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Knowledge Base

Fault AR207 doesn't fragment packets on PPPoA scenario

Publication Date:  2013-05-01  |   Views:  267  |   Downloads:  0  |   Author:  b00745015  |   Document ID:  EKB1000027405

Contents

Issue Description

AR 207 V200R00C00 in PPPoA scenario.

Topology:

Computer -----LAN-----AR207 (or Cisco C877)----ADSL----VPN---Ethernet---Cisco1921---LAN---Server
                                                                                                                         |
                                                                                                                         |_____Internet

PC1 from LAN was unable to ping with packets bigger than 1468 the server  and web service for internet was severly degraded.

Alarm Information

Pinging from computer towards server we have the fallowing results:
[AR]ping -s 1468 -a 192.168.20.1 192.168.7.1
  PING 192.168.7.1: 1468  data bytes, press CTRL_C to break
    Reply from 192.168.7.1: bytes=1468 Sequence=1 ttl=248 time=100 ms
    Reply from 192.168.7.1: bytes=1468 Sequence=2 ttl=248 time=90 ms
    Reply from 192.168.7.1: bytes=1468 Sequence=3 ttl=248 time=90 ms
    Reply from 192.168.7.1: bytes=1468 Sequence=4 ttl=248 time=90 ms
    Reply from 192.168.7.1: bytes=1468 Sequence=5 ttl=248 time=90 ms

  --- 192.168.7.1 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 90/92/100 ms

[AR]ping -s 1469 -a 192.168.20.1 192.168.7.1
  PING 192.168.7.1: 1469  data bytes, press CTRL_C to break
    Request time out
    Request time out
    Request time out
    Request time out
    Request time out

Handling Process

Escalated the issue to RnD, he made low level debugging on chip and concluded that chip had some limitation that didn't permit a normal functionalities. RnD team and chip vendor team started to work together to solve this and after couple of weeks they issue the patch
AR207-V200R002C00CP1001T.pat  which solved the problem: 

Root Cause

Customer tried with a cisco device, everything was working fine. So we concentrate the attention on AR200.

I've tried to tune AR configuration adjustion MTU and MSS, web service improved but fragmentation still no working.  Also asked customer to enable clear ip df to enable forcibly fragmentation

interface Atm0/0/0
tcp adjust-mss 1200
#
interface Atm0/0/0.1
mtu 1492
tcp adjust-mss 1200
pvc 1/33
  map bridge Virtual-Ethernet0/0/0
#

interface Dialer1
link-protocol ppp
ppp chap user **************
ppp chap password simple **********
mtu 1492
tcp adjust-mss 1200

ip address ppp-negotiate
clear ip df
dialer user *********
dialer bundle 1
dialer-group 1
nat outbound 2000 interface LoopBack 1
interface Vlanif1
mtu 1492
tcp adjust-mss 1200
ip address 192.168.20.1 255.255.255.0
clear ip df
dhcp select interface
dhcp server dns-list --------------------
dhcp server lease day 0 hour 2 minute 0

interface Virtual-Ethernet0/0/0
pppoe-client dial-bundle-number 1
tcp adjust-mss 1200

Suggestions

none.