Comcast Backbone is AS7922
Local Preference
By default all received customer routes are assigned a local preference of 300 in the Comcast Backbone. Customers have the ability to alter this local preference using the following BGP communities:
| Community | Local Pref | Effect
|
| 7922:290 | 290 | Used for customer backup when multi-homed to Comcast
|
| 7922:250 | 250 | Sets local preference equal to peer routes
|
| 7922:150 | 150 | Sets local preference equal to transit routes
|
| 7922:100 | 100 | Lowest possible value. Used for backup when multi-homed to multiple providers
|
Route Suppression
The use of these communities allow a customer to prevent their prefixes from being advertised to specific peers. Only those ASes that have a peering relationship with Comcast are available for suppression. An AS that is a customer of Comcast is not enabled for route suppression. Given that this list changes on a regular basis the best way to verify what ASes are available for route suppression, a customer should look at the full Internet feed received from Comcast and look for routes tagged with 7922:3000. Any AS directly connected to AS7922 and is tagged with 7922:3000 is a candidate for route suppression.
| Community | Effect
|
| 7922:999 | Prefixes are not sent to anyone. They are contained entirely within AS7922 only
|
| 7922:888 | Prefixes are not sent to ALL peers. Prefixes still sent to customers
|
| 65100:XXX | Do not announce prefixes to AS XXX
|
| 65200:XXX | Announce to AS XXX if 7922:888 is also set
|
Route Prepending
The use of these communities allow a customer to AS Path Pad advertisements to all or a subset of Comcast Peers. As with route suppression, Only those ASes that have a peering relationship with Comcast are available for prepending.
| Community | Effect
|
| 65101:0 | Prepend x1 to ALL PEERS
|
| 65101:XXX | Prepend x1 to AS XXX
|
| 65102:0 | Prepend x2 to ALL PEERS
|
| 65102:XXX | Prepend x2 to AS XXX
|
| 65103:0 | Prepend x3 to ALL PEERS
|
| 65103:XXX | Prepend x3 to AS XXX
|
| 65104:0 | Prepend x4 to ALL PEERS
|
| 65104:XXX | Prepend x4 to AS XXX
|
Blackhole
The use of this community should only be done in an extreme circumstances. This allows a customer to force traffic destined to any prefixes tagged with 7922:666 to be blackholed ingress to the Comcast Backbone. In order to protect our infrastructure and the Internet from misuse of this, Comcast will only allow a customer to blackhole traffic to a specific /32 that is already permitted by the customers existing ingress prefix filter. This is typically used when a customer is under a DDOS attack, causing congestion on their links from Comcast.
| Community | Effect
|
| 7922:666 | Blackhole traffic (only allowed on /32 routes)
|
