Saturday, March 28, 2015

PPP encapsulation failure on ASR 1k router (Cisco Bug : CSCuo35867)

"The ASR 1k bug that can screw your ISP link, if triggered". 

My ASR 1k was hit by this bug the other day. I suddenly found my ISP link flapping continuously. The logs and the debugs suggested an issue with the PPP encapsulation. 

The "show interface" command for a couple of interfaces showed something like this :

Serial0/1/3 is up, line protocol is down 
  Hardware is SPA-4XT-SERIAL
  Description: 512Kbps_xxx_to_xxx
  Internet address is
  MTU 1500 bytes, BW 2016 Kbit/sec, DLY 20000 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Closed, crc 16, loopback not set
  Keepalive set (10 sec)
  Last input 00:00:01, output 3d08h, output hang never

A couple of debug commands can provide you more insight into the issue.

The interface came up fine on changing the encapsulation to HDLC (of course on both the ends).

Later, after having the issue looked at by Cisco TAC, we realized that this was because of the bug CSCuo35867.


1. Rebooting the router can provide a temporary fix, however, the issue surfaces every couple of days.

2. The best option is to simply go for an IOS upgrade. The version that we were using was 15.3(3)S, which is affected by this bug. This issue has been fixed in 15.3(3)S3 and later.