[c-nsp] bgp PIC Edge & neighbor shutdown

[adamv0025 at netconsultings.com]

> Vladimir Troitskiy
> Sent: Friday, August 11, 2017 10:59 AM
> 
> Hi all,
> 
> BGP PIC Edge on an egress PE tracks PE-CE interface and switches to backup
> path immediately when link to the CE goes down. But what if I manually
> shutdown the PE-CE eBGP session? Does it trigger immediate CEF update on
> the egress PE (ASR9k)?
> --
Interesting question, to get the exact numbers for your setup I suggest you
hook it up in a loop with Ixia or Spirent to get the actual numbers. 
Let's do a little thought experiment to see what might be going on under the
hood. 
Let's assume you shut down the session on local (A)end. 
If BGP session is shut down, then BGP on local (A) end will immediately
withdraw all prefixes learned via that session, (BGP on remote (B) end will
terminate the session after receiving BGP Cease NOTIFICATION). 
 -now I think what you're really asking is whether this is done in a PIC way
(i.e. triggers artificial BGP-NH invalidation thus all prefixes at once) or
not (BGP withdraws prefix by prefix and notifies RIB for each), and my
answer is I don't know. 
But I think it actually doesn't matter because the data-plane is still
operational on both ends (remember the interface is still up and can serve
traffic that was not rerouted yet). 
Imagine that node A has the following FIB: 
10.2.0.0/24
...
100.2.0.0/24
...
200.2.0.0/24
All pointing to B and programed with backup path via A2. 

In time t0 BGP already managed to withdraw B's NH for 10.2.0.0/24 (cause BGP
always walks the table from top to bottom) so this prefix now points to A2. 
Also in time t0 A received a one packet destined to each of three prefixes
above.  
- Packet to destination in 10.2.0.0/24 will be using the alternate next-hop
(A's backup) that is A2. 
- Packets to destinations in 100.2.0.0/24 and 200.2.0.0/24 will still be
routed to B in time t0 - but that's ok since the link is operational so
packets will be delivered to B and B can route these packets to correct
destinations (cause it most likely learned routes for 100.2.0.0/24 and
200.2.0.0/24 via iBGP session -so these are unaffected by the A-B session
shut). 
So no loss in this direction. 

But in the opposite direction it depends on how B is set up. 
If B manages to withdraw route, for source addresses of the above packets,
as a result of BGP session to A going down and does not yet have any usable
alternative path via B2,  then the return traffic from B to A will be
dropped on B until the B's AS converges and traffic starts flowing through
B2. 

 

adam


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