IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

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IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

Proactive Address Resolution Protocol and Neighbor Discovery

Updated: August 12, 2026

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Explains how proactive ARP and ND preemptively resolve incomplete Layer 2 next-hop information for CEF routes, reducing first-packet software forwarding and helping prevent resolution failures and traffic drops.

Proactive Address Resolution Protocol (ARP) and Neighbor Discovery (ND) are CEF adjacency-resolution capabilities that

  • detect routes with incomplete Layer 2 next-hop information

  • start ARP or ND resolution before the first packet requires the adjacency, and

  • retry resolution every 15 seconds until the next-hop information is complete.

Incomplete next-hop information

When Cisco Express Forwarding (CEF) installs a route without Layer 2 adjacency information, it creates an incomplete Layer 3 next hop and programs the entry in hardware.

Without proactive resolution, the first packet uses the software forwarding path. Feature-specific information in the Layer 2 header can prevent the software path from removing the header completely, which can cause ARP or ND resolution to fail and traffic to be dropped.

How proactive ARP and ND work

Without proactive resolution, a packet with feature-specific Layer 2 information can reach the software forwarding path before CEF has complete adjacency information.

Summary

The process involves these components:

  • CEF: Detects the incomplete next hop and programs it in hardware.

  • Address Resolution Protocol (ARP) or Neighbor Discovery (ND): Resolves the missing Layer 2 adjacency.

Workflow

These stages describe proactive address resolution:

  1. CEF installs a route without complete Layer 2 adjacency information.

  2. CEF proactively starts ARP or ND resolution instead of waiting for the first packet.

  3. CEF retries resolution every 15 seconds until it resolves the next hop.

Result

CEF obtains the adjacency information before traffic depends on the incomplete next hop.

Enable proactive ARP and ND

Enable CEF to start ARP or ND resolution when a static route has incomplete next-hop information.

Procedure

  1. Enable proactive ARP and ND resolution in global configuration mode.

    Example:

    Router# configure
Router(config)# cef proactive-arp-nd enable
Router(config)# commit

  2. Display the running configuration.

    Example:

    Router# show running-config
cef proactive-arp-nd enable
end