Wednesday, 16 May 2007

Port Indexes on Cisco Switches

I spent today building some Cisco MDS9513 switches. It's good to get into the nuts and bolts of technology once in a while. They are big beasts - 2nd generation technology and with 11 usable slots, the biggest available line cards (48 ports), 528 ports in a single chassis. However the build had one fly in the ointment. As part of the (inherited) design, the chassis included a "generation 1" eight ethernet port IP line card for implementing FCIP.

The issue revolves around a feature called port indexes, which are used to track the ports installed in a MDS chassis. Generation 1 chassis have a maximum of 252 port indexes, generation 2 technology supports up to 1020 port indexes. However, a generation 1 line card inserted into a generation 2 chassis dumbs down the whole switch to generation 1 port indexes. So, with 252 indexes, 32 taken up by the FCIP card, only 240 port indexes are left which directly translates to 240 ports - less than half the switch capacity! If line cards are installed that take the port count above 252 then these additional cards won't come online - they will initially power up then power down.

In this instance the solution will be to move IP services to another (smaller) switch and hopefully Cisco will bring out a generation 2 version of the IP blade soon. There is a bigger problem though, and that is for any customers looking to take 9513s and use them with the SSM module, which supports products such as EMC Invista and Kashya (now EMC RecoverPoint). As far as I am aware, there's no plans for a generation 2 SSM module any time soon, so using the SSM module in the 9513 chassis will create the same port restriction issues.

I don't see why Cisco couldn't simply produce a gen 2 version of the old gen 1 blades which did nothing more than re-jig the port indexing. Come to think of it, surely they could patch the firmware on the gen1 line-cards to fix the problem. Obviously it is not that simple, or perhaps not that many people are using Invista and RecoverPoint to make it worthwhile.

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