| ARP ACD/ICMPv6 NUD* | SBC SWe Cloud Platform | Standby ports only | Performs active checking of two-way traffic through at least the local Ethernet interface, the cable, and the adjacent layer 2 switching function. Checks are accomplished using ARP (for IPv4) or Neighbor Discovery (for IPv6) mechanisms to probe an arbitrary, operator-specified target IP address on a local IP subnet, typically an address of a router (Gateway IP address). Depending on the address family (IPv4/IPv6) of the gateway IP address configured, either ARP ACD or ICMPv6 NUD probing messages are sent in such a way that explicit assignments of IP addresses to the standby ports are not required. Checks the link state between port and the adjacent router. |
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* Address Resolution Protocol - Address Conflict Detection / Internet Control Message Protocol Version 6 – Neighbor Unreachability Detection
ARP ACD/ICMPv6 NUD Methods for Standby Ports
IPv4 ARP ACD
If the destination address configured is an IPv4 address, then IPv4 probing is initiated by sending an ARP Probe requests and listening for the responses.
ARP request probes are sent with:
- Sender IP address of 0.0.0.0. The use of 0.0.0.0 is compatible with rfc 5227 on IPv4 Address Conflict Detection. This is convenient to use on standby ports since IP addresses are not assigned for standby ports.
- Sender hardware address containing the current local MAC address assigned to the sending port.
- Target IP address containing the configured target IP address to be probed.
- Target hardware address containing all zeros. The ARP request is sent on the LAN using L2 broadcast.
The target can be expected to respond with an ARP Response using L2 unicast.
IPv6 ICMPv6 NUD
If the destination address configured is an IPv6 address, then IPv6 probing would be initiated using Neighbor Unreachability Detection mechanism (RFC 4861 section 7). This is based on Neighbor Solicitation and Neighbor Advertisement ICMPv6 messages.
Because these are IP packets, the
needs IP addresses to send/receive them. The uses auto-generated link local IPv6 address from the current local MAC address.Neighbor Solicitation messages are sent with:
- IP source address containing auto-generated link local IP address
- IP destination address containing configured target IP address
- ICMP layer target address containing configured target IP address
- ICMP layer source link layer address containing PKT port MAC address
The Neighbor Solicitation message is sent on the LAN via L2 unicast to the system with the target IP address.
The target can be expected to respond with a Neighbor Advertisement using L2 unicast. Received messages are validated per RFC 4861 section 7.1.2: Check that the S bit = 1 (solicited) and that the target address = our configured target IP address.