Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) User Manual
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Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
53-1003031-02
IP Traceroute over MPLS
1
MPLS Label=1026 Exp=7 TTL=1 S=0
MPLS Label=500000 Exp=7 TTL=1 S=1
4 <1 ms <1 ms <1 ms 10.34.22.8
5 <1 ms <1 ms <1 ms 10.1.3.8
NOTE
The traceroute output reports information on a traceroute packet only when its TTL equals 1. Label
stack information associated with subsequent routing of the ICMP message along the LSPs to the
destination and back to the source is not displayed.
In the
scenario, the traceroute operation can be described as follows:
6. CE1 in San Jose sends a traceroute probe with a TTL of 1 to its peer in Broomfield, CE2, with
the destination IP address of 10.1.3.8. PE1 decrements the packet’s TTL by one and drops the
expired packet. It then generates an ICMP ttl-exceeded message, and sends it back to the
source IP address embedded in the IP header of the discarded datagram. Traceroute reports
the PE1 IP address at hop 1, but there is no label information.
1 <1 ms <1 ms <1 ms 10.51.3.7
7. CE1 sends a second traceroute probe to CE2, this time with a TTL value of 2. PE1 decrements
the TTL to 1 and pushes an inner VPN label of 500000 and an outer label of 1048 onto the
packet to route it to CE2 by way of R2. PE1 also copies the TTL value from the IP header into
the TTL field of the labels (recall that TTL propagation must be enabled on the ingress PE).
R2 decrements the TTL, drops the expired packet, and generates an ICMP ttl-exceeded
message. Before dropping the packet, and using the ICMP extension mechanism, R2 copies
the packet’s label stack plus its IP header and appends both to the ICMP message. The
message destination is CE1, the device that emitted the traceroute probe. But because R2
cannot return the ICMP message directly to CE1, R2 uses label-switching to forward the
encapsulated ICMP response in the direction of the original traceroute probe along the
configured LSPs and back to CE1. R2 sets the TTL in the topmost label to the maximum
supported value of 225 to ensure that the message can reach its destination before it times
out.
Traceroute reports the IP address of R2, plus the label stack that was pushed onto the
traceroute packet by PE1 and received by R2 when the packet’s TTL was 1. Note PE1s inner
VPN label of 50000 at the bottom of the stack and the outer label of 1048 at the top.
2 <1 ms <1 ms <1 ms 10.56.1.2
MPLS Label=1048 Exp=7 TTL=1 S=0
MPLS Label=500000 Exp=7 TTL=1 S=1
8. The third traceroute probe (TTL=3) is label-switched until it expires at R3. R3 (the Penultimate
Hop Popping (PHP) LSR) generates the ICMP message, appends the label stack from the
expired traceroute packet, and passes it on to PE2 without imposing a label. PE2 forwards the
ICMP message back to CE1 along the return LSP.
Traceroute reports the IP address of R3, plus the label stack which R3 received with the
traceroute packet from R2 when the packet’s TTL was 1.The packet’s label stack includes the
inner VPN label at the bottom of the stack, and the outer label 1026, which R2 imposed when
it swapped R1s 1048 label.
3
<1 ms <1 ms <1 ms 10.52.10.4
MPLS Label=1026 Exp=7 TTL=1 S=0
MPLS Label=500000 Exp=7 TTL=1 S=1