MTU on CISCO886VA-SEC-K9

[DirkDietz5932]

Hi all,

we are running many CISCO886VA-SEC-K9 on german ADSL lines. Currently we are running this IOS: 154-3.M8

Our config is basic:

interface ATM0
no ip address
no atm ilmi-keepalive
!
interface ATM0.7 point-to-point
mtu 1600
pvc 1/32
bridge-dot1q encap 7
pppoe-client dial-pool-number 1
!
interface Dialer1
 mtu 1492
 dialer pool 1

With this config we are able to ping our LNS with 1492 Byte:

Router#ping 2.17.0.163 size 1492 df-bit
Type escape sequence to abort.
Sending 5, 1492-byte ICMP Echos to 2.17.0.163, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/32/36 ms
Router# 

After upgrading IOS to 15.7(3)M5 max. packet size is now 1474 Byte.

Router#do ping 2.17.0.163 size 1474
Type escape sequence to abort.
Sending 5, 1474-byte ICMP Echos to 2.17.0.163, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/32/32 ms
Router#

Router#do ping 2.17.0.163 size 1475
Type escape sequence to abort.
Sending 5, 1475-byte ICMP Echos to 2.17.0.163, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
Router#

So i checked different IOS for this behavior:

c880data-universalk9-mz.157-3.M4a.bin: max 1474 Byte

c880data-universalk9-mz.156-3.M7.bin: max 1474 Byte

c880data-universalk9-mz.155-3.M10.bin: max 1492 Byte 

Does anybody know why getting MTU issues starting with 15.6?

If someone has an idea please let me know to share more input.

Kind Regards

Dirk

[marce1000]

 - Could you check the (default) MTU values in the running config on the ATM and Dialer interfaces after the upgrade ? If they would be accidentally higher then reduce them again to the working values from the config before the upgrade. Check if this helps.

 M.

[DirkDietz5932]

Hi,

the configured MTU after IOS-Upgrade are:

ATM0:  1600 Byte

Dialer1:  1492 Byte

It doesn't change when removing and adding again. We still get 1474 Byte only. So what happens with 18 Byte?

This starts with IOS 15.6.

Cheers

Dirk

[DirkDietz5932]

Hi all (including Cisco TAC),

i have now tested a bit more between Software versions.

Last Image of tree 15.5:   c880data-universalk9-mz.155-3.M10.bin

Everything works fine as expected, MTU of 1492 Byte are available.

Router#ping 2.17.0.163 size 1474
Type escape sequence to abort.
Sending 5, 1474-byte ICMP Echos to 2.17.0.163, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/32/32 ms
Router#

Router#ping 2.17.0.163 size 1492 df-bit
Type escape sequence to abort.
Sending 5, 1492-byte ICMP Echos to 2.17.0.163, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/30/32 ms
Router# 



Router#show run int atm0 | i mtu
 mtu 1600
Router#

Router#show int atm0 | i MTU
  MTU 1600 bytes, sub MTU 1600, BW 2044 Kbit/sec, DLY 180 usec,
Router#  



Router#show int dialer 1 | i MTU
  MTU 1492 bytes, BW 56 Kbit/sec, DLY 20000 usec,
  MTU 1492 bytes, BW 2044 Kbit/sec, DLY 20000 usec,
Router#

Router#show run int dialer 1 | i mtu
 mtu 1492
Router#   

First Image of tree 15.6:   c880data-universalk9-mz.156-3.M.bin

Max. MTU of 1474 available, no config changes done, same MTU values as with previous image.

Router#ping 2.17.0.163 size 1474
Type escape sequence to abort.
Sending 5, 1474-byte ICMP Echos to 2.17.0.163, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/32/36 ms
Router#


Router#ping 2.17.0.163 size 1475
Type escape sequence to abort.
Sending 5, 1475-byte ICMP Echos to 2.17.0.163, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
Router#



Router#show run int atm0 | i mtu
 mtu 1600
Router#  

Router#show int atm0 | i MTU
  MTU 1600 bytes, sub MTU 1600, BW 2044 Kbit/sec, DLY 180 usec,
Router#  



Router#show int dialer 1 | i MTU
  MTU 1492 bytes, BW 56 Kbit/sec, DLY 20000 usec,
  MTU 1492 bytes, BW 2044 Kbit/sec, DLY 20000 usec,
Router#    

Router#show run int dialer 1 | i mtu
 mtu 1492
Router#  

Looks like  a bug for me, or a new unknown feature.

Cisco, please help.

Many thanks

Dirk