1. An MPLS echo request packet containing a single VPN IPv4 sub-TLV is sent from
PE 1 with the following attributes:
10.1.1.1Source address
127.0.0.0/8Destination address
3503UDP port
255TTL
Randomly generated 32-bit number used to match the replySender’s handle
Integer that is incremented for each echo request packetSequence number
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:
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. 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 on page 373.
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
377Copyright © 2010, Juniper Networks, Inc.
Chapter 5: Monitoring MPLS
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