Packet Flow Example for the ping mpls Command

The following example illustrates the packet flow that results when you issue the ping mpls ip command from router PE 1 (10.1.1.1) to router PE 2 (10.2.2.2) over an LDP base tunnel.

1. PE 1 sends an MPLS echo request UDP packet that contains an LDP IPv4 sub-TLV. The packet is sent as a labeled packet over the target LSP. The packet has the following attributes:

   Source address	10.1.1.1
   Destination address	127.0.0.0/8
   UDP port	3503
   TTL	255
   IPv4 prefix in the TLV	10.2.2.2/32
   Sender’s handle	Randomly generated 32-bit number used to match the reply
   Sequence number	Integer that is incremented for each echo request packet

2. Router P 1 label-switches the packet to P 2.
3. Router P 2 label-switches the packet to PE 2 (assuming PHP is not configured).
4. Router PE 2 pops the label 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 out. If the values match, the CLI displays an exclamation point (!).

The following sample output represents what you might see when you issue the ping mpls ip and ping mpls ip detail commands for the topology shown in Figure 67.

host1:pe1#ping mpls ip 10.2.2.2/32
Sending 5 UDP echo requests for LDP IPv4 prefix, timeout = 2 sec
!!!!!     
Success rate = 100% (5/5), round-trip min/avg/max = 4294967295/4/0 ms

host1:pe1#ping mpls ip 10.2.2.2/32 detail
Sending 5 UDP echo requests for LDP IPv4 prefix, timeout = 2 sec
  MplsNextHopIndex 32 handle 8073311
    '!' - success, 'Q' - request not transmitted,
    '.' - timeout, 'U' - unreachable,
    'R' - downstream router but not destination
    'M' - malformed request, 'N' - downstream router has no mapping

Sending MPLS ping echo request, handle 8073311 seq 21241
! 10.2.2.2 Replying router is an egress for the FEC at stack depth/0 seq 21241
Sending MPLS ping echo request, handle 8073311 seq 21242
! 10.2.2.2 Replying router is an egress for the FEC at stack depth/0 seq 21242
Sending MPLS ping echo request, handle 8073311 seq 21243
! 10.2.2.2 Replying router is an egress for the FEC at stack depth/0 seq 21243
Sending MPLS ping echo request, handle 8073311 seq 21244
! 10.2.2.2 Replying router is an egress for the FEC at stack depth/0 seq 21244
Sending MPLS ping echo request, handle 8073311 seq 21245
! 10.2.2.2 Replying router is an egress for the FEC at stack depth/0 seq 21245

Success rate = 100% (5/5), round-trip min/avg/max = 4/4/0 ms

Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice.