Troubleshooting: Cisco 877 on Eircom ADSL2+

During the actual configuration of the Cisco 877s 2 weeks ago some issues arose which needed some debugging! All commands shown below were entered via global configuration mode.


Cisco 877 with a Linksys WAG160N on top!

DNS lookups and SSL connection issues:

Initially we had some issues with DNS lookups and connections to SSL secured services like HTTP and IMAPS. It turned out that I had failed to alter the MTU values! I didn't actually think that I had to! My bad!


The internal VLAN interfaces needed their maximum segment size (MSS) values  adjusted to compensate for the 8 extra bytes needed by the PPPoE header.

int vlan1
ip tcp adjust-mss 1452

The GRE tunnel interfaces also needed some adjustment. This presented as being unable to access services on the other end of the VPN. Again, the MSS values needed to be lowered to account for the GRE header and IPsec headers.


int tun0
ip tcp adjust-mss 1300


ADSL2+ sync problems

There was also some issues getting DSL sync and proper downstream/upstream speeds. Initially, with the ATM interface set to automatic, it was only getting 7meg down. To achieve the full speed I set the DSL operating mode explicitly to ADSL2+.


Still no joy though! I lost DSL sync. This was countered by setting the DSL sync mode to ANSI on the ATM interface. Once that was done, the DSL sync'd and proper speeds were achieved.

Below are the commands used on the interface.

int atm0
dsl sync mode ansi
dsl operating-mode adsl2+