UDLD — to Global or Per Port

I was dis­cuss­ing UDLD today, and think­ing about mer­its of glob­ally enable UDLD on all our switches or should we con­sider enabling UDLD per port ?

UDLD is the pro­cess of con­firm­ing that a bi-​​directional com­mu­nic­a­tion chan­nel is avail­able. The best example is an eth­er­net port that uses two fibres and data flows uni­direc­tion­ally on each fibre.

The dia­gram shows the two eth­er­net ports con­nec­ted and one of the fibres has broken e.g. con­nector, bend radius, back­hoe atten­u­ation etc. However, the Ethernet port will be UP when the receive line is receiv­ing. The left side will show the phys­ical port as UP, but the right side will show the port as DOWN.

This causes spanning-​​tree and rout­ing pro­tocol prob­lem because they do not detect that the inter­face is down. (note: Bidirectional Forwarding Detection should also detect this type of fail­ure). Troubleshooting can be tricky too.

So UDLD is a Layer ¹⁄₂ soft­ware pro­tocol that estab­lishes con­nectiv­ity on the inter­face. It is not lim­ited to eth­er­net or fibre optic, but that is the most com­mon use. Both inter­faces must sup­port UDLD and have the same set­tings. Other vendors also sup­port UDLD, Cisco has sub­mit­ted it to the IETF in a Draft on April 2007, and other vendors also use it. I have per­son­ally used UDLD on Foundry quite successfully.

Global or not global

Cisco allows con­fig­ur­a­tion of UDLD as a global default or on a port by port basis. When using global mode, cop­per eth­er­net are excep­ted, the assump­tion is that these will be com­puter ports. This means that cop­per con­nec­ted switches will require UDLD to be expli­citly con­figured. Copper cables have the same prob­lem of fail­ure if you lose the trans­mit, but this is uncom­mon. This neg­ates the bene­fit of UDLD

If only one side is con­figured for UDLD, then the con­nec­tion will not go act​ive​.It will also go into err-​​disable state So a switch with Global UDLD enabled will not inter­op­er­ate will with a switch that has the default set­tings on the other side.

Autonegotiation

If you have enabled eth­er­net auto-​​negotiation on both ports, then the link pulse pro­cess will achieve a uni­direc­tional detec­tion at star­tup, how­ever, if the link fails after nego­ti­ation the prob­lem will reoccur

Conclusion

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Because enabling UDLD would cause too much of devi­ation from ‘expec­ted norm’ I recom­mend con­fig­ur­ing UDLD on a per port basis. However, it is vital that you must audit the con­fig­ur­a­tion on an ongo­ing basis to ensure that new switch-​​to-​​switch con­nec­tions have been con­figured. Because UDLD is one of those ‘it just works’ tech­no­lo­gies, people for­get to imple­ment the UDLD fea­ture and this will cause prob­lems in the future.

Postscript

If you miss enabling UDLD at first deploy­ment, you can enable it later without caus­ing ser­vice inter­rup­tion, provided that you con­fig­ure both sides within 30 seconds or so.

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