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remote address in local and remote reachability table (DLSW)


I have been testing a DLSW ethernet session between a mainframe IBM and a Tandem. I noticed that It gets connected after a long time, I found in my dlsw reachability table that I have the remote mac-address in the Local and remote reachability. I think it is the problem, sometimes when I clear the reachability table it gets connected but not always.

I don't understand why I saw the mac-address in local, I have some services to the same ibm host with serial ports (SDLC).

Is there a way to force the remote mac-address in the remote reachability?

Thank you.

Cisco Employee

Re: remote address in local and remote reachability table (DLSW)

Hi Arturo,

Whenever you see a MAC address found both local and remote, it indicates a problem. This pretty much breaks DLSw. What causes this is usually a redundant DLSw topology where there are two DLSw routers on the same ethernet segment or VLAN. In other words, there are two paths over DLSw between the IBM and Tandem mainframes. The explorer from the remote is being sent out from both ethernet routers onto the same LAN segment or VLAN. This causes the mac-address table on the switch to learn the MAC address from one router ( port ) and then the other.

The explorer packet is also seen by the other DLSw router on the same LAN segment or VLAN and now it thinks the MAC is both local and remote.

Here is a reference on Cisco Online:

You have to find another way to do redundancy in an ethernet environment other than two DLSw routers on the same LAN segment or VLAN. Maybe DLSW Ethernet Redundancy, a spanning tree soulution or multiple adapters with the same MAC but in different VLANs, such sa two OSAs with the same MAC, each one is only connected to one DLSw router each OSA is in a different VLAN.

Best regards,


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