Document local-pref of prefixes (#24347)

* Document local-pref of prefixes

Documents local-pref of prefixes even in case of overlap with more specific external route

Closes PCX-15583

* Correct misspelling

* Longer explanation

I provided a longer explanation on the warning

* Small correction

* Clarified which route would be selected

* Apply suggestion from @marciocloudflare

* Apply suggestion from @marciocloudflare

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Co-authored-by: marciocloudflare <[email protected]>

Author: ricardomacas

src/content/docs/magic-transit/how-to/advertise-prefixes.mdx

@@ -143,6 +143,11 @@ When you use AS prepending to migrate traffic away from Magic Transit, the typic
 BGP has different mechanisms to control route priorities which are set by the peered network, not by Cloudflare. As such, this is a best effort feature. Cloudflare cannot guarantee that peers will honor AS prepends on Cloudflare's transit and peering connections.
 :::
 
+:::caution
+For traffic originated from Cloudflare's services, Cloudflare's internal network will enforce local preference for traffic delivery to Magic Transit. This applies even if a more specific or shorter path route is available on the public Internet. You should withdraw the prefix from Cloudflare to avoid delivery of Cloudflare-sourced traffic over Magic Transit. This preference is local to our internal network and does not impact the route decision process of other networks.
+For example, if you have a CDN zone onboarded with a Magic Transit-protected origin that is part of a Cloudflare-advertised `/22` prefix, and you later opt to advertise a more specific and shorter `/24` prefix route directly from your network, Cloudflare's servers will continue to route proxied CDN traffic to your Magic Transit network, which will follow configured routes to your tunnel(s). This is specific to Cloudflare services: traffic from other sources will converge as expected by BGP to the direct route, because it is the most specific.
+:::
+
 ## BGP control with Cloudflare Route Reflectors
 
 Optionally, you can use BGP to control the advertisement status of your prefix — advertised or withdrawn — from Cloudflare's global network for on-demand deployment scenarios. BGP Control works by establishing BGP sessions to Cloudflare's globally distributed Route Reflectors, which will initiate propagation of your prefix advertisement across Cloudflare's global network. You can peer with Cloudflare's Route Reflectors via Internet or CNI. CNI peering is available through your account team.