W5500 VRRP

Question

Background: An H3C core switch is used with the VRRP function enabled. H3C switch IP address: 172.20.0.255; Physical MAC address: 10:b6:5e:aa:f0:de; Virtual MAC address: 00:00:5e:00:01:01 (only exists when VRRP is enabled) PC host IP address: 172.20.0.119; Physical MAC address: e8:cf:83:91:b7:b6 W5500 embedded device IP address: 172.20.20.20; Physical MAC address: aa:bb:e4:ae:45:dc Objective: The PC host sends ping packets to the W5500 embedded device through the H3C switch and expects a response. When the PC host sends a ping packet to the W5500 embedded device, the PC detects that the target is on a different network segment. Therefore, the destination MAC address at the Ethernet layer is set to the MAC address of the H3C gateway. Since VRRP is enabled, the final destination MAC address is the virtual MAC address of the H3C switch: 00:00:5e:00:01:0a. 2.After receiving the ping packet, the H3C switch modifies the source MAC address to its physical MAC address (10:b6:5e:aa:f0:de) and the destination MAC address to the MAC address of the W5500 embedded device (aa:bb:e4:ae:45:dc). It then decreases the TTL by 1 and forwards the data. 3. When the W5500 receives the ping packet, it needs to send a response. Thus, it sets its own physical MAC address (aa:bb:e4:ae:45:dc) as the source MAC address. However, an error occurs here: the destination MAC address is incorrectly set to the physical MAC address of the H3C switch (10:b6:5e:aa:f0:de). In reality, the destination MAC address should be the virtual MAC address of the H3C switch (00:00:5e:00:01:0a). Because after enabling VRRP, the H3C switch only recognizes data packets with the virtual MAC address 00:00:5e:00:01:0a, it will discard this response packet and not forward it. As a result, the PC host cannot receive the reply, leading to a ping timeout. 4. A similar issue occurs when the W5500 acts as a TCP server. 5. Strangely, the function works normally when the W5500 acts as a TCP client. That is, when the W5500 sends cross-network segment data, the destination MAC address is the virtual MAC address of the H3C switch.

TheoIm · Answer

Hello, Normal MAC Setting In a VRRP environment, the MAC address associated with the gateway IP (e.g., 172.20.0.255) must be the virtual MAC. Therefore, when the W5500 sends a reply to the PC, the destination MAC must be the VRRP virtual MAC (00:00:5e:00:01:0a). This ensures that the VRRP-enabled gateway accepts the packet and forwards it to the PC correctly. Current MAC Setting Issue When the H3C switch sends a packet to the W5500, it sets the source MAC to its physical MAC (10:b6:5e:aa:f0:de). The W5500 then incorrectly stores Gateway IP ↔ Physical MAC in its ARP table. As a result, when the W5500 replies, the destination MAC becomes the physical MAC (10:b6:5e:aa:f0:de). In a VRRP-enabled environment, the gateway only accepts packets addressed to the virtual MAC, so it discards the reply and never forwards it to the PC. Key Error Correct: W5500 → Destination MAC = 00:00:5e:00:01:0a (VRRP Virtual MAC) Current: W5500 → Destination MAC = 10:b6:5e:aa:f0:de (Gateway Physical MAC)