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Knowledge Base

The Traffic Cannot Be Imported into a Tunnel in FA Mode Because It Is Not a Bidirectional One

Publication Date:  2019-07-18  |   Views:  499  |   Downloads:  0  |   Author:  w93777  |   Document ID:  EKB0000359669


Issue Description

Refer to the appendix for the networking diagram.
An MPLS TE tunnel was established between R1 and R3 and the status of the tunnel was Up. After FA was configured for MPLS TE on R1 and the metric was changed to less than the cost of the physical link ― R1-R2-R3, the traffic could not be imported into the tunnel yet.
The configurations on R1 for the tunnel are as follows:
interface Tunnel1/0/0
ip address unnumbered interface loopback 0
tunnel-protocal mpls te
mpls te record-reroute label
mpls te path explicit-path pri-path
mpls te tunnel-id 201
mpls te igp advertise
mpls te igp metric absolute 800
mpls te fast-reroute
mpls te commit
isis enable 64 


Alarm Information


Handling Process

After a tunnel was established from R3 to R1 on the R3, the traffic could be imported into the tunnel normally. 

Root Cause

IGP FA in MPLS TE functions in both directions, that is, a tunnel needs to be established and advertised in both directions. Otherwise, the bidirectional reachability check by OSPF fails at the time of SPF calculation and the tunnel is not allowed to engage in the calculation of IGP SPF. 


In shortcut mode, this problem does not exist. Shortcut functions uni-directionally, so a tunnel can be used on the head end if the tunnel is Up on one end and shortcut is configured.