Packet Flows for ping and trace to L3VPN IPv4 Prefixes

This example describes packet flow for an MPLS ping is sent from VRF PE 11 on router PE 1 to the IPv4 prefix 10.99.99.21/32. For validation at the remote end, the source address of the echo request packet must be the same as the update-source address of BGP peer.

1. An MPLS echo request packet containing a single VPN IPv4 sub-TLV is sent from PE 1 with the following attributes:

   Source address	10.1.1.1
   Destination address	127.0.0.0/8
   UDP port	3503
   TTL	255
   Sender’s handle	Randomly generated 32-bit number used to match the reply
   Sequence number	Integer that is incremented for each echo request packet

   The VPN IPv4 sub-TLV has the route distinguisher set to that of the VRF and the IPv4 prefix set to 10.99.99.21/32. The packet exits PE 1 with two labels.

2. Router P 1 switches labels based on the outer label of the packet and forwards the packet to P 2.
3. Router P 2 switches labels based on the outer label of the packet and forwards the packet to PE 2.
4. Router PE 2 pops both labels and determines that the destination address is in the 127.0.0.0/8 subnet. PE 2 sends the packet up to the control plane. The MPLS ping application on the control plane then creates an MPLS echo reply to the received echo request. The echo reply packet has a return code of 3, which means that the replying router is an egress for the FEC at stack depth. The echo reply packet includes the Interface and Label Stack TLV to indicate both the interface on which the request packet was received and the incoming label stack. The MPLS echo reply packet is sent back as a (labeled) UDP packet with the following attributes:

   Source address	10.2.2.2
   Destination address	10.1.1.1
   UDP port	3503

5. When the MPLS echo reply reaches router PE 1, the router matches the sender’s handle and the sequence number to the echo request packet that PE 1 sent. The CLI displays an exclamation point (!).

Packet flow for an MPLS trace to an L3VPN IPv4 prefix is the same as for an IPv4 prefix except that the echo request packets and echo reply packets contain the VPN IPv4 sub-TLV instead of the LDP IPv4 sub-TLV. The following sample output represents what you might see when you issue the trace mpls l3vpn and trace mpls l3vpn vrf commands for the topology shown in Figure 67.

host1:pe1:pe11#ip8:pe1#trace mpls l3vpn 10.99.99.21/32 detail
Tracing VPN IPv4 prefix, timeout = 2 sec, Max TTL 32
  MplsNextHopIndex 73 handle 8073322

1 0ms 10.33.33.33 Label switched at stack-depth/2
  TLV Interface and Label stack 20 bytes
    Router 10.33.33.33 Intf 10.10.10.2
    [L34 EXP 0 TTL 1] [L68 EXP 0 S TTL 1] 
  TLV Downstream mapping 24 bytes
    Router 10.31.31.2 Intf 10.31.31.1 mtu 9180
    [L56 EXP 0 LDP] [L68 EXP 0 S Unknown] 
  TLV Downstream mapping 24 bytes
    Router 10.34.34.2 Intf 10.34.34.1 mtu 1500
    [L79 EXP 0 LDP] [L68 EXP 0 S Unknown] 
2 2ms 10.55.55.55 Label switched at stack-depth/2
  TLV Interface and Label stack 20 bytes
    Router 10.55.55.55 Intf 10.34.34.2
    [L79 EXP 0 TTL 1] [L68 EXP 0 S TTL 2] 
  TLV Downstream mapping 24 bytes
    Router 10.120.120.2 Intf 10.120.120.1 mtu 1500
    [L43 EXP 0 LDP] [L68 EXP 0 S Unknown] 
3 3ms 10.2.2.2 Replying router is an egress for the FEC at stack depth
  TLV Pad 20 bytes
  TLV Interface and Label stack 20 bytes
    Router 10.2.2.2 Intf 10.120.120.2
    [L43 EXP 0 TTL 1] [L68 EXP 0 S TTL 3]

host1:pe1#trace mpls l3vpn vrf pe11 10.99.98.21/32 reply pad-tlv exp-bits 5 
detail
Tracing VPN IPv4 prefix, timeout = 2 sec, Max TTL 32
  Handle 1921136  MplsNextHopIndex 78 [L68,L34]

1 0ms 10.33.33.33 Label switched at stack-depth/2
  TLV Pad 20 bytes
  TLV Interface and Label stack 20 bytes
    Router 10.33.33.33 Intf 10.10.10.2
    [L34 EXP 5 TTL 1] [L68 EXP 0 S TTL 1] 
  TLV Downstream mapping 24 bytes
    Router 10.31.31.2 Intf 10.31.31.1 mtu 9180
    [L56 EXP 5 LDP] [L68 EXP 0 S Unknown] 
  TLV Downstream mapping 24 bytes
    Router 10.34.34.2 Intf 10.34.34.1 mtu 1500
    [L79 EXP 5 LDP] [L68 EXP 0 S Unknown] 
2 2ms 10.55.55.55 Label switched at stack-depth/2
  TLV Pad 20 bytes
  TLV Interface and Label stack 20 bytes
    Router 10.55.55.55 Intf 10.34.34.2
    [L79 EXP 5 TTL 1] [L68 EXP 0 S TTL 2] 
  TLV Downstream mapping 24 bytes
    Router 10.120.120.2 Intf 10.120.120.1 mtu 1500
    [L43 EXP 5 LDP] [L68 EXP 0 S Unknown] 
3 3ms 10.2.2.2 Replying router is an egress for the FEC at stack depth
  TLV Pad 20 bytes
  TLV Interface and Label stack 20 bytes
    Router 10.2.2.2 Intf 10.120.120.2
    [L43 EXP 5 TTL 1] [L68 EXP 0 S TTL 3] 

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