Subject: (ION) Re: Checking NHRP cache information

From: "Yu Ning" <[email-masked]>
Date: Tue, 13 Jan 1998 10:45:00 +0800

>Yu Ning wrote:
>>
>> Hi friends,
>> I've got a question about NHRP.
>> In NHRP draft, page 6:
>>
>>    example). Station S first determines the next hop to station D
>>                        ^^^^^^^
>>    through normal routing processes (for a host, the next hop may simply
>>    be the default router; for routers, this is the "next hop" to the
>>    destination internetwork layer address).  If the destination's
>>    address resolution information is already available in S's cache then
>>    that information is used to forward the packet.  Otherwise, if the
>>    next hop is reachable through one of its NBMA interfaces, S
>>    constructs an NHRP Resolution Request packet (see Section 5.2.1)
>>    ....
>>
>> Why should S use normal routing processes to determines the next hop
>> to station D? I don't think the intermediate IP address is useful for
>> creating a cut-through NBMA link. Instead, I think S should check its
>> cache first to see if the dest IP address has already its NBMA address
>> in the cache. Then, if not hit, it should send NHRP req to preconfigured
>> NHS for resolution.
>>
>
>One has to look at the bigger picture, not just the NBMA subnetwork when
>reading this paragraph. S cannot assume that the station it wishes to
>reach is reached via the NBMA network. It may be connected to other
>subnetworks (e.g. Ethernet) and station D could be reachable via one of
>those subnetworks. Once it determines that it should use the NBMA
>network as the next hop, then it would check its cache before sending
>the NHRP Resolution Request.

That's also the first impression of my reading that paragraph, but further
reading
confused me. So I think that paragraph should be re-written like this:

"Station S should first verify that if it's NBMA direct-attached, if so, it
will further
check its local resolution cache for the NBMA address of the dest IP. If not
hit, it will initiate a NHRP resolution query to its preconfigured(default)
NHS.
The NHS will further prorogate the request till the end.
Otherwise(non-direct-
NBMA-attached), the forward the packet in normal routing path. "

Why the paragraph should be re-written like that is because: there is always
a routed path between S and D, and that path should be the last
consideration.
If making routing decision for the next hop(of network layer) first, NBMA
short-cut
will be neglected in the case of direct-NBMA-attached host. So whether
direct
or in-direct NBMA-attached should be decide first.The original paragraph may
not really so absurdity, but it lead to confusion.

What's more, can't we really do anything if the S is
non-direct-NBMA-attached?
Can S ROUTED(not using pre-configured PVC) the NHRP request to a NHS,
then the NHS is responsible for establishing a cut-through between itself
and
the dest NHS. So through the data path, only the src NHS-> dest NHS is cut-
through, while the S->src NHS, dest NHS->D is routed? It seems in the MPOA
1.0
model (which can be seen as a NHRP realization), the edge device can
create a cut-through for its sub-attached hosts. And the hosts are not
nessicarily
direct-NBMA-attached.
Thanks for your response.

Sincerely, Yu Ning
ATM R&D Center of BUPT
Beijing, P.R.C.
email: [email-masked]

>> That's just like what the ATM ARP dose, and the only diff is ARP relay
was
>> restricted within LIS, while NHRP req/replay can be send via routing
>> path to the des.
>>
>
>Once the host/router ATMARP client determines that the destination is
>reachable via the ATM network, then it checks the cache and if not
                               ^^^^^^^^^^^^^^^^
    Should be "the same LIS within the phyisical ATM network"

>there, sends the ARP request packet to the ATMARP server.

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