Inverse ARP should be disabled before bringing up the Frame Relay interface

Last Updated on Wed, 28 Jun 2023 | CCIE

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Cisco CCIE Prep v1.0—Module 2-19

Once the interface is brought up with the no shutdown command, the Frame Relay switch will use Local Management Interface (LMI) to communicate the Data-Link Connection Identifier (DLCI) information to the router. Once the DLCIs have attained an active state, meaning that both sides of the connection are up and the Frame Relay switch has the correct Frame Relay route statements, Inverse Address Resolution Protocol (ARP) is performed to map the remote Layer 3 addresses to local DLCIs. Inverse ARP entries are noted in the Frame Relay address mapping table with the keyword dynamic. You can view this table by entering the show frame-relay map command.

Inverse ARP works very well in a full mesh Frame Relay topology. However, Inverse ARP has many shortcomings. Inverse ARP will not provide a complete mapping solution in a partial mesh (hub and spoke) topology. Also, Inverse ARP will resolve the Internet Protocol (IP) address of the next-hop router's physical interface even if this IP address is not part of the same IP subnet. This can cause problems in the CCIE Lab and in the real world.

It is recommended that Inverse ARP be disabled on all of your Frame Relay routers in the CCIE Lab. It is also recommended that you disable Inverse ARP before actually bringing up your Frame Relay interfaces with the no shutdown command. If Inverse ARP is not disabled before bringing up the interface, you will have to manually clear the Inverse ARP mappings using the clear frame-relay-inarp command once your static mappings are in place. Some versions of the Internetwork Operating System (IOS) will actually require a reload to clear the Inverse ARP entries out of the Frame Relay address mapping table, even after entering this command.

Table 2-2:

Step

Command

Description

Step 1:

no frame-relay inverse-arp

Disables the sending of Inverse ARP requests on the physical interface.

Step 2:

no arp frame-relay

Prevents the router from responding to Inverse ARP requests on an interface.

Step 3:

no shutdown

Brings up the physical interface.

Continue reading here: Use subinterfaces to avoid split horizon issues

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Readers' Questions

  • Raine
    Why using inverse arp before interface no shutdown?
    1 year ago
    • Inverse address resolution protocol (Inverse ARP) is a protocol used by some routed protocols to dynamically map a Layer 3 (network layer) address to a Layer 2 (data link layer) address. It is used when two routers have no knowledge of each other, but need to communicate. It is useful when configuring Frame Relay, ATM or ISDN networks, because it allows routers to obtain the data link layer address of the remote router without manual intervention. Therefore, using inverse ARP before interface no shutdown helps to configure Frame Relay and other networks without any manual intervention.
    • Veijo
      When should inverse arp be disabled?
      1 year ago
      • Inverse ARP should be disabled if it is not required for the particular application or protocols being used in the environment. It can also be disabled if it poses a security risk or if it is causing problems within the network.