add ghost route protection

pages/01.home/bgp-communities/default.md

@@ -52,3 +52,24 @@ burble.dn42 implements [large BGP communities](http://largebgpcommunities.net/),
 | ( 4242422601 : 140 : _DN42 region_ )     | Route learned in this DN42 region | 
   
   
+### Ghost Route Protection
+
+In a large network like burble.dn42 it can take some time for route updates
+to be distributed and for the network to settle following changes. This can
+be particularly noticeable when routes are deleted as it can create a
+cascade of ghost updates, each with an increasing internal path length as
+the route removal spreads across the network. 
+
+To protect against ghost routes, a community is used to track how many internal
+hops a route has. In a fully meshed network like burble.dn42, a genuine route should
+never have more than two internal hops, so an export filter is used to prevent
+distribution of longer paths. 
+
+|Community  |Description|
+|:---|:---|
+| ( 4242422601 : 100 : 1 )     | Added on first internal re-distribution |
+| ( 4242422601 : 100 : 2 )     | Added on second re-distribution and prevents further re-distribution within burble.dn42 |
+ 
+
+The ( 4242422601 : 100 : x ) communities are not exported to peers.
+

pages/01.home/maintenance-log/default.md

@@ -15,6 +15,9 @@ A log of changes to the burble.dn42 network.
 uk-lon1 has been upgraded. If you are peered on this node, please update
 your IP address accordingly.
 
+burble.dn42 now includes some limited protection against ghost route updates.
+See the [communities](/home/bgp-communities) page for more details.
+
 #### 9th September 2020
 
 uk-lon1 will be upgraded this weekend (12/13th