[c-nsp] TCN's - Causing brief outages on ASR1K

CiscoNSP List cisconsp_list at hotmail.com
Sun Dec 14 19:31:57 EST 2014

Hi Everyone,

We have had an issue for the past 2 weeks, where randomly OSPF+LDP sessions(But not all sessions) would drop for ~3-9 seconds on an ASR1006. Any link connected directly to the ASR was not experiencing any issues....only links going via a 4948(Only doing L2) were being impacted.

To further eliminate the ASR, we moved one of the links that was experiencing the OSPF/LDP flaps from the 4948 directly to the ASR....flaps stopped occurring for this link.

TAC have been working on the problem for over a week, and Friday identified that the flaps were coinciding with TCN's on the 4948 - There suggestion was to enable "spanning-tree portfast trunk" on the link from the 4948 -> ASR....this was done, and we havent experienced a flap for over 2 days(But we have gone for 2-3 days, without a flap previously)

Ive asked TAC (But they have yet to respond to my question), so Im hoping someone on the list can:

Is it expected behaviour for a TCN to cause a flap on an ASR...We have many other POP's with switches 4948's/4500's etc(trunk)->ASR+7200's and do not have spanning-tree portfast trunk enabled, and they do not experience any flaps?

Is enabling spanning-tree portfast trunk on the 4948 trunk port to the ASR simply stopping the TCN's from getting to the ASR, and therefore stopping the flaps....but we still have (a yet to be identified) issue?

Thanks in advance for any suggestions/comments. 		 	   		  

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