We have two geographically distinct locations that currently both fall under
the same ASN.
At site 1 we have a particular set of ip networks (/20 and bigger) in use
only locally to this site
At site 2 we have a separate set of ip networks (/20 and bigger) in use only
locally to this site
Each site has at least one upstream internet connection advertising with
BGP.
There is also a (reliable) private link between to the two sites where our
routers at each site are all talking iBGP (as well as ospf). There is a
router subnet (/27) that spans the two sites.
We currently advertise all subnets out all upstream connections as if both
sites were only one and traffic routes between sites without issue via the
private link.
If the private link between the two sites fails, will BGP allow for us to
access the IP subnets at site 2 from site 1 via the internet given that both
sites are advertising under the same ASN?
Is this a case where having multiple ASNs makes sense to treat each site as
remote peers to each other?
Thanks,
Justin
Justin Krejci wrote:
If the private link between the two sites fails, will BGP allow for us to
access the IP subnets at site 2 from site 1 via the internet given that both
sites are advertising under the same ASN?
No, because your router at site 2 will not accept any prefix with its
own AS in the AS_PATH (which site 1 would be advertising from).
Is this a case where having multiple ASNs makes sense to treat each site as
remote peers to each other?
Unless someone else has any better advice (I'm sure they do), you will
need two separate public ASNs. Site 1 advertises it's space out of AS1,
and site 2 advertises it's space from AS2.
If you do that, it may be best if you have an eBGP session between the
two PoPs using med/pref to ensure the direct link is preferred if it is
up. (I've never had to do iBGP between two sites like this before, but I
do know that eBGP is preferred over iBGP).
Steve
Once upon a time, Steve Bertrand <steve@ibctech.ca> said:
Unless someone else has any better advice (I'm sure they do), you will
need two separate public ASNs. Site 1 advertises it's space out of AS1,
and site 2 advertises it's space from AS2.
I don't know that it's better advice, but another way to link the two
sites is via a tunnel (GRE or IPIP). Use the upstream IP on each router
as the local endpoint, and then run some routing protocol over the
tunnel.
Maybe. Especially if both sites are connected to the same ISP, you
can tweak some BGP knobs to allow your own ASN to appear in the AS
PATH N times where N > 1, and accept the routes anyway.
Depending on your security policies you may want to encrypt said tunnel also.
Other than that, it all depends on it all depends. For example - if you receive / or have a default route pointing to the ISP, then the fact you have the same AS and won't receive the other site's routes in BGP doesn't matter at all - you'll follow a default from site 1 to the ISP, and the ISP will have a route to site 2 and can pass the traffic in the right direction. If you don't mind your traffic being passed unencrypted over the Internet, that is. You'll obviously need to adapt your firewall policies to allow for that flow as well.
j.
Personally, I don't really like the tunnel idea... I've had to deal with
them for v6 connectivity, and they seem so 'ugly'.
My first thoughts were about de-aggregation, but since he's already
advertising different space out of each site, that became irrelevant.
I was just thinking that two AS numbers would be the cleanest, easiest
to maintain method for him to take.
Certainly tunnelling did go through my mind though to ensure
site-to-site peering over the Internet.
Steve
This is a good concept but if the ISP route is a Juniper then as I recall by default it looks ahead, sees the as-path routing loop if it were to send it to the other router, and doesn't send it. So while you might be able to configure it on the receiving router, if the sending router won't send it, you're SOL.
j.
Steve,
Agreed. I'm not suggesting that a tunnel is the ultimate best solution, but rather just pointing out that if you go with a tunnel, it's worth remembering that it's going unencrypted over a public network rather than site to site over a private link.
j.
True, the ISP in this case would have to cooperate 
If you're running Cisco with the right IOS it looks like you could use the
'neighbor x.x.x.x allowas-in' command to accept your own AS. Then you would
just have to set your local route origination so that the appropriate routes
were withdrawn when the backnet link goes down.
