Document local-pref of prefixes

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

Closes PCX-15583

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, even if a **shorter path** or even a **more specific route** is available over the Internet. You should withdraw a prefix completely from Cloudflare to avoid delivery of Cloudflare traffic over Magic Transit. 
+For example, if you have a CDN zone onboarded with a Magic Transit-protected origin, the proxied CDN traffic from Cloudflare's servers will continue to be delivered to your Magic Transit network and follow its route to your configured tunnesl, even if a more specific or shorter route is publicly available on the internet. 
+:::
+
 ## 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 Optionally, you can use BGP to control the advertisement status of your prefix — advertised or withdrawn — from Cloudflare's global network for on-demand deployment scenarios.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.
@@ -251,4 +256,4 @@ neighbor 173.245.63.66 {
 neighbor 141.101.67.22 {
     description "CF RR#3 CDG";
 }
-```
+```