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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 on page 373.

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

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.

host1:pe1#ping mpls l3vpn vrf pe11 10.99.99.21/32

JunosE 11.2.x BGP and MPLS Configuration Guide

Juniper JUNOSE 11.2.X BGP AND MPLS User Manual

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