8310 + IST + STG + loop prevention 3

[Navara]

I have two 8310 units with a four port IST working fine. However, if I create a simple loop with a normal CAT5 cable connected to both 8310's (seperate ports)the CPU goes balistic and I end up with a useless solution (loop).

If I create a loop on the SAME 8310 the loop is correctly detected (and doesn't affect the network) and blocks the lower priority port. If the loop goes via the IST - then a loop exists AND takes down the network.

Nortel's standard STG on the IST allows this condition to continue until the cable is withdrawn.

If you disable Nortel's STG on the IST and use Cisco compatable STG then after a period of time the CPU disables the port creating the loop due to excessive traffic (CP limit)and the condition to normality returns.

It would appear that spanning tree traffic does not traverse the IST or is filtered??

Any ideas?

 

[acloe]

When you configure an IST, STP is disabled on the IST ports by default. This is because with SMLT, the two aggregation boxes appear as a single device to the edge switches. This allows for the sub-second L2 failovers.

There is another protocol called SLPP, which is on the 8600 which helps prevent loops in a SMLT network when spanning tree is not used, however that protocol hasn't been implemented on the 8300 yet. For now, just be careful where you are plugging in cables on the aggregation boxes =).

 

[Navara]

I thought the purpose of Nortel's version of spanning tree is to tag the BPDU's so they can be passed over the IST?

Indeed, a direct CAT5 connections doesn't always create a loop where the CPU is max'd out - but one of the CAT5 ports is disabled (CP limit exceeded?).

I have also seen a successful 'blocking' statement but this was a one off.

Is there no other method for solidly preventing an accedental loop with 8310's and an IST?

Regards

 

[anthonyanderberg]

I don't have any 8300s, but on 8600s the IST ports don't participate in spanning tree calculations and can't be shutdown, but they can still be members of multiple spanning tree groups - I assume 8300s are the same.

You'd know it if ports got CP-limit disabled, there are log file messages and the ports have to be manually re-enabled.

 

[Navara]

Yes the syslog message does say the port has been shutdown and you do have to re-enable it.

However, there is NOT a consistent response to a loop.

Sometimes the loop is caught and a correct 'blocking' message is displayed by the lower priority switch/port; sometimes one of the ports gets disabled and has to be disabled/re-enabled and I then have the worst case scenario which is a loop (created by normal traffic or BPDU's) which sends both CPU's to the max and the switches no longer perform correctly until the loop is removed.

Your thoughts....

 

[Alainv]

You are confusing two concepts.
First NSTP. This is proprietary, just as PVST+ of cisco is prprietary.
If you want to talk with other vendors, you need to deploy MSTP.

Yes nortel tags its bpdu vlan's (except for group 1), but this technology has nothing to do with SMLT.

Second:
SMLT, is a proprietary extention of 802.3ad. If you implement it, you don't need STP.


SMLT works without and won't support STP on its SMLT or IST links.
True, sometimes it is a pitty, but that's how it works. Perhaps nortel will adapt something in the future, but now it ain't possible.
STP can only be used on the access, read client ports to prevent local loops, due patching errors etc.

I don't use the 8300, but also this chassis will support SLPP in the near future and this must prevent looping problems.

If you want to check it out, look for the 8600 documentation.