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MPLS (BGP-LU) traceroute

 

 

 

 

RP/0/0/CPU0:02-PE2#traceroute mpls ipv4 172.16.21.3/32 fec-type bgp source 172.16.10.2
Mon Jul  7 13:15:42.447 UTC
 
Tracing MPLS Label Switched Path to 172.16.21.3/32, timeout is 2 seconds
 
Codes: '!' - success, 'Q' - request not sent, '.' - timeout,
  'L' - labeled output interface, 'B' - unlabeled output interface, 
  'D' - DS Map mismatch, 'F' - no FEC mapping, 'f' - FEC mismatch,
  'M' - malformed request, 'm' - unsupported tlvs, 'N' - no rx label, 
  'P' - no rx intf label prot, 'p' - premature termination of LSP, 
  'R' - transit router, 'I' - unknown upstream index,
  'X' - unknown return code, 'x' - return code 0
 
Type escape sequence to abort.
 
  0 10.1.0.2 MRU 1500 [Labels: implicit-null/implicit-null/16018 Exp: 0/0/0]
L 1 10.1.0.3 MRU 1500 [Labels: implicit-null/16019 Exp: 0/0] 0 ms
L 2 10.9.0.3 MRU 1500 [Labels: 16012/implicit-null/16015 Exp: 0/0/0] 0 ms
L 3 10.2.0.2 MRU 1500 [Labels: implicit-null/implicit-null/16015 Exp: 0/0/0] 10 ms
L 4 10.2.0.7 MRU 1500 [Labels: 301792 Exp: 0] 0 ms
F 5 172.16.21.1 MRU 0 [No Label] 10 ms
 
 
On the first hop I see 3 labels (ImpNull/ImpNull/16018), and I don't understand why? On First hop the route in question is reachable as BGP-LU route (one label) over single-hop RSVP tunnel (again, one ImpNull). From where I get second ImpNull?
 
 
RP/0/0/CPU0:02-PE2#show cef 172.16.21.3/32 detail 
Mon Jul  7 13:08:51.966 UTC
172.16.21.3/32, version 3138, internal 0x4004001 0x0 (ptr 0xacbb75a4) [1], 0x0 (0xacbb2d24), 0x410 (0xacc338bc)
 Updated Jul  7 12:59:17.265
 Prefix Len 32, traffic index 0, precedence n/a, priority 4
  gateway array (0xacb6405c) reference count 15, flags 0xf0, source rib (6), 0 backups
                [6 type 5 flags 0x10111 (0xacc38408) ext 0x0 (0x0)]
  LW-LDI[type=5, refc=3, ptr=0xacbb2d24, sh-ldi=0xacc38408]
   via 172.16.10.3, 3 dependencies, recursive [flags 0x6000]
    path-idx 0 NHID 0x0 [0xacec1a24 0x0]
    next hop 172.16.10.3 via 16008/0/21
     local label 16019 
     next hop 172.16.10.3/32 tt31         labels imposed {ImplNull 300640}  <========== goes via RSVP tunnel 31
 
 
    Load distribution: 0 (refcount 6)
 
    Hash  OK  Interface                 Address
    0     Y   Unknown                   16008/0        
 
 
 
RP/0/0/CPU0:02-PE2#show mpls traffic-eng tunnels 31 detail 
Mon Jul  7 13:11:55.793 UTC
 
 
Name: tunnel-te31  Destination: 172.16.10.3 
  Signalled-Name: LSP-COR-PE2--->ASBR1
  Status:
    Admin:    up Oper:   up   Path:  valid   Signalling: connected
 
