The MPLS TTL Acreage and MPLS TTL Propagation
The IP header’s TTL acreage supports two important features: a apparatus to analyze looping
packets, and a adjustment for the traceroute command to acquisition the IP abode of anniversary router in a
particular end-to-end route. The MPLS header’s TTL acreage food the aforementioned features—in fact,
using all defaults, the attendance or absence of MPLS LSRs in a arrangement has no appulse on the end
results of either of the TTL-related processes.
MPLS needs a TTL acreage so that LSRs can absolutely avoid the encapsulated IP attack when
forwarding IP packets. Essentially, the LSRs will cutback the MPLS TTL field, and not the IP
TTL field, as the packet passes through the MPLS network. To accomplish the accomplished action work,
using all absence settings, admission E-LSRs, LSRs, and departure E-LSRs assignment as follows:
■ Admission E-LSRs— After an admission E-LSR decrements the IP TTL field, it pushes a characterization into
an unlabeled packet and again copies the packet’s IP TTL acreage into the new MPLS header’s
TTL field.
■ LSRs—When an LSR swaps a label, the router decrements the MPLS header’s TTL field, and
always ignores the IP header’s TTL field.
■ Departure E-LSRs—After an departure E-LSR decrements the MPLS TTL field, it ancestor the final
MPLS attack and again copies the MPLS TTL acreage into the IP attack TTL field.
Figure 19-5 shows an archetype in which a packet arrives at PE1, unlabeled, with IP TTL 4. The
callouts in the amount account the capital accomplishments for the three roles of the LSRs as declared in the
previous list.
Figure 19-5 Archetype of MPLS TTL Propagation
The appellation MPLS TTL advancement refers to the accumulated argumentation as apparent in the figure. In effect,
the MPLS routers bear the aforementioned TTL amount beyond the MPLS network—the aforementioned TTL values
that would accept occurred if MPLS was not acclimated at all. As you ability expect, a absolutely looping packet
would eventually cutback to TTL 0 and be discarded. Additionally, a traceroute command
CE1 PE1 P1 PE2
Ingress
E-LSR
LSR Egress
E-LSR
CE2
1 IP TTL 4 2 4 IP TTL 1
IP TTL 3
MPLS TTL 3 3
IP TTL 3
MPLS TTL 2
Decrement IP TTL to 3
Push New MPLS Header
Copy to New MPLS TTL
Decrement MPLS TTL to 1
POP MPLS Header
Copy MPLS TTL to IP TTL
Ignore IP TTL (Still 3)
Decrement MPLS TTL to 2
MPLS Unicast IP Forwarding 703
would accept ICMP Time Exceeded letters from anniversary of the routers in the figure, including
the LSRs.
However, abounding engineers do not appetite hosts alfresco the MPLS arrangement to accept afterimage into the
MPLS arrangement with the traceroute command. SPs about apparatus MPLS networks to create
Layer 3 WAN services, and the SP’s barter sit alfresco the MPLS network. If the SP’s
customers can acquisition the IP addresses of the MPLS LSRs, it may abrade the chump who wants to
see alone chump routers, and it may actualize a aegis acknowledgment for the SP.
Cisco routers can be configured to attenuate MPLS TTL propagation. When disabled, the ingress
E-LSR sets the MPLS header’s TTL acreage to 255, and the departure E-LSR leaves the aboriginal IP
header’s TTL acreage unchanged. As a result, the absolute MPLS arrangement appears to be a distinct router
hop from a TTL perspective, and the routers central the MPLS arrangement are not apparent from the
customer’s traceroute command. Amount 19-6 shows the aforementioned archetype as in Amount 19-5 but now
with MPLS TTL advancement disabled.
Figure 19-6 Archetype with MPLS TTL Advancement Disabled
Cisco supports the adeptness to attenuate MPLS TTL advancement for two classes of packets. Most
MPLS SPs may appetite to attenuate TTL advancement for packets forwarded by customers, but allow
TTL advancement for packets created by the SP’s routers. Using Amount 19-5 afresh for an example,
an SP architect may be logged in to router PE1 in adjustment to affair a traceroute command. PE1 can
be configured to use TTL advancement for locally created packets, which allows the traceroute
command issued from PE1 to account all the routers in the MPLS cloud. At the aforementioned time, PE1 can be
configured to attenuate TTL advancement for “forwarded” packets (packets accustomed from
customers), preventing the chump from acquirements router IP addresses central the MPLS network.
(The command is no mpls ttl-propagation [local | forwarded].)
NOTE Although the PE1 router has TTL-Propagation disabled, all routers in the MPLS
domain should additionally accept TTL disabled for constant achievement of the TTL propagation.
CE1 PE1 P1 PE2
Ingress
E-LSR
LSR Egress
E-LSR
CE2
1 IP TTL 4 2 4 IP TTL 3
IP TTL 3
MPLS TTL 255 3
IP TTL 3
MPLS TTL 254
Decrement IP TTL to 3
Push New MPLS Header
with TTL = 255
Decrement MPLS TTL to 253
POP MPLS Header
Ignore IP TTL (Still 3)
Decrement MPLS TTL to 254