• gcharriere

    Dear Erwin, could you please clarify what happens when frames tagged by a Cisco Equipment on one side are received by a Brocade device on the other side. To be more precise, I am referring about the interconnection between UCS (FEX) and a Brocade device. By default, the FEX will tag traffic in VSAN 1. It means that somehow this tag should be included in the extension header. In this case, what happens when the brocade switch receives it. He simply does not take it into account and accept the traffic?

    I was under the impression that VSAN were working the same way than VLAN. With VLAN,when a port is in access mode or untagged more, the frame is sent without 802.1q header at all. However, with VSAN, I did some tests that made me think that it behaves differently. When we connect a FEX in VSAN 1 to a MDS switch, there is communication only if the VSAN 1 is configured on the other side as well. It means that the VSAN ID is included in the header. However when we connect it to a Brocade switch that is not able to “speak” VSAN, there is still communication. Any idea how?

    Thanks in advance.

    • Hello Gael,

      I think the communication is limited to the ELP and EFP parameters. During initialization of the ISL the ELP (Exchange Link Parameters) checks whats on the other side. If its a switch it will pop into E-port mode and it resumes with EFP (Exchange Fabric Parameters) negotiation. It might well be that the ISL itself if fully operational but the fabrics will segment. I think a Cisco FEX sits in NPV mode by default and thus is unaware of any VSAN configuration.

      Secondly don’t get mixed up with VLAN and VSAN. These are totally different and have nothing in common. Fibre-Channel N-ports do not “tag” frames per-se. There is only field they might use (CS_CTL) which is an obsolete byte in the frame header which was used in Class 1 and Class 4. I’ve personally never seen this byte being used. An N-port NEVER sets an extended or routing header.

      Hope this helps.


      • gcharriere

        Thanks Erwin. Cisco FEX sits in NPV mode by default, but most of the time, f-port-trunking feature is used as well. And in this case the traffic from the FEX goes into the switch with the VSAN tag. It is still not clear for me how a Brocade switch will process a frame with a VSAN tag from a Cisco equipment.

        According to the white paper from Cisco (http://www.cisco.com/c/dam/en/us/products/collateral/servers-unified-computing/ucs-b-series-blade-servers/whitepaper_C07-730016.pdf), the tag is simply ignored by the
        Brocade device, but there is no details about the way it works.


        • Hi Gael,

          There is no VSAN identifier between the FEX and the Brocade switch. The vHBA, VIC and FEX determine to which VSAN they belong but this is not propagated further. You can compare it a bit like portgroups in a Brocade terminology where the F-ports on the Access Gateway (NPIV switch) can be grouped to use one or more N-ports going to the rest of the fabric. These portgroups are also not propagated to the rest of the fabric. With the terminology differences across these two platforms it doesn’t make it easier but I hope I explained it a bit here.


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