    path option 1,  type dynamic  (Basis for Setup, path weight 1000)
    Last Signalled Error: Mon Jul  7 09:35:12 2014
      Info: [40] PathErr(23,4)-(system) at 10.1.0.11
    G-PID: 0x0800 (derived from egress interface properties)
    Bandwidth Requested: 0 kbps  CT0
    Creation Time: Wed Jul  2 15:50:21 2014 (4d21h ago)
  Config Parameters:
    Bandwidth:        0 kbps (CT0) Priority:  7  7 Affinity: 0x0/0xffff
    Metric Type: TE (default)
    Hop-limit: disabled
    AutoRoute:  enabled  LockDown: disabled   Policy class: not set
    Forward class: 0 (default)
    Forwarding-Adjacency: disabled
    Loadshare:          0 equal loadshares
    Auto-bw: disabled
    Fast Reroute: Disabled, Protection Desired: None
    Path Protection: Not Enabled
    BFD Fast Detection: Disabled
    Reoptimization after affinity failure: Enabled
    Soft Preemption: Disabled
  SNMP Index: 13
  History:
    Tunnel has been up for: 00:13:04 (since Mon Jul 07 12:58:52 UTC 2014)
    Current LSP:
      Uptime: 00:12:55 (since Mon Jul 07 12:59:01 UTC 2014)
    Reopt. LSP:
      Last Failure:
        LSP not signalled, identical to the [CURRENT] LSP
        Date/Time: Mon Jul 07 12:59:01 UTC 2014 [00:12:55 ago]
    Prior LSP:
      ID: 44 Path Option: 1
      Removal Trigger: reoptimization completed
  Current LSP Info: 
    Instance: 45, Signaling Area: OSPF core area 0
    Uptime: 00:12:55 (since Mon Jul 07 12:59:01 UTC 2014)
    Outgoing Interface: GigabitEthernet0/0/0/3, Outgoing Label: implicit-null  <===================== tunnel has implicit null 
    Router-IDs: local      172.16.10.2
                downstream 172.16.10.3
    Soft Preemption: None
    Path Info:
      Outgoing:
        Explicit Route:
          Strict, 10.1.0.11
          Strict, 172.16.10.3
 
      Record Route: Empty
      Tspec: avg rate=0 kbits, burst=1000 bytes, peak rate=0 kbits
      Session Attributes: Local Prot: Not Set, Node Prot: Not Set, BW Prot: Not Set
                          Soft Preemption Desired: Not Set
    Resv Info:
      Record Route:
        IPv4 10.1.0.11, flags 0x0
      Fspec: avg rate=0 kbits, burst=1000 bytes, peak rate=0 kbits
Displayed 1 (of 3) heads, 0 (of 0) midpoints, 0 (of 3) tails
Displayed 1 up, 0 down, 0 recovering, 0 recovered heads
 
 
​I have even captured the LSP ping itself, and see following:
 
Multiprotocol Label Switching Echo
    Version: 1
    Global Flags: 0x0000
        0000 0000 0000 0... = Reserved: 0x0000
        .... .... .... ...0 = Validate FEC Stack: False
        .... .... .... ..0. = Respond only if TTL expired: False
        .... .... .... .0.. = Validate Reverse Path: False
    Message Type: MPLS Echo Request (1)
    Reply Mode: Reply via an IPv4/IPv6 UDP packet (2)
    Return Code: No return code (0)
    Return Subcode: 0
    Sender's Handle: 0x000052cd
    Sequence Number: 1
    Timestamp Sent: Jul  6, 2014 19:10:49.967036000 UTC
    Timestamp Received: Jan  1, 1970 00:00:00.000000000 UTC
    Vendor Private
        Type: Vendor Private (64512)
        Length: 12
        Vendor Id: ciscoSystems (9)
        Value: 0001000400000004
    Target FEC Stack
        Type: Target FEC Stack (1)
        Length: 12
        FEC Element 1: BGP labeled IPv4 prefix
            Type: BGP labeled IPv4 prefix (12)
            Length: 5
            IPv4 Prefix: 172.16.21.3 (172.16.21.3)
            Prefix Length: 32
            Padding
    Downstream Mapping
        Type: Downstream Mapping (2)
        Length: 28
        MTU: 1500
        Address Type: IPv4 Numbered (1)
        DS Flags: 0x00
            0000 00.. = MBZ: 0x00
            .... ..0. = Interface and Label Stack Request: False
            .... ...0 = Treat as Non-IP Packet: False
        Downstream IP Address: 10.1.0.11 (10.1.0.11)
        Downstream Interface Address: 10.1.0.11 (10.1.0.11)
        Multipath Type: no multipath (0)
        Depth Limit: 0
        Multipath Length: 0
        Downstream Label Element 1, Label: 3 (Implicit-Null), Exp: 0, BOS: 0, Protocol: 0 (Unknown)
            Downstream Label: Implicit-Null (3)
            Downstream Exp: 0
            Downstream BOS: 0
            Downstream Protocol: Unknown (0)
        Downstream Label Element 2, Label: 3 (Implicit-Null), Exp: 0, BOS: 0, Protocol: 0 (Unknown)
            Downstream Label: Implicit-Null (3)
            Downstream Exp: 0
            Downstream BOS: 0
            Downstream Protocol: Unknown (0)
        Downstream Label Element 3, Label: 304336, Exp: 0, BOS: 1, Protocol: 0 (Unknown)   <=========================
            Downstream Label: 304336
            Downstream Exp: 0
            Downstream BOS: 1
            Downstream Protocol: Unknown (0)
 
 
 
So, two ImpNull Downstream Label Elements are included.
 
 
Why?
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