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Maxium recommended AS-Path Prepend

Is there a recommended maximum that I can prepend my public IP address adverts, to the WAN.

The reason for me asking is I have three ISP's.

ISPA: no prepend

ISPB: As-path prepend x x x x

ISPC: As-path prepend x x x x x x x x

Is prepending 8 times frowned on?

The design goal is any internet traffic comes in via ISPA, unless it is offline, then ISPB , would be used final fall back is ISPC.

The issue is how many prepends to add when I don't know how many AS-paths will be involved between the Internet client and ISP A B C to make sure that ISP A is chosen first.

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Hall of Fame Super Silver

Re: Maxium recommended AS-Path Prepend

Hello Martin,

AS path prepending is an attempt to influence return traffic on the internet but results are not guaranteed.

Routing policies and peering arragments between service providers may cause some traffic to return on the less preferred ISPs even when ISP A  link is operational.

Consider that there are BGP criteria that are checked before AS path length like local preference.

You can use the proposed AS path prepending schema, but you have be ready for real world behaviour

Hope to help


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