BGP and anycast

Mike Schoenecker MSchoenecker at yipes.com
Tue Jan 30 05:35:16 UTC 2001


If I understand this correctly you are trying to advertise one /24 out to 2
separate providers on the internet.
If this is the case you will need to make sure that the 2 providers in this
scenario are the same and will allow you to advertise smaller subnets of
this block.  If you advertise the same block out of 2 separate regions BGP
will not know where to send traffic.  BGP will select the most specific and
route to this destination.  If there are 2 similar advertisements there will
be routing anomalies. If you are trying to connect the sites together [ one
subnet ] across the internet, the best way to do this is to establish a VPN
between sites and advertise the entire /24 out of one region and share the
subnet between regions over the VPN.  The points of the VPN will need to be
of public address space that is either advertised or routed to you from your
provider.  Netscreen has a solution for this.  This will enable you to
receive traffic destined to your network at one location and forward the
necessary traffic across the internet to your other region over the VPN.  I
have found it very difficult to get anyone to listen to advertisements less
than a /24 this is why I suggest that the carrier between regions be the
same it would be easier to get them to satisfy this request. I thought of
the use of IBGP but you will still experience the same issues of
reachability i.e the transit carrier would need to advertise no less than
the /24.

Hope this helps
PS. get Internetwork Routing Architectures by Cisco it is the best book on
BGP.

-----Original Message-----
From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu]On Behalf Of
Swede
Sent: Monday, January 29, 2001 5:18 PM
To: nanog at merit.edu
Subject: BGP and anycast



How does one announce the same net (with the same
origin AS) from different places on the Internet? Or
should the "anycast" networks be announced from
different origin AS:es?
(Can't find "anycast" setup in my BGP for Dummies<tm>)

----------------------------------------
AS12345
IGP
(announces net 1.2.3/24 among others)
Connected to several major networks (P, Q, W, Z)
----------------------------------------
Isolated* site 1 (one unique routable net and
1.2.3/24)
Router connected to a major network X, announced as
AS12345
----------------------------------------
Isolated* site 2 (one unique routable net and
1.2.3/24)
Router connected to a major network Y, announced as
AS12345
----------------------------------------
* Isolated - No contact to main AS via IGP, tunnels or
telepathy

So when communicating among the sites (doing zone
transfers etc) I use the unique routable network...
piece of ca...
...but won't my BGP routers at the different locations
be a bit puzzled when they see the announcements from
another AS12345 for my unique networks (and more so
for the anycast)?
The config above does seem to break the concept of an
AS.

Feel free to bash my Yahoo mail if this post is
utterly stupid or seems way out of scope
/Swede - still among the clueless  aka Anders Plym,
presently without *real* mail access


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