Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k
New Member

BGP dual-homing - can I advertise the same /24 with two different ASNs?

I currently have an Internet link to AT&T. I am leasing a Class C from AT&T (I'll call it A.B.C.0/24), receiving full Internet routes, and I am advertising their Class C using their ASN, yyyy.

I want to dual home this with a lower speed Verizon link. VZ said they will advertise the AT&T Class C and I will receive full Internet routes. The question I have is which ASN I will use to advertise A.B.C.0/24 to VZ.

To complicate matters, I have another pair of Internet links that I have dedicated for my company's eCommerce where I am dual homed to AT&T and Verizon and I have 4 Class Cs (X.Y.Z.0/22) and a ASN qqqqq from ARINs. These 4 Class Cs are different than the Class C above.

The issue is that VZ wants to advertise the single A.B.C.0/24 Class C using the ASN qqqqq associated with X.Y.Z.0/22 as opposed to the AT&T ASN yyyy.

Is this legal/possible and not violate Internet protocol?

Thanks.

Everyone's tags (4)
1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: BGP dual-homing - can I advertise the same /24 with two diff

Hello Kbyrd,

that would need a three parties agreement between you, ATT and VZ.

In other words if you present the prefix with an ATT AS VZ should consider you a peer not a client ...

I don't know how big a customer you are for ATT, but usually these very big providers don't like other big ones to do these kind of things, also because it messes up their complex routing policies.

Hope to help

Giuseppe

4 REPLIES
Hall of Fame Super Silver

Re: BGP dual-homing - can I advertise the same /24 with two diff

Hello Kbyrd,

if the A.B.C.0/24 belongs to ATT the yyy AS is probably a private AS  number  ( in range 64512-65535 for 16 bits AS numbers).

ATT probably overrides AS yyy with one of their public  AS numbers.

You can check this by yourself using looking glasses a good site for them is

http://www.traceroute.org

Verizon should not originate the A.B.C.0/24 with one of their AS numbers. Asking to you to originate prefix A.B.C.0/24 with your own public AS number just moves the problem to you as you have noted.

You should find out what AS number is used by ATT to publish the prefix in the internet and also to check if it exists as a /24: if ATT uses a less specific aggregate over the internet the risk is that new link would become the primary link for return traffic (most specific route wins).

The only clean ways are:

you get a fifth public /24 and you advertise it to both ATT and VZ with your own AS number

OR

you get a second public address block from Verizon and you make NAT part of the game as explained here

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a0080091c8a.shtml

quite a camplex job but the document is actually complete and covers also DNS aspects

in a given time a public IP prefix should be originated by only one public AS number this is a consistency rule for the internet.

Hope to help

Giuseppe

New Member

Re: BGP dual-homing - can I advertise the same /24 with two diff

giuslar, thanks for your response.

I am advertising A.B.C.0/24 with a public AT&T ASN in the 2xxx range (not 7018).

Is there a reason why VZ wouldn't allow me to advertise it to them with the same AT&T ASN for consistency?

Hall of Fame Super Silver

Re: BGP dual-homing - can I advertise the same /24 with two diff

Hello Kbyrd,

that would need a three parties agreement between you, ATT and VZ.

In other words if you present the prefix with an ATT AS VZ should consider you a peer not a client ...

I don't know how big a customer you are for ATT, but usually these very big providers don't like other big ones to do these kind of things, also because it messes up their complex routing policies.

Hope to help

Giuseppe

New Member

Re: BGP dual-homing - can I advertise the same /24 with two diff

I'm on the phone with VZ tech now. They will give me one of their ASNs (not 701) to use. I guess I'm not that big of a customer!

One thing, there must be a "split-horizon" or route-loop prevention type of issue when I used my own AS (qqqqq) to advertise these AT&T routes to VZ. The problem was that I could not see my own /22 range (that is on a different Internet link) unless I used the VZ ASN instead.

Thanks for your assistance.

1677
Views
10
Helpful
4
Replies
CreatePlease to create content