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BL e-class blade switches with packet discard on cross connect

Rene van Wolferen
Occasional Visitor

BL e-class blade switches with packet discard on cross connect

Hello we use BL e-class blade switches. Inside the enclosure is a cross connect bus to connect switch A and B. If both switches are connected to our lan the cross connect is blocked bij STP. This functions normal.
But when the cross connect is blocked we see a lot of packets discard on it. If both switches function normally no traffic should be sent to the cross connect because they know a better way to our lan.
But why is the counter still increasing. Can somebody help me about this.
Thankx
2 REPLIES
Connery
Trusted Contributor

Re: BL e-class blade switches with packet discard on cross connect

Hi Rene,
When spanning tree blocks a link, only one side of the link is actually being blocked. For instance, if you look at the spanning tree state for the crossconnect ports, you'll most likely see that Switch B is blocking the cross connects, but Switch A is not. Switch B is most likely blocking the ports since Switch B has a higher MAC address (it comes this way from the factory).

Spanning tree only blocks one side of the link for several reasons: 1. at least one switch needs to continue to receive BPDUs or no one will ever realize the link needs to be unblocked. 2. Blocking only one end of the link is as effective as blocking both ends of the link - in either case, no frames can completely traverse the link.

So, back to your question - why are you seeing discards? Most likely because the switch that is not blocking the link (most likely Switch A) is still transmitting broadcast, multicast and unknown unicast frames down that link and the OTHER switch (most likely Switch B) is the one responsible for discarding them.

Bottom line is that what you are seeing is almost always normal behavior.

I hope this answers your question. If not, please post a reply.

Best regards,
-sean
Anders_35
Regular Advisor

Re: BL e-class blade switches with packet discard on cross connect

Another reason one port is still open is that there needs to be a way out from that "lan-segment" that sits between the switches.
I.e. in ancient times, computers were physically clamped on to the cable. If both exits from the cable segment were blocked, nothing would work.