Manual Dynamic Mac Address

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You must have enough media access control (MAC) addresses to assign to the number of logical domains, virtual switches, and virtual networks you are going to use. You can have the Logical Domains Manager automatically assign MAC addresses to a logical domain, a virtual network (vnet), and a virtual switch (vsw), or you can manually assign MAC addresses from your own pool of assigned MAC addresses. The ldm subcommands that set MAC addresses are add-domain, add-vsw, set-vsw, add-vnet, and set-vnet. If you do not specify a MAC address in these subcommands, the Logical Domains Manager assigns one automatically.

When a logical domain or a device associated with an automatic MAC address is removed, that MAC address is saved in a database of recently freed MAC addresses for possible later use on that system. These MAC addresses are saved to prevent the exhaustion of Internet Protocol (IP) addresses from a Dynamic Host Configuration Protocol (DHCP) server. Although Cisco switches dynamically build the MAC address table by using the source MAC address of the received frames, you can also manually add a MAC address to the switch’s MAC address table. The static MAC entries will be retained even after the switch is restarted. To configure a static MAC address, the following command is used. Since MAC addresses can only be used within their local segment, you can't use them for port forwarding or similar. When your IP address is dynamic you can either pay your ISP for a static IP or use a dynamic DNS service that always points to your current IP. 21 a4bb.6d10.5555 DYNAMIC Gi2/0/3 DYNAMIC Gi2/0/3 MAC Address Method Domain Status Fg Session ID-Gi2/0/3 a4bb.6d10.5555 dot1x DATA Auth 0A67FE04000004FE7E292422 Gi2/0/3 0004.f2aa.1111 N/A UNKNOWN Unauth 0A67FE04000004FF7E292D25. #show mac address-table int g2. When your Mac is connected to a private network in a home or office, it’s probably assigned what’s known as a dynamic IP address. (To check, see How to Find Your Mac’s IP Address.) That’s not a problem for the majority of users - most people don’t care whether their IP addresses changes or not.

The advantage to having the Logical Domains Manager assign the MAC addresses is that it utilizes the block of MAC addresses dedicated for use with logical domains. Also, the Logical Domains Manager detects and prevents MAC address collisions with other Logical Domains Manager instances on the same subnet. This frees you from having to manually manage your pool of MAC addresses.

MAC address assignment happens as soon as a logical domain is created or a network device is configured into a domain. In addition, the assignment is persistent until the device, or the logical domain itself, is removed.

Range of MAC Addresses Assigned to Logical Domains Software

Logical domains have been assigned the following block of 512K MAC addresses:

00:14:4F:F8:00:00 ~ 00:14:4F:FF:FF:FF

The lower 256K addresses are used by the Logical Domains Manager for automatic MAC address allocation, and you cannot manually request an address in this range:

00:14:4F:F8:00:00 - 00:14:4F:FB:FF:FF

You can use the upper half of this range for manual MAC address allocation:

00:14:4F:FC:00:00 - 00:14:4F:FF:FF:FF

Automatic Assignment Algorithm

When you do not specify a MAC address in creating logical domain or a network device, the Logical Domains Manager automatically allocates and assigns a MAC address to that logical domain or network device. To obtain this MAC address, the Logical Domains Manager iteratively attempts to select an address and then checks for potential collisions.

Before selecting a potential address, the Logical Domains Manager first looks to see if it has a recently freed, automatically assigned address saved in a database for this purpose (see Freed MAC Addresses). If so, the Logical Domains Manager selects its candidate address from the database.

Manual Dynamic Mac Address

If no recently freed addresses are available, the MAC address is randomly selected from the 256K range of addresses set aside for this purpose. The MAC address is selected randomly to lessen the chance of a duplicate MAC address being selected as a candidate.

The address selected is then checked against other Logical Domains Managers on other systems to prevent duplicate MAC addresses from actually being assigned. The algorithm employed is described in Duplicate MAC Address Detection. If the address is already assigned, the Logical Domains Manager iterates, choosing another address, and again checking for collisions. This continues until a MAC address is found that is not already allocated, or a time limit of 30 seconds has elapsed. Rom emulators for mac. If the time limit is reached, then the creation of the device fails, and an error message similar to the following is shown.

Manual Dynamic Mac Address App


Duplicate MAC Address Detection

To prevent the same MAC address from being allocated to different devices, one Logical Domains Manager checks with other Logical Domains Managers on other systems by sending a multicast message over the control domain's default network interface, including the address that the Logical Domain Manager wants to assign to the device. The Logical Domains Manger attempting to assign the MAC address waits for one second for a response back. If a different device on another LDoms-enabled system has already been assigned that MAC address, the Logical Domains Manager on that system sends back a response containing the MAC address in question. If the requesting Logical Domains Manager receives a response, it knows the chosen MAC address has already been allocated, chooses another, and iterates.

By default, these multicast messages are sent only to other managers on the same subnet; the default time-to-live (TTL) is 1. The TTL can be configured using the Service Management Facilities (SMF) property ldmd/hops.

Each Logical Domains Manager is responsible for:

Manual Dynamic Mac Address Labels

  • Listening for multicast messages

  • Keeping track of MAC addresses assigned to its domains

  • Looking for duplicates

  • Responding so that duplicates do not occur

If the Logical Domains Manager on a system is shut down for any reason, duplicate MAC addresses could occur while the Logical Domains Manager is down.

Automatic MAC allocation occurs at the time the logical domain or network device is created and persists until the device or the logical domain is removed.

Freed MAC Addresses

Manual Dynamic Mac Address Generator

When a logical domain or a device associated with an automatic MAC address is removed, that MAC address is saved in a database of recently freed MAC addresses for possible later use on that system. These MAC addresses are saved to prevent the exhaustion of Internet Protocol (IP) addresses from a Dynamic Host Configuration Protocol (DHCP) server. When DHCP servers allocate IP addresses, they do so for a period of time (the lease time). The lease duration is often configured to be quite long, generally hours or days. If network devices are created and removed at a high rate without the Logical Domains Manager reusing automatically allocated MAC addresses, the number of MAC addresses allocated could soon overwhelm a typically configured DHCP server.

When a Logical Domains Manager is requested to automatically obtain a MAC address for a logical domain or network device, it first looks to the freed MAC address database to see if there is a previously assigned MAC address it can reuse. If there is a MAC address available from this database, the duplicate MAC address detection algorithm is run. If the MAC address had not been assigned to someone else since it was previously freed, it will be reused and removed from the database. If a collision is detected, the address is simply removed from the database. The Logical Domains Manager then either tries the next address in the database or if none is available, randomly picks a new MAC address.

In this lab I used a Cisco Catalyst WS-C3560G-24TS switch [IOS Version 12.2(40)SE]. Using Cisco Port Security it is possible to associate a static MAC address to a physical port on a switch. This only allows one host with that specific MAC address to connect physically to the specified port. The interface configuration command you would use to accomplish this and shutdown the port if the rule is violated is:

– switchport port-security mac-address [host_mac_address]
– switchport port-security violation shutdown

However, if you are attempting to assign static MACs to many ports, this can quickly become tedious and a dynamic approach would be more appropriate. The more efficient way to accomplish the task would be to apply the below commands using the “range” command option on all interested interfaces. This will permanently associate the first MAC address learned on the port to that port. If another host attempts to connect to the port after the association is made, the port will be shutdown. In the example below I demonstrate with GigabitEthernet ports 1 – 24.

Manual Dynamic Mac Address Software

Cisco Port Security with Dynamic MAC Address Learning