05-12-2008 07:38 AM - edited 03-03-2019 09:55 PM
Hello
My customer has multiple sites, each with an internet connection using a different ISP for each site.
They also have their own public IP address range and AS# that will be used.
- For discussion, let's assume the range they own is 100.100.0.0/16 and they own AS# 999
The would like to sub-divide the public IP range over multiple sites, but use the same AS# for each site.
Site 1 would advertise 100.100.0.0/27 via asn 999 to ISP "A"
Site 2 would advertise 100.100.8.0/27 via asn 999 to ISP "B"
Site 3 would advertise 100.100.16.0/27 via asn 999 to ISP "C"
In a failure case, Site 1 would advert Site2's subnets .. etc
No Site would ever advert the whole 100.00.0.0/16 class B
Is it 'legal' to use the same ASN like this?
Thanks
Solved! Go to Solution.
05-12-2008 07:52 AM
Wes,
It is certainly "legal" to have three independant sites using the same ASN to advertise different prefixes. Here are a few questions for you.
1. Is there any need for inter-site communication?
2. Are any of these sites dual-homed?
3. In a case of failure, are you planning on the subnet for the other site to be announced automatically. It would probably be easier just to advertise the specific /27 from each site and then the full /16 (or an aggregate covering the 3 sites) as well from the 3 sites, which would provide full redundancy in case of failure.
Regards,
05-20-2008 09:13 AM
Wes,
You do not necessarily need to run iBGP in this scenario. Each router can run standalone and make its own routing decisions.
iBGP would only be required if the 3 routers needed to have a certain level of cohesion in making their routing decisions.
Regards,
05-12-2008 07:52 AM
Wes,
It is certainly "legal" to have three independant sites using the same ASN to advertise different prefixes. Here are a few questions for you.
1. Is there any need for inter-site communication?
2. Are any of these sites dual-homed?
3. In a case of failure, are you planning on the subnet for the other site to be announced automatically. It would probably be easier just to advertise the specific /27 from each site and then the full /16 (or an aggregate covering the 3 sites) as well from the 3 sites, which would provide full redundancy in case of failure.
Regards,
05-12-2008 09:12 AM
Hi Harold
Thanks for answering.
1. Is there any need for inter-site communication?
###>>> Not via the Internet ... There is an internal/nat'd path for this if required.
2. Are any of these sites dual-homed?
###>> Each site has one ISP connection.
###>> Internet redundancy via the internal network.
3. In a case of failure, are you planning on the subnet for the other site to be anounced automatically.
### > yes .. that is the plan. Playing with both methods. Probably will use an aggregate as you described.
Again, thanks for the help !
05-12-2008 12:54 PM
Most ISP's wont allow prefixes lower than /24
Get it verified by the ISP as the /27 may get rejected by other upstream ISPs
Narayan
05-13-2008 12:07 PM
Arg... Those subnets were supposed to read /22 not /27.
Thanks!
05-20-2008 08:31 AM
I have a 2nd question on this.
In the case above the 3 sites are all part of the same ASN.
In this case, do all the routers in this ASN still need to be I-BGP peers ?
05-20-2008 08:52 AM
05-20-2008 09:13 AM
Wes,
You do not necessarily need to run iBGP in this scenario. Each router can run standalone and make its own routing decisions.
iBGP would only be required if the 3 routers needed to have a certain level of cohesion in making their routing decisions.
Regards,
05-20-2008 09:33 AM
Thanks Harold
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