[c-nsp] SR-TE

[aaron1 at gvtc.com]

oh wow, i brought down a link accidentally today in my lab (between r24 and
r23) and i discovered something quite interesting (or concerning)

an srte tunnel can progress in a direction and actually hairpin back on
itself!

is this news to anyone else?  i'm pretty sure that rsvp-te didn't allow for
looping ero's... and I wonder if it was disallowed due to signalling that
was able to track on the ero/rro?

notice the hops

traceroute source is r10 and destination is r30

r10#traceroute 1.1.1.2
Type escape sequence to abort.
Tracing the route to 1.1.1.2
VRF info: (vrf in name/id, vrf out name/id)
  1 1.0.0.1 47 msec 37 msec 39 msec
  2 10.20.1.21 [MPLS: Labels 16023/16022/24002 Exp 0] 257 msec 251 msec 257
msec
  3 10.20.1.22 [MPLS: Labels 16023/16022/24002 Exp 0] 268 msec 250 msec 244
msec
  4 10.20.1.2 [MPLS: Labels 16023/16022/24002 Exp 0] 257 msec 249 msec 148
msec
  5 10.20.1.6 [MPLS: Labels 16023/16022/24002 Exp 0] 122 msec 109 msec 111
msec
  6 10.20.1.10 [MPLS: Labels 16022/24002 Exp 0] 118 msec 111 msec 111 msec
  7 10.20.1.9 116 msec 111 msec 117 msec
  8 1.1.1.2 197 msec *  191 msec


r10 - initiates traceroute to r30
r20 - l3vpn then inserts traffic into srte path and pushes sid stack
r24 - sid 1
r23 - sid 2
r22 - sid 3 - egress l3vpn 
r30 - destination of traceroute

so the traceroute above shows the traffic flowing like this...

r10--->r20--->r24--->r20(again)--->r21--->r22--->r23--->r22(again)--->r30


r10                  r30
|                        |
|                        |
r20-----r21-----r22
|                        |
|                        |
r24------X------r23


i put an X where I accidentally brought down the connection

RP/0/RP0/CPU0:r20#sh segment-routing traffic-eng policy
Sun Sep 20 00:09:51.167 CDT

SR-TE policy database
---------------------

Color: 1, End-point: 10.20.0.22
  Name: srte_c_1_ep_10.20.0.22
  Status:
    Admin: up  Operational: up for 00:20:10 (since Sep 19 23:49:40.492)
  Candidate-paths:
    Preference: 1 (configuration) (active)
      Name: my-srte-policy
      Requested BSID: 100
      Explicit: segment-list my-srte-sidlist-2 (valid)
        Weight: 1, Metric Type: TE
          16024 [Prefix-SID, 10.20.0.24]
          16023 [Prefix-SID, 10.20.0.23]
          16022 [Prefix-SID, 10.20.0.22]
  Attributes:
    Binding SID: 100
    Forward Class: Not Configured
    Steering BGP disabled: yes
    IPv6 caps enable: yes


RP/0/RP0/CPU0:r20#sh run segment-routing
Sun Sep 20 00:14:33.060 CDT
segment-routing
 traffic-eng
  segment-list my-srte-sidlist-2
   index 1 address ipv4 10.20.0.24
   index 2 address ipv4 10.20.0.23
   index 3 address ipv4 10.20.0.22
  !
  policy my-srte-policy
   binding-sid mpls 100
   color 1 end-point ipv4 10.20.0.22
   autoroute
    include all
   !
   candidate-paths
    preference 1
     explicit segment-list my-srte-sidlist-2

-Aaron