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:
10.1.1.1Source address
127.0.0.0/8Destination address
3503UDP port
255TTL
10.2.2.2/32IPv4 prefix in the TLV
Randomly generated 32-bit number used to match the replySender’s handle
Integer that is incremented for each echo request packetSequence number
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:
10.2.2.2Source address
10.1.1.1Destination address
3503UDP port
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 on
page 370.
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
Packet Flow Examples for Verifying MPLS Connectivity â– 371
Chapter 4: Monitoring MPLS
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