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Getting ARP tables in the Resource Manager deployment model

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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 in this article.

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 the following information for both the primary and secondary interfaces for each peering types:

  1. Mapping of on-premises router interface ip address to the MAC address
  2. Mapping of ExpressRoute router interface ip address to the MAC address
  3. Age of the mapping

ARP tables can help validate layer 2 configuration and troubleshooting basic layer 2 connectivity issues.

Example ARP table:

Age InterfaceProperty IpAddress  MacAddress    
--- ----------------- ---------  ----------    
 10 On-Prem           10.0.0.1   ffff.eeee.dddd
  0 Microsoft         10.0.0.2   aaaa.bbbb.cccc

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 the following information is true before you progress further:

  • A valid ExpressRoute circuit configured with at least one peering. The circuit must be fully configured with the connectivity provider. You or your connectivity provider must have configured at least Azure private, Azure public, or Microsoft peering on this circuit.
  • IP address ranges used to configure the peerings. Review the ip address assignment examples in the ExpressRoute routing requirements page to understand how ip addresses get mapped to interfaces. 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, refresh the circuit configuration using the refresh button in 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.

Note

Depending on the hardware platform, the ARP results may vary and only display the On-premises interface.

ARP tables for Azure private peering

The following cmdlet provides the ARP tables for Azure private peering

# Required Variables
$RG = "<Your Resource Group Name Here>"
$Name = "<Your ExpressRoute Circuit Name Here>"

# ARP table for Azure private peering - Primary path
Get-AzExpressRouteCircuitARPTable -ResourceGroupName $RG -ExpressRouteCircuitName $Name -PeeringType AzurePrivatePeering -DevicePath Primary

# ARP table for Azure private peering - Secondary path
Get-AzExpressRouteCircuitARPTable -ResourceGroupName $RG -ExpressRouteCircuitName $Name -PeeringType AzurePrivatePeering -DevicePath Secondary

Sample output for one of the paths:

Age InterfaceProperty IpAddress  MacAddress    
--- ----------------- ---------  ----------    
 10 On-Prem           10.0.0.1   ffff.eeee.dddd
  0 Microsoft         10.0.0.2   aaaa.bbbb.cccc

ARP tables for Microsoft peering

The following cmdlet provides the ARP tables for Microsoft peering

# Required Variables
$RG = "<Your Resource Group Name Here>"
$Name = "<Your ExpressRoute Circuit Name Here>"

# ARP table for Microsoft peering - Primary path
Get-AzExpressRouteCircuitARPTable -ResourceGroupName $RG -ExpressRouteCircuitName $Name -PeeringType MicrosoftPeering -DevicePath Primary

# ARP table for Microsoft peering - Secondary path
Get-AzExpressRouteCircuitARPTable -ResourceGroupName $RG -ExpressRouteCircuitName $Name -PeeringType MicrosoftPeering -DevicePath Secondary

Sample output for one of the paths:

Age InterfaceProperty IpAddress  MacAddress    
--- ----------------- ---------  ----------    
 10 On-Prem           20.33.0.1   ffff.eeee.dddd
  0 Microsoft         20.33.0.2   aaaa.bbbb.cccc

How to use this information

The ARP table of a peering can be used to determine and validate layer 2 configuration and connectivity. This section provides an overview of how ARP tables look under different scenarios.

ARP table when a circuit is in operational state (expected state)

  • The ARP table has an entry for the on-premises side with a valid IP address and MAC address. The same can be seen for the Microsoft side.
  • The last octet of the on-premises ip address is an odd number.
  • The last octet of the Microsoft ip address is an even number.
  • The same MAC address appears on the Microsoft side for all three peerings (primary / secondary).
Age InterfaceProperty IpAddress  MacAddress    
--- ----------------- ---------  ----------    
 10 On-Prem           20.33.0.1   ffff.eeee.dddd
  0 Microsoft         20.33.0.2   aaaa.bbbb.cccc

or

Age InterfaceProperty IpAddress  MacAddress    
--- ----------------- ---------  ----------    
 10 On-Prem           20.33.0.1   ffff.eeee.dddd

ARP table when on-premises / connectivity provider side has problems

If a problem with the on-premises or connectivity provider occurs, the ARP table shows one of two things. You see the on-premises MAC address show incomplete or only see the Microsoft entry in the ARP table.

Age InterfaceProperty IpAddress  MacAddress    
--- ----------------- ---------  ----------   
  0 On-Prem           20.33.0.1   Incomplete
  0 Microsoft         20.33.0.2   aaaa.bbbb.cccc

or

Age InterfaceProperty IpAddress  MacAddress    
--- ----------------- ---------  ----------    
  0 Microsoft         20.33.0.2   aaaa.bbbb.cccc

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:

  1. 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.
  2. 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 don't 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're still experiencing issues.