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


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)

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