Subject: BFD for IGPs

From: Chris Nogradi <[email-masked]>
Date: Wed, 9 Mar 2005 16:27:43 -0500

Dave,

With regards to the "BFD for IPv4 and IPv6 (Single Hop)" draft, I was 
wondering if you could clarify what is meant in the 1st paragraph of section 
7.3 by, "... but the protocols must share a common topology,..."?  It is not 
clear to me if you are referring to the routing protocol (IS-IS) advertising 
multiple data protocols (IPv4 or IPv6) sharing a common topology or if you 
are referring to the routing protocols (IS-IS and OSPF) sharing a common BFD 
session.  If you mean the former, are you saying that only one BFD session 
should be created for both data protocols? Or are you saying that a session 
would be created for each data protocol but a failure in either would cause 
both to be considered down?

Also in the the second paragraph of the same section, I am not sure what is 
meant by "... the failing data protocol SHOULD no longer be advertised in 
Hello packets in order to signal a lack of connectivity for the protocol."  
Are you saying that the router would no longer advertise supporting the 
failed data protocol in its Hellos?  Would this not affect other routers on a 
multiaccess network since we are still capable of supporting that data 
protocol?

Thanks,

Chris

---

From: Dave Katz <[email-masked]>
Date: Wed, 9 Mar 2005 16:17:19 -0700

On Mar 9, 2005, at 2:27 PM, Chris Nogradi wrote:

> session.

I was speaking of the data protocols sharing a routing topology.

There are some multiprotocol routing protocol variants that can 
calculate only a single topology for use by all data protocols.  In 
that case, if one data protocol fails, you are faced with either 
tearing down connectivity for all protocols (bad) or black-holing 
traffic for the failed protocol (worse.)  So in this case you tear down 
the IGP neighbor so that all data protocols reroute.

> If you mean the former, are you saying that only one BFD session

The latter, more specifically, the failure of one BFD session would 
cause the IGP to consider both data protocols to be down.  Remember 
that every data protocol gets its own BFD session, as BFD is testing 
the *data* path.  The point of these paragraphs is to define how those 
sessions interact with the control plane.

> multiaccess network since we are still capable of supporting that data

Other routing protocol variants can calculate different topologies for 
each protocol.  In this case the response to a failing BFD session is 
to stop advertising the data protocol in hellos, which will cause the 
topology of the failed protocol to be recalculated, and only that 
protocol reroutes.

In a multiaccess network, this will typically mean that the router in 
question will be bypassed for that particular data protocol, even if it 
still has working paths to other routers.  This is an inherent 
limitation of the way the routing protocols model multiaccess networks, 
namely, that full connectivity is guaranteed.  If full connectivity is 
not present, things fall apart in ugly ways.  Fixing this is outside of 
the scope of BFD.  ;-)

I will flesh all this out a little bit in the spec.

--Dave