Re: [dpdk-dev] Questions about vm2vm vhost-user/virtio-net test

[Min Hu (Connor)]

Hi, David, thanks for your reply,
 other question as below,

在 2021/10/1 2:25, David Christensen 写道:

On 9/30/21 2:07 AM, Min Hu (Connor) wrote:

Hi, all,
     I got the answer: testpmd fwd mode should be set "io", then ping
OK.

     IO fwd mode, it will not change packet MAC address info, and
ping OK
     MAC fwd mode, it will change packet MAC address info, like:
     "rte_ether_addr_copy(&peer_eth_addrs[fs->peer_addr],
                 &eth_hdr->d_addr);
     rte_ether_addr_copy(&ports[fs->tx_port].eth_addr,
                 &eth_hdr->s_addr)"
     Then, ping failed.

     So, everyone, I got one question: how could this happen?
     router change packet src MAC and dst MAC, but ping OK,
     testpmd vhost, treated as switch, also changes packert src MAC
     and dst MAC, but ping failed ?

Running testpmd in this configuration is more like a virtual wire, not a switch, though in both cases the SRC/DST MAC addresses aren't typically

Well, if SRC/DST MAC addresses modifed in some case, what will happen?
That is, A send ping request to B， while SRC/DST MAC addresses
modified，B can get the packet, but B cannot send ping reply.
This is the result of my test. Why?

modified by the wire/switch when the frame is in transit. Both endpoints reside in the same Ethernet broadcast domain and can talk to each other directly.

A router typically connects two different broadcast domains, which may or may not be using Ethernet, so the router usually changes the SRC/DST MAC address to work correctly as the frame transitions from one broadcast domain into another. See https://en.wikipedia.org/wiki/Broadcast_domain.

Dave
.