Packet Flow Example for the trace mpls Command

The following example illustrates the packet flow that results when you issue the trace 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 and a Downstream Mapping TLV. The packet has the following attributes:

   Source address	10.1.1.1
   Destination address	127.0.0.0/8
   UDP port	3503
   TTL	1
   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. The TTL expires on router P 1. P 1 exceptions the packet up to the control plane. Router P 1 then creates an MPLS echo reply packet in reply to the received MPLS echo request. The MPLS echo reply packet has a return code of 8, which means that the packet would have been label-switched at the outermost label (label-stack depth 1). The Downstream Mapping TLV is set to indicate the path that the packet would have taken from the router. The Interface and Label Stack TLV is included in the echo reply packet. The MPLS echo reply packet is sent back as a labeled UDP packet with the following attributes:

   Source address	10.3.3.3
   Destination address	10.1.1.1
   UDP port	3503

3. 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 the router ID of the router that sent the echo reply. The detail version of the command displays the downstream mapping TLV contained in the MPLS echo reply.
4. Steps 1–3 are repeated with a TTL of 2 and the destination address set to router P 2’s router ID, 10.4.4.4.
5. Router PE 1 next sends an MPLS echo request with a TTL of 3. This packet’s TTL expires on router PE 2. PE 2 exceptions the packet up to the control plane. The MPLS trace 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 Downstream Mapping TLV is not included in the echo reply packet.
6. When PE 2’s echo reply packet reaches router PE 1, the router matches PE 2’s handle and the sequence number to the echo request packet that PE 1 sent. The CLI displays the router ID for PE 2, indicating that PE 2 is the target router.

The following sample output represents what you might see when you issue the trace mpls ip command for the topology shown in Figure 67.

host1:pe2#trace mpls ip 10.1.1.1/32
Tracing LDP IPv4 prefix, timeout = 2 sec, Max TTL 32
  MplsNextHopIndex 60, handle 8073312

1 2ms 10.44.44.44 Label switched at stack-depth/1
2 1ms 10.33.33.33 Label switched at stack-depth/1
3 2ms 10.1.1.1 Replying router is an egress for the FEC at stack depth/0

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