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in the echo reply packet. The MPLS echo reply packet is sent back as a labeled
UDP packet with the following attributes:
10.3.3.3 Source address
10.1.1.1 Destination address
3503 UDP port
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 366.
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
Packet Flow Examples for Verifying MPLS Connectivity
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Chapter 4: Monitoring MPLS