[dpdk-dev] max_rx_pkt_len

[Balázs Lugossy]

Hi DPDK developers,

We have an external DPDK KNI interface in our product, and we experienced that ethernet payload of size 1500 cannot be received, only 1496. It is important that the traffic is VLAN tagged. We have encountered this report: https://dev.dpdk.narkive.com/xGGOQC8R/dpdk-dev-issue-with-mtu-max-rx-pkt-len-handling-by-different-nics-pmd-drivers, which pretty much similar to our case. We started to experiment with max_rx_pkt_len, but no matter what we set it to it won't work with 1500 payload only 1496. Then we checked rte_eth_dev_configure and realized that max_rx_pkt_len is defaulted to ETHER_MAX_LEN in case max_rx_pkt_len is set to > ETHER_MAX_LEN. ETHER_MAX_LEN is 1518 and thus doesn't contain enough bytes for possible vlan header. As a workaround we have enabled JUMBO frames and set max_rx_pkt_len to 2048. This way traffic is fine. But I want to know what should be the official way to support 1500 payload in VLAN tagged frames with standard frame size?

Regards,
Balazs