Technical data

Brocade MLX Series and NetIron Family Documentation Updates 7

53-1002805-03

IPv6 Traceroute over an MPLS network

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.

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. The fourth traceroute probe (TTL=4) is forwarded until it expires at the egress provider edge

device PE2. PE2 drops the packet and generates a ttl-exceeded ICMPv6 message without label

stack extension since there is no label stack to report.

Traceroute reports only the IP address of PE2. The transit router P2 popped the outer label

before passing the traceroute packet on to the egress PE2 and PE2 pops the VPN label before

sending the ICMPv6 message back to the customer source device CE1.

4 <1 ms <1 ms <1 ms 2001:DE8:300::2

5 <1 ms <1 ms <1 ms 2001:DB8:2::2

5. The fifth traceroute probe (TTL=5) has a TTL large enough for the packets to reach the

customer destination device CE2. CE2 generates an ICMPv6 port unreachable message, which

CE2 sends back to CE1.