We are using Anycast, and I plan to implement the "nearest access" by appending myself AS path to impact my peer BGP routers.

For example I have two POPs(they are same ip), Pop1 in London and POP2 in Japan. If I found all of the worlds' traffic go to London but rare traffic go to Japan, then I will append myself AS path longer to London ISP than Japan ISP.

But my question is that when I append my AS path length, how long the London ISP can get to know it and take effect. I know BGP convergence time may take a few minutes, but for my case it just peer to peer convergence, so I guess it should be shorter, right?

  • 1
    Rather than append your own AS to make a path longer (less preferred), you prepend your AS to the path.
    – Ron Maupin
    Feb 7, 2019 at 1:23
  • what's the difference between append and prepend? for example my as# is: 4402, I'd like to add two more for London ISP router, it would be 4402,4402,4402, so no difference between append or prepend. right?
    – Jack
    Feb 7, 2019 at 3:40
  • 1
    Append means you add it to the end, but prepend means you add it to the beginning. ASes are added to the beginning of the list. The correct BGP terminology is prepend.
    – Ron Maupin
    Feb 7, 2019 at 4:08

1 Answer 1


The answer, unfortunately, isn't really quantifiable without knowing a lot more about how one constituent route fails, where you're measuring from and how that given remote is connected.

First off - your question isn't really a function of being located in a particular country as much as the specific providers in use, their connections to one another and where you're testing from.

As an example - if both London and Japan have connections to the same carrier then there likely isn't a whole lot of reconvergence required beyond the AS boundary of that carrier and any delay is going to be due to the mechanism of the route disappearing (e.g. explicit retraction vs dead peer) and the subsequent propagation within the carrier's network (usually fairly quick).

If London and Japan are connected to two different carriers then convergence is going to be a function of the above (means of route removal) plus the particulars of how a given point sees the routes in question. This is going to vary quite a bit. Changes to the routing table may need to propagate through a single AS or through a half-dozen. Each AS could consist of two or three routers or a dozen.

TL;DR - It's hard to quantify this, as it will vary tremendously by how you're connected and from where you're measuring. Best-case is convergence in a few seconds, worst-case several minutes.

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