Mike
Michael K. Smith wrote:
Justin Krejci wrote:
If the private link between the two sites fails, will BGP allow for us to
access the IP subnets at site 2 from site 1 via the internet given that both
sites are advertising under the same ASN?
No, because your router at site 2 will not accept any prefix with its
own AS in the AS_PATH (which site 1 would be advertising from).
If you're running Cisco with the right IOS it looks like you could use the
'neighbor x.x.x.x allowas-in' command to accept your own AS. Then you would
just have to set your local route origination so that the appropriate routes
were withdrawn when the backnet link goes down.
I stand corrected.
I've read about this, but does anyone have operational experience with
it that they can share?
Even though we are a very small SP, I always feel that going against the
traditional grain when doing things like this may leave a trail of
undocumented, hard-to-troubleshoot issues in the future.
To rephrase the OP's question, would it be BCP to acquire a second ASN,
and without further de-aggregating, continue advertising each site's IP
space to the DFZ, but from dissimilar ASs as opposed to the same one?
Steve
Have you ever known an ISP to not co-operate when it comes to
requesting a BGP session?
yes. this problem is rampant with colonialist telcos in the poorer
countries.
randy
Randy Bush wrote:
Have you ever known an ISP to not co-operate when it comes to
requesting a BGP session?
yes. this problem is rampant with colonialist telcos in the poorer
countries.
Yeah, well, I don't live in a poorer country, and I deal with it here.
*cough*
Steve
Have you ever known an ISP to not co-operate when it comes to
requesting a BGP session?
yes. this problem is rampant with colonialist telcos in the poorer
countries.
Yeah, well, I don't live in a poorer country, and I deal with it here.
*cough*
you asked a question. you are not required to like the answer.
randy
Have you ever known an ISP to not co-operate when it comes to
requesting a BGP session?
yes. this problem is rampant with colonialist telcos in the poorer
countries.
Yeah, well, I don't live in a poorer country, and I deal with it here.
*cough*
you asked a question. you are not required to like the answer.
oh, and i belive there was a north american incident of this discussed
on this list in the last year. i am just too soaked to have the energy
to search. i think it was due to living in a sparsely served area, so
the isp could get away with <bleep>.
randy
To rephrase the OP's question, would it be BCP to acquire a
second ASN, and without further de-aggregating, continue
advertising each site's IP space to the DFZ, but from
dissimilar ASs as opposed to the same one?
This would definitely be the best approach. You're not introducing new IP
prefixes and you're not extending AS paths, so the net effect on the global
BGP routing is zero (OK, you might have to use the 4 byte AS number :).
Just make sure that both ISPs you connect to allow you to advertise
"transit" prefixes. If site A public link goes down, but the private link is
up, site B will advertise its own address space plus site A's address space
with an extra AS number in the AS path (and the upstream ISP might filter
that).
Ivan
http://www.ioshints.info/about
For a given interconnection between the upstream ISPs for the two site, once
the direct link goes down, the time required for site A to learn the new
route to site B and vice versa would be different with the different
proposed solutions, right?
Thanks and best regards
Hi all,
We actually have a very similar setup to what Justin asked about, with the exception that we advertise only some of our netblocks to one provider and the rest to the other. If one of the providers fails, we then advertise all netblocks through the provider which is still up. If the private link between our two locations fails, the two halves of our network communicate via the Internet.
From what Justin described, I would think he would be able to keep a single
ASN and configure his network so that if the private link goes down, the two newly disconnected halves of his network advertise only the netblocks they can still "see" (i.e. the ones on their half). As long as his internal network is set up with dynamic routing (iBGP / OSPF) the two halves should realize they have to get to the other half via the Internet.
In our case, we don't get full routing tables from our providers, just default routes. Perhaps in Justin's case something as simple as a floating static route via the Internet to the other half of the network would take care of any ASN weirdness. It doesn't sound like he really needs his border routers to speak BGP with each other while the private link is down. If he wanted to remove the BGP session entirely under these circumstances, he could do the iBGP peering between RFC 1918 addresses and thus force the iBGP session to go down if the private link fails.
Thanks,
Adam