VLAN Assignment Issues

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• when you configure a VLAN,
  • the switch ports assigned to that VLAN are in a segmented network
  • VLAN is likely to be assigned its own subnet address
  • any device connecting to a port in the VLAN must have an appropriate IP configuration for that subnet
    • i.e., IP address, subnet mask, default gateway, DNS servers
  • hosts in the VLAN must use the router to contact other VLANs or other remote hosts
• VLAN’s router is likely configured as a virtual interface
  • if router has a single physical trunk link to the switch hosting the VLAN,
    • then that virtual interface will be configured as a subinterface of the physical interface
    • i.e., interface G0.16
  • If a layer 3 switch is used to implement the VLAN,
    • the default gateway address will usually be configured as a Switched Virtual Interface (SVI)
      • i.e., interface VLAN16
    • use show commands on the router or switch to verify:
      • parameters have been set correctly
      • ports have been assigned to the correct VLANs
• host must be physically connected to the correct switch port configured with the appropriate VLAN ID
  • if it has correct VID, and the host is still assigned to the wrong VLAN,
    • suspect a physical cabling problem
    • verify the cable from the patch panel is connected to the correct switch port
  • if continued problems,
    • verify that the patch panel labeling correctly identifies the wall jack that the computer is connected to
• If cannot diagnose issue by looking at configuration:
  • use ping to test connectivity
  • use same process as you would with physical LAN:
    1. ping the loopback address, then the host’s own IP address
       • verifies that TCP/IP is working and the host IP is configured correctly
    2. Ping the default gateway to verify it is contactable
       • optionally, also ping another host on the same VLAN/subnet to verify that local communications can be established
       • if cannot ping the default gateway, look for a problem with either the host or switch/router configuration
       • if can rule out one of these and no connectivity with any local hosts, check:
         • patch cable for the workstation’s wall port is connected to the correct switch port
         • for physical cabling issues
    3. ping a remote host or server to verify that routing is available
       • if cannot ping any remote hosts,
         • verify the switch and router configuration
         • determine if the problem is isolated to single VLAN or spread across network
         • if can ping some remote hosts but not others, use traceroute to determine cause

Info

• hosts in each VLAN must be able to reach DHCP servers and DNS servers
• one option is to place these hosts within the VLAN
  • but could mean provisioning more servers
• typically will be in a server VLAN
  • hosts in the client VLAN must use the default gateway to contact
  • DHCP relay must also be configured