DMLT "failures"


I've been tasked with putting together a new LAN infrastructure around 4 core stacks of 5510 switches. Each Stack has 3 switches in it. I have used 2-link DMLTs to link the stacks together.

I am testing the network with PCs pinging all of the different VLAN addresses on the switch stacks and a couple of other devices (routers, DHCP server etc) via the Multiping utility.

I have found that when I unplug a link from a DMLT all traffic is rerouted over the other link in the DMLT without any problems.

However if I power off an actual switch in a stack, that contains a DMLT link, some traffic streams do not recover.

I also get different (but consistent) results by using different NICs. i.e. NIC#1 will be able to ping some addresses but not others while NIC#2 will be able to ping addresses not seen by NIC#1 but not some addresses that that NIC#1 can!!!!!!, even when they are configured with the same IP address and patched into the same switch port and under the same test conditions. (I'm using an onboard NIC and a PCMCIA card NIC for these tests).

I can recover all traffic streams by unplug / plug in the other link of the DMLT, thereby forcing some kind of reorganisation.

Is this right?????

i.e Should all traffic streams going through a DMLT recover if a switch hosing one of the DMLT actually fails through hardware / power? All streams are recovered when a link is lost (unpatched) suggesting that the switch that has lost its physical link does some kind of reorganisation with the other DMLT hosting switch, a reorganisation which cannot be carried out if one of the switches has actually failed??.

Hope that makes sense. Any pointers would be most welcome.



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Dave Ward
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MLT or DMLT is an extened 802.3ad. The path selection of each client connected to a switch/stack which performs MLT/DMLT is done on a SRC/DST MAC address basis. Therefore, you got different result on different NICs. The traffic should recover after a period and the lengh of this period depends on what causes this failure. When you unplug and plug a link, this action force the switch which has MLT/DMLT to re-calculate the Layer 2 path and Layer 3 routing, so you can get link recovery quickly.

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