LDP label (No Label):
When we enable mpls ip on the tunnel, we now see XRv12’s label for 11.11.11.11/32 (24000) as the outgoing LDP label, plus the [T] indication.
What if the PE to P tunnel has a headend of XR instead of XE? I created a TE tunnel between XRv11 and CSR6. XRv11 just has to enable LDP on the interface, and CSR6 has to accept the targetted hello:
Tunneling SR inside of RSVP-TE
Tunneling SR inside RSVP-TE works by default. If the route in the RIB is via a TE-tunnel and the router is running SR, the router will automatically put the tailend’s SR label for the final PE in the MPLS label stack.
For example, in this lab I switched over to SR. When XR14 sends XR13 traffic, CSR8 automatically pushes 16011 under the RSVP-TE label (16).
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