> -----邮件原件----- > 发件人: L3VPN [mailto:[email protected]] 代表 Joe Touch > 发送时间: 2013年12月4日 3:11 > 收件人: Pedro Roque Marques > 抄送: Thomas Morin; [email protected] > 主题: Re: on limitations of draft-xu-l3vpn-virtual-subnet > > > > On 12/3/2013 10:47 AM, Pedro Roque Marques wrote: > > Joe, > > > > On Dec 3, 2013, at 7:54 AM, Joe Touch <[email protected]> wrote: > > > >> > >> > >> On 12/3/2013 12:55 AM, Pedro Roque Marques wrote: > >>> Is the fundamental IP behavior that you are referring to the TTL decrement > between two addresses on the same "subnet" ? > >>> I believe you are wrong: to my knowledge proxy ARP is compliant with Host > and Router requirement RFCs. > >> > >> For IPv4, RFC1812 says that all 1's "limited broadcast" > >> (255.255.255.255) MUST NOT be forwarded. RFC2644 updates that RFC to > >> say that even subnet-directed broadcasts MUST NOT be forwarded (as > >> configured by default). > >> > >> So by default, a router would never forward the broadcast for which > >> the proxy ARP would be an appropriate response. > > > > yes. > > > >> Which would mean that the > >> proxy would work only if already populated at the router; > > > > I don't follow. Lets assume i've a device that is an IP compliant > > router which has interface A and interface B. > > The quote above means that it should not forward a packet address to > > 255.255.255.255 between link A and link B (or vice-versa). This has > > nothing no relationship to whether this device is capable of > > generating ARP requests or proxy ARP replies. > > ARP requests would never be forwarded across such a device. This means that > proxy ARP responses would either have to be manually configured or learned by > the actions of some other party. There's no opportunity to have the proxy ARP > be a short-cut for a relayed ARP - which limits the cases where proxy ARP can > be > used.
Hi Joe, It seems that you have mistakenly taken the ARP proxy as the ARP cache since ARP requests have never been expected to be forwarded across such a device in the ARP proxy case. Xiaohu > >> there would be > >> no means to forward the initial request if the cache were empty. So > >> proxy ARP would work if manually configured, but would be unreliable > >> if automatically populated if used on a router that spans two groups > >> of hosts on the same subnet. > > > > Proxy ARP is unrelated to subnet broadcasts. > > See the example above; not always. > > Joe
