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-->This article walks you through the steps to learn the ARP tables for your ExpressRoute circuit.
Important
This document is intended to help you diagnose and fix simple issues. It is not intended to be a replacement for Microsoft support. You must open a support ticket with Microsoft support if you are unable to solve the problem using the guidance described below.
The steps and examples in this article use Azure PowerShell Az modules. To install the Az modules locally on your computer, see Install Azure PowerShell. To learn more about the new Az module, see Introducing the new Azure PowerShell Az module. PowerShell cmdlets are updated frequently. If you are not running the latest version, the values specified in the instructions may fail. To find the installed versions of PowerShell on your system, use the Get-Module -ListAvailable Az cmdlet.
Address Resolution Protocol (ARP) and ARP tables
Address Resolution Protocol (ARP) is a layer 2 protocol defined in RFC 826. ARP is used to map the Ethernet address (MAC address) with an ip address.
The ARP table provides a mapping of the ipv4 address and MAC address for a particular peering. The ARP table for an ExpressRoute circuit peering provides the following information for each interface (primary and secondary)
- Mapping of on-premises router interface ip address to the MAC address
- Mapping of ExpressRoute router interface ip address to the MAC address
- Age of the mapping
ARP tables can help validate layer 2 configuration and troubleshooting basic layer 2 connectivity issues.
Example ARP table:
The following section provides information on how you can view the ARP tables seen by the ExpressRoute edge routers.
Prerequisites for learning ARP tables
Ensure that you have the following before you progress further
- A Valid ExpressRoute circuit configured with at least one peering. The circuit must be fully configured by the connectivity provider. You (or your connectivity provider) must have configured at least one of the peerings (Azure private, Azure public and Microsoft) on this circuit.
- IP address ranges used for configuring the peerings (Azure private, Azure public and Microsoft). Review the ip address assignment examples in the ExpressRoute routing requirements page to get an understanding of how ip addresses are mapped to interfaces on your side and on the ExpressRoute side. You can get information on the peering configuration by reviewing the ExpressRoute peering configuration page.
- Information from your networking team / connectivity provider on the MAC addresses of interfaces used with these IP addresses.
- You must have the latest PowerShell module for Azure (version 1.50 or newer).
Note
If layer 3 is provided by the service provider and the ARP tables are blank in the portal/output below, refresh the Circuit configuration using the refresh button on the portal. This operation will apply the right routing configuration on your circuit.
Getting the ARP tables for your ExpressRoute circuit
This section provides instructions on how you can view the ARP tables per peering using PowerShell. You or your connectivity provider must have configured the peering before progressing further. Each circuit has two paths (primary and secondary). You can check the ARP table for each path independently.
ARP tables for Azure private peering
The following cmdlet provides the ARP tables for Azure private peering
Sample output is shown below for one of the paths
Microsoft Mac Torrent
ARP tables for Azure public peering
The following cmdlet provides the ARP tables for Azure public peering
Sample output is shown below for one of the paths
ARP tables for Microsoft peering
The following cmdlet provides the ARP tables for Microsoft peering
Sample output is shown below for one of the paths
How to use this information
The ARP table of a peering can be used to determine validate layer 2 configuration and connectivity. This section provides an overview of how ARP tables will look under different scenarios.
ARP table when a circuit is in operational state (expected state)
The ARP table will have an entry for the on-premises side with a valid IP address and MAC address and a similar entry for the Microsoft side.
The last octet of the on-premises ip address will always be an odd number.
The last octet of the Microsoft ip address will always be an even number.
The same MAC address will appear on the Microsoft side for all 3 peerings (primary / secondary).
ARP table when on-premises / connectivity provider side has problems
If there are issues with the on-premises or connectivity provider you may see that either only one entry will appear in the ARP table or the on premises MAC address will show incomplete. This will show the mapping between the MAC address and IP address used in the Microsoft side.
or
Note
Open a support request with your connectivity provider to debug such issues.If the ARP table does not have IP addresses of the interfaces mapped to MAC addresses, review the following information:
- If the first IP address of the /30 subnet assigned for the link between the MSEE-PR and MSEE is used on the interface of MSEE-PR. Azure always uses the second IP address for MSEEs.
- Verify if the customer (C-Tag) and service (S-Tag) VLAN tags match both on MSEE-PR and MSEE pair.
ARP table when Microsoft side has problems
- You will not see an ARP table shown for a peering if there are issues on the Microsoft side.
- Open a support ticket with Microsoft support. Specify that you have an issue with layer 2 connectivity.
Next Steps
- Validate Layer 3 configurations for your ExpressRoute circuit
- Get route summary to determine the state of BGP sessions
- Get route table to determine which prefixes are advertised across ExpressRoute
- Validate data transfer by reviewing bytes in / out
- Open a support ticket with Microsoft support if you are still experiencing issues.
Applies To: Windows Server (Semi-Annual Channel), Windows Server 2016, Windows Server 2012 R2, Windows Server 2012
Displays and modifies entries in the Address Resolution Protocol (ARP) cache. The ARP cache contains one or more tables that are used to store IP addresses and their resolved Ethernet or Token Ring physical addresses. There is a separate table for each Ethernet or Token Ring network adapter installed on your computer. Used without parameters, arp displays help information.
Syntax
Parameters
| Parameter | Description |
|---|---|
/a [<Inetaddr>] [/n <ifaceaddr>] | Displays current arp cache tables for all interfaces. The /n parameter is case-sensitive. To display the arp cache entry for a specific IP address, use arp /a with the Inetaddr parameter, where Inetaddr is an IP address. If Inetaddr is not specified, the first applicable interface is used. To display the arp cache table for a specific interface, use the /nifaceaddr parameter in conjunction with the /a parameter where ifaceaddr is the IP address assigned to the interface. |
/g [<Inetaddr>] [/n <ifaceaddr>] | Identical to /a. |
[/d <Inetaddr> [<ifaceaddr>] | Deletes an entry with a specific IP address, where Inetaddr is the IP address. To delete an entry in a table for a specific interface, use the ifaceaddr parameter where ifaceaddr is the IP address assigned to the interface. To delete all entries, use the asterisk (*) wildcard character in place of Inetaddr. |
/s <Inetaddr> <Etheraddr> [<ifaceaddr>] | Adds a static entry to the arp cache that resolves the IP address Inetaddr to the physical address Etheraddr. To add a static arp cache entry to the table for a specific interface, use the ifaceaddr parameter where ifaceaddr is an IP address assigned to the interface. |
/? | Displays help at the command prompt. |
Remarks
The IP addresses for Inetaddr and ifaceaddr are expressed in dotted decimal notation.
The physical address for Etheraddr consists of six bytes expressed in hexadecimal notation and separated by hyphens (for example, 00-AA-00-4F-2A-9C).
Entries added with the /s parameter are static and do not time out of the arp cache. The entries are removed if the TCP/IP protocol is stopped and started. To create permanent static arp cache entries, place the appropriate arp commands in a batch file and use Scheduled Tasks to run the batch file at startup.
Examples
To display the arp cache tables for all interfaces, type:
To display the arp cache table for the interface that is assigned the IP address 10.0.0.99, type:
Microsoft Protocol Analyzer
To add a static arp cache entry that resolves the IP address 10.0.0.80 to the physical address 00-AA-00-4F-2A-9C, type: