ARP announcements
因上架一台AscenLink 330 做 HA，發現 HA 生效換 MAC後，
D-Link DFL-210 竟然沒有刷新 ARP Cache，以致無法對外互動！
經過一番的查察，才找到如下文章！
結論：
AscenLink 330 的ARP announcements 機制不符慣例，
經調整DFL-210參數，放寬相關設限後，得以正常運作！
ARP may also be used as a simple announcement protocol. This is useful for updating other host's mapping of a hardware address when the sender's IP address or MAC address has changed. Such an announcement, also called a gratuitous ARP message, is usually broadcast as an ARP request containing the sender's protocol address (SPA) in the target field (TPA=SPA), with the target hardware address (THA) set to zero. An alternative is to broadcast an ARP reply with the sender's hardware and protocol addresses (SHA and SPA) duplicated in the target fields (TPA=SPA, THA=SHA).
An ARP announcement is not intended to solicit a reply; instead it updates any cached entries in the ARP tables of other hosts that receive the packet. The operation code may indicate a request or a reply because the ARP standard specifies that the opcode is only processed after the ARP table has been updated from the address fields.[4][5][6]
Many operating systems perform gratuitous ARP during startup. That helps to resolve problems which would otherwise occur if, for example, a network card was recently changed (changing the IP-address-to-MAC-address mapping) and other hosts still have the old mapping in their ARP caches.
Gratuitous ARP is also used by some interface drivers to effect load balancing for incoming traffic. In a team of network cards, it is used to announce a different MAC address within the team that should receive incoming packets.
ARP announcements can be used to defend link-local IP addresses in the Zeroconf protocol (RFC 3927), and for IP address takeover within high-availability clusters.
沒有留言:
張貼留言