Checking L2/L3VPN traffic path through SP network (for ECMP)

Question

Folks,
Scenario:
CE1-----PE1=====P1=====P2=====PE2-------CE2
Lets say CE1 and CE2 are doing L2VPN and all hops between PE1, P1,P2 and PE2 have more than one equal cost paths (ECMP).
I am trying to ascertain a way of knowing what path the EoMPLS traffic would take inside the SP core.
Some vendors say the way the hashing works is that if a PE finds its got more than one path to the egress PE, it would do hashing based on src/dst MAC and in other cases if a P device finds its got more than one path to egress PE, it would do hashing based on VC-label.
In either case, lets say we know what hashing method the P or PE device is using, obviously we would need an easier method to determine what path a pseudowire would take inside the provider network - Again, some vendors use what is called a "pseudowire traceroute" to determine this path. A pre-requisite of this is that at the time of setting up of the PW, the control word needs to be turned on.
I am looking for more knowledge on whether someone knows how the pseudowire traceroute would work and the process behind the PW traceroute which uses the control word ? more like how we know a normal traceroute works is through UDP pakcets with incremenyting TTL... and so forth
Anyone ??

Answer 1

Hello Ulatif,
it looks like that mpls traceroute for a pseudowire is not possible.
Actually the VCCV should be under the implementation of ping mpls and ping mpls pseudowire. The following document is a little old but explains the basic concepts under ping mpls and traceroute mpls.
http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/gslsppt.html#wp1156080
However, sh mpls l2transport vc detail provides the choice for a specific pseudowire between two parallel paths
see this example from our network:
sh mpls forw 10.80.0.25
Local  Outgoing      Prefix            Bytes Label   Outgoing   Next Hop   
Label  Label or VC   or Tunnel Id      Switched      interface             
21     295           10.80.0.25/32     0             Te1/2      10.82.0.233
       341           10.80.0.25/32     0             Te1/6      10.82.0.237
sh mpls l2transport vc det
Local interface: Te1/7 up, line protocol up, Ethernet up
  Destination address: 10.80.0.25, VC ID: 1, VC status: up
    Output interface: Te1/2, imposed label stack {295 372}
    Preferred path: not configured 
    Default path: active
   Next hop: 10.82.0.233
  Create time: 7w4d, last status change time: 6w4d
  Signaling protocol: LDP, peer 10.80.0.25:0 up
    Targeted Hello: 10.80.0.24(LDP Id) -> 10.80.0.25
    Status TLV support (local/remote)   : enabled/supported
      Label/status state machine        : established, LruRru
      Last local dataplane   status rcvd: no fault
      Last local SSS circuit status rcvd: no fault
      Last local SSS circuit status sent: no fault
      Last local  LDP TLV    status sent: no fault
      Last remote LDP TLV    status rcvd: no fault
    MPLS VC labels: local 1429, remote 372
    Group ID: local 0, remote 0
    MTU: local 9216, remote 9216
    Remote interface description:
  Sequencing: receive disabled, send disabled
  VC statistics:
    packet totals: receive 5172156, send 5361948
    byte totals:   receive 676971483, send 917397631
    packet drops:  receive 0, seq error 0, send 610
This solves the question at source PE or destination PE of the pseudowire but I agree that in the middle in your scenario there are other possible choices of intermediate nodes.
All I can say is that once a path is chosen by source PE it determines a complete path because intermediate nodes will make a choice and keep it.
Hope to help
Giuseppe