Brocade Multi-Service IronWare Administration Guide (Supporting R05.6.00) - Manual (Page 225)

Multi-Service IronWare Administration Guide 207
53-1003028-02
IPv6 Traceroute over an MPLS network
6
1 <1 ms <1 ms <1 ms 2001:DB8:1::2
2 <1 ms <1 ms <1 ms 2001:DC8:200::3
MPLS Label=1026 Exp=0 TTL=1 S=0
MPLS Label=794624 Exp=0 TTL=1 S=1
3 <1 ms <1 ms <1 ms 2001:DD8:300::2
MPLS Label=1029 Exp=0 TTL=1 S=0
MPLS Label=794624 Exp=0 TTL=2 S=1
4 <1 ms <1 ms <1 ms 2001:DE8:300::2
5 <1 ms <1 ms <1 ms 2001:DB8:2::2
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 Figure 14 scenario, the traceroute operation can be described as follows:
1. CE1 sends a traceroute probe with a TTL of 1 to its peer, CE2, with the destination IP address
of 2001:DB8:2::2. PE1 decrements the packet’s TTL by one and drops the expired packet. It
generates a ttl-exceeded ICMPv6 message, and sends it back to CE1 with the source IPv6
address embedded in the IPv6 header of the expired packet. Traceroute reports the PE1 IPv6
address at hop 1, but there is no label information.
1. <1 ms <1 ms <1 ms 2001:DB8:1::2
2. CE1 sends a second traceroute probe to CE2, with an incremented TTL value of 2. PE1
decrements the TTL value to 1, and adds the 6PE label and the Label Distribution Protocol
(LDP) label onto the packet to route it to CE2 by way of the transit router P1. 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).
The transit router P1 decrements the TTL, drops the expired packet since the TTL value is 0,
and generates a ttl-exceeded ICMPv6 message. Before dropping the packet, and using the
ICMPv6 extension mechanism, P1 copies the packet’s label stack plus its IP header and
appends both to the ICMPv6 message. Though the message destination is CE1, P1 cannot
return the ICMPv6 message directly to CE1. It 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. P1 sets the maximum TTL value of 255 to ensure that the message can reach its
destination before it times out.
Traceroute reports the IP address of P1, plus the label stack that was pushed onto the
traceroute packet by PE1 and received by P1 when the packet’s TTL was 1.
2 <1 ms <1 ms <1 ms 2001:DC8:200::3
MPLS Label=1026 Exp=0 TTL=1 S=0
MPLS Label=794624 Exp=0 TTL=1 S=1
3. The third traceroute probe (TTL=3) is forwarded until it expires at the transit router P2. P2 (the
Penultimate Hop Popping (PHP) LSR) generates the ICMPv6 message, appends the label stack
from the expired traceroute packet, and passes it on to PE2 without imposing a label. PE2
forwards the ICMPv6 message back to CE1 along the return LSP.
Traceroute reports the IP address of P2, plus the label stack which P2 received with the
traceroute packet from P1 when the packet’s TTL was 1.