BGP - Same ASN, but different subnets

Answered Question
May 12th, 2008
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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

Correct Answer by Harold Ritter about 9 years 1 month ago

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,

Correct Answer by Harold Ritter about 9 years 1 month ago

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,

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Correct Answer
Harold Ritter Mon, 05/12/2008 - 07:52
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  • Cisco Employee,

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,

Wes Smith Mon, 05/12/2008 - 09:12
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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 !



royalblues Mon, 05/12/2008 - 12:54
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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

Wes Smith Tue, 05/13/2008 - 12:07
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Arg... Those subnets were supposed to read /22 not /27.

Thanks!

Wes Smith Tue, 05/20/2008 - 08:31
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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 ?

Correct Answer
Harold Ritter Tue, 05/20/2008 - 09:13
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  • Cisco Employee,

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,

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