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

["Min Hu (Connor)" <humin29@huawei.com>]

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
> .