Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

BGP Question - Advertising class C assigned from ISP 1 through ISP 2

To All,

I have a customer with a single router dual-homed to ISP 1 and ISP 2 (leaving the names of the ISP's out). The customer has their own BGP ASN number assigned by ARIN, but they do not have their own class C assigned from ARIN.

ISP 1 assigned a class C from their address space to the customer. We would like to use ISP 2 as the primary path (it has more bandwidth) and ISP 1 as the secondary path. As-Prepends have been setup accordingly to accomplish this task.

Customer can get to all of their sites across the internet through ISP 2 except one location. ISP 2 states that since the class C is allocated to ISP 1, they need to get their own Class C from ARIN.

Below is a url that links to a document from Sprint published in 2001. On page 8 it shows a scenario (Case 1 and Case 2), whereby this practice is acceptable. The class C from ISP 1 would be a "traffic magnet" through ISP 2, but this outcome is fine by us.

It's my belief that at one time, a class C (whether it's assigned from an ISP or ARIN), may be advertised from any one point to any other point across the internet.

Has this changed since 2001? (I know it's been 8 years.)

Any feedback would be greatly appreciated.


Hall of Fame Super Silver

Re: BGP Question - Advertising class C assigned from ISP 1 throu

Hello Brian,

I agree with ISP2 people:

after being multihomed your customer could get an ASN.

Now they should apply for getting their own IP address block.

The reason for this is that if the /24 is part of ISP1 address space a correct BGP advertisement about this prefix should be originated from ASN of ISP1.

from any point in the internet the path should be:

ASNk ASNj .... ASN_ISP1 ì|?

instead there is your customer ASN in the righmost position.

if someone performs consistency checks this advertisement fails.

There have been several improvements in policies like for example RPSL that enforces policies and can implement information taken from regional registries like ARIN

Internet is now so great that is not possible to allow this kind of things.

I mean the AS path can differ on the first ASN hops but everyone should agree on the final ASN (the originator ASN) and that ASN should be the legitimate owner of the prefix.

Hope to help


New Member

Re: BGP Question - Advertising class C assigned from ISP 1 throu


Thank you very much for your response. It is much appreciated.