Need help with Cisco to Nortel ISDN connection

[Jeffreyala]

We have a remote site on our Frame Relay that has a Nortel AN router. Our hub site is using a Nortel ASN router. We are replacing the AN with a Cisco 2611XM. This site has an ISDN line for backup purposes. Does anyone here have experience setting up a backup ISDN going from Cisco to Nortel??

 

[gdschertz]

actualy no I dont but seeings as you are trying to communicte via isdn there should be no capatiblity isues. just configure the cisco to use the isdn line as a backup and if you using ip to route set the correct address on the int.

 

[Jeffreyala]

Thanks for the reply. Actually, another member has pointed out that there is a compatibility problem.

Cisco and Bay interpret PPP MTU size differently. Bay MTU size includes PPP headers and in Cisco routers, the headers are not counted. This results in a 6-octet difference in MTU interpretation (If Bay says 1600, Cisco sees 1594).
If the two are set up with the same size MTU and MRU, when the Cisco sends a fully-loaded packet to the Bay, the Bay will read it as an oversize packet and drop it. If you set the Bay MTU to 1506 and the Cisco to 1500, PPP will negotiate down to 1500 and the Bay buffers will be large enough to accommodate the whole packet from the Cisco.

Solution:
Activating RFC1661 compliance with MTU and MRU defaults enables the Cisco to correctly see the Bay MRU as 1500 to properly synchronize with the Cisco MTU of 1500. The RFC 1661 option can be found in Site Manager as a PPP Line option-box in Site Manager: Protocols->PPP->Line->RFC 1661 compliance), Enable "RFC1661." (Enabled by default in recent BayRS releases).

I figured I'd post this incase someone ever needs it. I can email the whole document if anyone needs it.


Jeff