Re: Why need single hop BFD? Does single Hop BFD requires to travese all possoble links beween the two neighbors?

[sadasgup <[e-mail]>]

>>Any physical media, like 802.3, SONET, DWDM wavelength all have physical
failure indication. Each neighbor can also use the physical failure indication
to declare the >>connectivity between two immediate neighbors, which is much
faster than a BFD session, isn¹t it? It also needs less processing on the
router/LSR, there won¹t be any >>proactive periodical sending BFD over the link
anymore.  
 
>>Can you explain (or add to the document) what is the reason for having single
hop BFD? 
>>Is single Hop BFD only for the Tunnel scenario?

Linda,

Have you considered scenarios where two routers are connected via a Ethernet
switch or equivalent? Something like this -

Router A -------L2 Switch-------Router B

If there is a problem on one side of the connectivity (e.g. Router A to L2
Switch), the switch will hide this failure from Router B. This will impact
the IGP convergence, since  Router B has no indicatation of failure at the
physical layer.

Regards,
Santanu



On 08/11/09 6:40 PM, "Linda Dunbar" <[e-mail]> wrote:

> Dave and David,
>  
> Forgive me for not aware of the history of the BFD development. Reading
> through the ³BFD for IPv4 and IPv6 (Single Hop)²
> (draft-ietf-bfd-v4v6-1hop-10.txt), I am not clear why single hop BFD is
> needed, especially for two immediately connected neighbors. There is Hello
> message between two immediate neighbors. If two immediate neighbors need
> logical layer to detect any failure between the two immediate neighbors, they
> can use the Hello message to achieve this purpose. Even though Hello message
> is from control Plane, it would be much less work for routers/LSRs to monitor
> the Hello messages than creating a new BFD session.
>  
> Any physical media, like 802.3, SONET, DWDM wavelength all have physical
> failure indication. Each neighbor can also use the physical failure indication
> to declare the connectivity between two immediate neighbors, which is much
> faster than a BFD session, isn¹t it? It also needs less processing on the
> router/LSR, there won¹t be any proactive periodical sending BFD over the link
> anymore.  
>  
> Can you explain (or add to the document) what is the reason for having single
> hop BFD? 
> Is single Hop BFD only for the Tunnel scenario?
>  
> In Section 2 (Application and Limitation), the last paragraph does indicate
> that the transmitted packets are immediately routed back towards the sender on
> the interface over which they where sent if BFD Echo function is used. But
> when Link Aggregation is used to bundle the multiple parallel links between
> two neighbors, how does the network layer enforce which link to send back the
> ³echo² message?
>  
> Even if BFD ECHO can be enforced to be sent back on the same interface port so
> that the individual link¹s failure can be detected, what can this fault do
> when this fault can¹t affect the connectivity between the two immediate
> neighbors in control plane¹s view?  All the links are bundled and two
> immediate neighbors are still connected?
>  
> Thank you very much,
>  
> Linda Dunbar
>  
> Advanced Technology Dept, Wireline Networks,
> Huawei Technologies, Inc.
>  
>