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

[Linda Dunbar <[e-mail]>]

Dave and David,

 

Forgive me for not aware of the history of the BFD development. Reading
through the "BFD for IPv4 and IPv6 (Single Hop)"
(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, isn't it? It also needs less
processing on the router/LSR, there won't 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 "echo" message?

 

Even if BFD ECHO can be enforced to be sent back on the same interface port
so that the individual link's failure can be detected, what can this fault
do when this fault can't affect the connectivity between the two immediate
neighbors in control plane's 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.