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Cisco Bug: CSCvv95741 - Call establishment failures due to selection of UP without chunks

Last Modified

Oct 09, 2020

Products (1)

  • Cisco ASR 5000 Series

Known Affected Releases

21.19.n4

Description (partial)

Symptom:
CP has Sx peers established with 5 UPs, out of which available ip-pool is allocated to only 4 UPF. UPF#5 does not have IP pools allocated (as per CLI show ip pool-chunks pool-all), but Sx association to UPF#5 is established because, for example, another APN in the same up-group has enough chunks for all UPs.

Then while establishing sessions, we observe that CP selects UPF#5 (that does not have ip pool) for session establishments.

We observe the same behaviour in both cases below:
1/ When UPF has 0 chunk, it is selected by CP for new session establishment
2/ When UPF has less chunks than the others (eg UPF1-4 has 3 chunks and UPF-5 has 2 chunks) and UPF-5 chunks are full, it is still selected by CP for new session establishment although there are available chunks on the other UPFs

This behaviour is blocking all new sessions establishments attempts, because the CP selects the UPF which has least number of sessions to create new sessions. It means UPF-5 will always be selected and the other available chunks on the other UPFs will not be used (apart for the first 1 or 2 sessions).

Conditions:
(1 or more UPF has no chunk allocated AND that UPF is still sx-associated (eg because another APN with more chunks than the number of UP is using the same UP group) 
OR
1 or more UPF has less chunk allocated than the other UPFs AND all the chunks of that 1 (or more) UPFs are full
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