ARP - Address Resolution Protocol

Enhanced Definition

Address Resolution Protocol (ARP) is a communication protocol used by the TCP/IP stack on z/OS to map an Internet Protocol (IP) address to a physical hardware address, known as a Media Access Control (MAC) address, on a local area network (LAN) segment. This mapping is essential for the z/OS system to send IP packets to the correct network interface controller (NIC) or Open Systems Adapter (OSA) port on the target device.

Key Characteristics

- Operates at the Data Link Layer (Layer 2) and Network Layer (Layer 3) of the OSI model, bridging the gap between logical IP addresses and physical MAC addresses.
- Uses ARP requests, which are broadcast messages sent to all devices on the local network segment, asking for the MAC address associated with a specific IP address.
- Responding devices send an ARP reply (unicast) containing their MAC address if their IP address matches the request.
- The z/OS TCP/IP stack maintains an ARP cache to store recently resolved IP-to-MAC mappings, reducing the need for repeated ARP requests.
- Crucial for the functioning of OSA (Open Systems Adapter) cards, which are the physical network interfaces connecting z/OS LPARs to Ethernet networks.
- Can be dynamic (learned automatically) or static (manually configured) for specific entries, though dynamic is far more common.

Use Cases

- Initializing TCP/IP Communication: When a z/OS application or service needs to send an IP packet to another device on the same LAN segment for the first time, ARP is used to discover the target device's MAC address.
- Gateway Resolution: Resolving the MAC address of the default router or gateway on the local subnet, allowing the z/OS system to send packets to destinations outside its immediate LAN.
- OSA Express Connectivity: Enabling an OSA Express adapter to correctly forward Ethernet frames by knowing the destination MAC address corresponding to an IP address.
- Troubleshooting Network Issues: Analyzing ARP cache entries or monitoring ARP traffic can help diagnose network connectivity problems on the mainframe, such as duplicate IP addresses or incorrect MAC mappings.

Related Concepts

ARP is an integral component of the z/OS TCP/IP stack, working beneath the IP layer to facilitate local network communication. It is fundamental for OSA Express adapters, as these physical interfaces rely on ARP to correctly encapsulate IP packets into Ethernet frames with the appropriate destination MAC address. ARP bridges the gap between IP addressing (logical addressing) and MAC addressing (physical addressing), making it possible for IP packets to traverse the physical network. While ARP handles local address resolution, it works in conjunction with IP routing to determine the next hop for packets destined for remote networks.

Best Practices:

Monitor ARP Cache: Regularly review the ARP cache on your z/OS system (e.g., using NETSTAT ARP commands) to identify any suspicious or incorrect entries that might indicate ARP poisoning or misconfigurations.
Ensure Network Configuration Accuracy: Verify that IP addresses, subnet masks, and default gateway settings are correctly configured for your OSA interfaces and the z/OS TCP/IP stack to prevent ARP resolution failures.
Understand ARP Behavior for Troubleshooting: When diagnosing network connectivity issues, remember that ARP resolution must succeed before IP communication can occur. Check for ARP requests/replies and cache entries.
Consider Static ARP for Critical Devices (Rarely): For highly stable, critical connections to specific devices on the same segment, a static ARP entry can be configured to bypass dynamic resolution, though this adds administrative overhead and is generally not recommended for most scenarios.
Clear Stale ARP Entries: If a device's MAC address changes or a network issue causes incorrect entries, clearing the ARP cache (e.g., via V TCPIP,,P ARP,DELETE,IPADDR=... or recycling the interface) can resolve communication problems.