Learn why your network may experience temporary “TTL expired in transit” messages when testing Loop-Free Alternate (LFA) fast convergence and how transient microloops can occur.
Table of Contents
Question
Your company implemented LFA in the network to help with fast convergence in case a failure occurs on the network. However, while the network is being tested to confirm that LFA is working properly, the test set starts receiving “TTL expired in transit” messages for a limited amount of time. What is the cause of these messages?
A. recursion
B. flooding of packets
C. microloops
D. interface dampening
Answer
C. microloops
Explanation
When a company implements Loop-Free Alternate (LFA) in their network to enable fast convergence after a failure, they may notice “TTL expired in transit” messages appearing for a short period during testing. The cause of these messages is microloops (Answer C).
LFA helps networks recover quickly when a link or node fails by pre-computing backup paths. However, immediately after a topology change, the forwarding tables on different routers may be temporarily inconsistent. One router may have converged to the new topology while another is still using the old information.
This inconsistency can lead to brief microloops – small forwarding loops where packets circle between two or more routers until the network fully converges. Packets caught in a microloop will have their TTL (Time to Live) decremented at each hop. If the loop persists long enough, the TTL reaches zero and the packet is discarded, generating a “TTL expired in transit” message.
Microloops caused by temporary inconsistencies are an expected behavior of LFA and will resolve themselves quickly once all routers converge to the new topology, typically within milliseconds or seconds. The “TTL expired” messages should subside after this short convergence period, confirming LFA is operating properly to provide fast rerouting around failures.
In summary, seeing transient “TTL expired in transit” messages when testing LFA fast convergence is likely due to temporary microloops and is an expected part of the convergence process. The messages should disappear once the network is fully synchronized to the new topology.
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