You manage an MPLS network where the PE devices consist of multiple vendors. You are asked to conceal the MPLS topology for all LSPs. Which global configuration parameter will accomplish this?
A.
Configure no-decrement-ttl on the ingress router only.
B.
Configure no-propagate-ttl on the ingress router only.
C.
Configure no-decrement-ttl on all routers within the MPLS network.
D.
Configure no-propagate-ttl on all routers within the MPLS network.
Answer is Using the no-propagate-ttl Command in all routes in the path
which is configured at the [edit protocols mpls] global hierarchy level. This option doesn’t signal any information in a Path message, which makes it interoperable with all other router vendors. the ingress router doesn’t copy the IP TTL value into the MPLS header
and instead puts a 255 value in its place. When the packet arrives at the penultimate router, the TTL value is not written back into the IP packet, which is forwarded natively to the egress router
( Extracted from JNCIS study guide)
The correct answer is D
Since the network consists of devices from multiple vendors, the no-propagate-ttl knob must be used as the no-decrement-ttl option is specific to Juniper RSVP-based LSPs. In addition, all routers in the network should be configured with the correct knob since consistent behavior is required on both the ingress and popping (normally the penultimate) router. If the knob is only configured on the ingress router, the IP TTL is not copied intothe MPLS header and a value of 255 is used instead. When the label is popped, however, the correct IP TTL is not written back into the IP header and the new (and very high) MPLS TTL is used instead.
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Awesome article.|
Nice description..