UCS Disjointed Layer 2 Network Configuration


The short definition of a disjointed Layer 2 network is when there are physical or logical Layer 2 network domains upstream that are not interconnected and which cannot be accessed by all the UCS Fabric Interconnect (FI) uplinks.

Before the release of Cisco UCS manager 2.0, one of the border interfaces per fabric was elected automatically to receive unknown unicast, broadcast, and multicast traffic for all blades and subsequently for all VMs in case the blades are used for virtualization workloads. In a disjointed Layer 2 this may pose a problem since not all FI uplinks carry the same traffic. If that is the case, some unicast, multicast or broadcast traffic will be dropped. The solution is to control which uplinks are used for which traffic.

To mitigate this issue, the network design might include implementing a Nexus 5k between the disjointed Layer 2 and FI ensuring that the northbound and southbound traffic flow through the N5K. Another alternative would be to have the FI configured in an Ethernet switching mode, which can result in spanning trees issues.

Cisco’s resolution for a disjointed layer2 network implementation is to have a designated receiver per VLAN. Implementing this requires some configuration changes in UCSM 2.0 and above. I will be providing screenshots on how to correctly introduce another network domain (backup, DMZ, etc.) into an existing production environment. The same steps can be followed for a Greenfield implementation.

In this example, we have VLAN2, 3, 20, and 30 defined globally in the UCS LAN Uplinks Manager. By default all VLANs will flow through all uplinks and vNIC or vNIC templates are already defined in the service profile.

The following diagram shows the network topology. This is a non-disjointed Layer 2 domain. It shows a normal network/Cisco UCS topology where all traffic flows through all uplinks:

UCS Disjointed Layer 2 Network Configuration

The screenshot below shows the LAN uplink manager configuration, which is the default when you create VLANs and configure Eth1/13 on both FI as uplink port. If you can click on the Fabric B tab, it should appear as shown below:

UCS Disjointed Layer 2 Network Configuration

A disjointed Layer 2 is introduced when you have a physically or logically separated network or networks introduced into the topology.

The diagram below shows the topology of a disjointed Layer 2 network.

UCS Disjointed Layer 2 Network Configuration

In the diagram above, eth 1/13 on both FIs can’t reach VLANs 4, 5, 40, and 50. Eth1/23 on both FIs can’t reach VLAN 2, 3, 20, and 30.

UCSM 2.0 and above solve this issue by assigning dedicated uplinks to VLANs that they reach. Introducing a disjointed layer changes the default configuration of the uplink switch. You can use LAN uplink manager to add uplink Eth1/23 and VLAN 4, 5, 40 and 50 to the same VLAN/VLAN Group. You can then isolate Eth1/13 and VLAN 2, 3, 20, 30 to their own VLAN/VLAN group.

This accounts for the way UCS operates. Normally, all VLANs are available on all uplinks by default. If you don’t want that, you must define/isolate them to their respective VLAN/VLAN Group. If you do not associate each uplink to its dedicated VLANs some of the blades will have network connectivity problems. If the blade runs as an ESXi host, some VMs will lose network connectivity.

The procedure to properly configure a disjointed Layer 2 is listed below:

  1. Login to UCS.
  2. Click on the LAN tab.
  3. Click LAN cloud.
  4. Click on LAN uplink manager.
    UCS Disjointed Layer 2 Network Configuration
  5. Click the VLAN tab.
  6. Click the VLAN manager subtab.
    UCS Disjointed Layer 2 Network Configuration
  7. Click on the uplink (Eth1/13) on the left pane and hold down the Ctrl key and click all the VLANs that will be associated with the uplink on the right pane.
    UCS Disjointed Layer 2 Network Configuration
  8. Next, click on the uplink (Eth1/23) on the left pane and hold down the Ctrl key and click all the VLANs that will be associated with the uplink on the right pane.
    UCS Disjointed Layer 2 Network Configuration
  9. Click Fabric B and repeat steps 6 to 9.
    UCS Disjointed Layer 2 Network Configuration
  10. At the end of the day, it should look like this:
    UCS Disjointed Layer 2 Network Configuration

This configuration will ensure that VLANs are associated with the right uplinks. Be careful that any VLAN that is added afterwards is associated with the correct uplink in LAN uplink manager. Remember when you create your vNICs or vNIC template, do not assign VLANs in the two separate network domains to one vNICs. vNICs will only associate with one network domain and not both.

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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.