Some doubts on large scale BGP/AS design and black hole routing risk

magicboiz at magicboiz at
Thu Mar 31 08:12:07 UTC 2016

Hi everybody!

as part of laboratory work at the university,  I'm working on a BGP 
design study, and I would like to post some questions regarding IP 
address space allocation and its impact on BGP which are breaking my mind :)

Let's suppose we have an ISP/AS with two POPs: PARIS and LONDON. These 
two POPs are connected with redundant leased lines. Each POP has a BGP 
router speaking eBGP to different ISP providers/upstreams and also, each 
POP run its own OSPF area/ISIS area. Something like this:

  <INET> ---eBGP---<LONDON POP-ospf area1>===redundant leased lines 
(ospf area0)===<PARIS POP- ospf area2>---eBGP---<INET>

Now, this AS/ISP gets one /22 prefix from it RIR (RIPE in this case), 
and starts to announce it to its upstreams in PARIS and LONDON at the 
same time.

My questions are:

1. What could happen in the case of total failure in the redundant 
leased lines? Black hole routing between POPs?

2. What are the best design methods to avoid this scenario?

    2.1: adding a third POP creating a triangle? What if a POP looses 
connection with the other two POPs at the same time? Another black hole?

    2.2: requesting another prefix and allocating 1:1 prefix:POP, so in 
the scenario each POP only would announce its prefix to the upstreams?

    2.3: other?

Thanks in advance!

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