I came across a bulletin from Nortel just recently that I thought was important enough to post here in case anyone reading this has 8648GTRs installed in his/her ERS 8600 chassis. I have about 10 of them installed at multiple locations, primarily in core switches feeding large server farms and other high-speed devices. I don’t believe I need to extol the pains that auto-negotiation sometimes reaps on network engineers. While modern network switches and NICs are definitely more compatible with respect to auto-negotiation, problems sometimes still arise. It would seem that a duplex mismatch on one port of the 8648GTR could potentially impact performance on up to 24 ports. Here’s the text from the bulletin;

Background:

An Ethernet port can operate either in Full or Half Duplex mode. A duplex mismatch is created when using inconsistent settings for duplex mode, i.e. full duplex on the port and half duplex on the connected device (or vise versa). This situation is most likely created when using inconsistent and inappropriate settings for auto-negotiation, i.e. auto-negotiation enabled on the port and disabled on the device connected to the port (or vise versa). The duplex mismatch problem can be corrected by setting consistent duplex mode on both the port and the connected device when hard setting the duplex mode or by enabling auto-negotiation on both the port and the connected device, when using auto-negotiation.

Ethernet ports of most devices today have auto-negotiation enabled as the default setting. When a device with auto-negotiation disabled is connected to a port that has auto-negotiation enabled, the port is not able to detect the duplex setting of the connected device and falls back to half duplex thus potentially causing a duplex mismatch. A duplex mismatch will cause physical layer errors and performance degradation of the connection. Any mixture of auto-negotiation enabled on one-side and auto-negotiation disabled on the other side is an “unsupported” configuration. The setting on both sides of any connection must match for proper operation. A problem has been identified when there is a duplex mismatch on one or more ports of an 8648 GTR module. For an 8648 GTR module, a duplex mismatch may cause complete communication issues on the port with the mis-matched duplex or occasionally on the entire lane (Port 1-24 or Port 25-48) that contains the port with mismatched duplex. The module can be recovered from the situation when physically reseated, but for complete recovery the mis-configuration must also be corrected. Correcting the duplex setting configuration alone will not recover the communication loss until the module is reseated as well.

Analysis:

A duplex mismatch may cause communication loss on a port or an entire lane of an 8648 GTR module. When there is such a communication loss, the debugging commands show that the ingress stats look normal with all traffic ingressing the impacted port(s) and the MAC addresses learned in the Forwarding Database Table for the devices connected to the port(s), but no traffic egressing the port(s).

Recommendations:

Nortel recommends proper configuration of auto-negotiation whenever possible to prevent a duplex mismatch situation. To avoid a duplex mismatch, auto-negotiation must be enabled on the port as well as the device connected to that port.

You can find a copy of the bulletin in PDF format right here. Interestingly there are quite a few restrictions and issues with the 8648GTR that I should probably discuss them here when the time allows.

Nortel is also in the process of releasing v5.0 software for the Ethernet Routing Switch 8600 along with four new IO modules (cards); 8612XLRS, 8634XGRS, 8648GBRS and 8648GTRS. I hope to talk about those in the very near future.

Cheers!