BGP Community String for Cogent Communication, Inc. AS174

   April 11, 2007  Leave a comment

Attention

This BGP Community string information might be outdated. Please contact Cogent Communication, Inc.  AS174 to get more recent one. This BGP communites is ONLY for the customer who has BGP with Cogent Communication, Inc.  AS174. ipBalance.com is not maintaining this BGP Community string.

Cogent will send the customer a complete BGP table without aggregating the internal Cogent routes. This is useful for customers who are multi-homed within the Cogent Network.

Local Preference

All customer routes announced to Cogent will have a local pref of 130. The customer can control the local preference for their announcements by using a BGP community string that is passed to Cogent in the BGP session. The following table lists the BGP community strings and the corresponding local preference that will be set when they are used.

BGP Community String Local Pref Effect
174:10 10
Set customer route local preference to 10
(below everything-least preferred)
174:70 70
Set customer route local preference to 70
(below peers)
174:120 120 Set customer route local preference to 120
(below customer default)
174:125 125 Set customer route local preference to 125
(below customer default)
174:135 135 Set customer route local preference to 135
(above customer default)
174:140 140

Set customer route local preference to 140
(above customer default)

 

 

AS Padding

Customers with public AS numbers may pad their AS path to control the desirability of their
connections. Customers with an assigned private AS number from Cogent may not pad their AS path. Cogent will ignore any padding of private AS numbers from customers.

 

No-Export

Customers can control the "no-export" settings of their routes with the following community
strings:

 

BGP Community String Effect
174:970 Do not send route to NA (North America) – not accepted in NA
174:971 Do not send route to NA peers.
174:975 Set local preference to 10 in NA.
174:980 Do not send route to EU (Europe) – not accepted in EU
174:981 Do not send route to EU peers.
174:985 Set local preference to 10 in EU.
174:990 Do not send route to BGP customers, or peers.
174:991 Do not send route to peers.

 

Peer Controls

Effective on ALL peer, non-granular.
BGP Community String Effect
174:3000 Do not announce.
174:3001 Prepend 174 1 time.
174:3002 Prepend 174 2 times.
174:3003 Prepend 174 3 times

 

Detailed Non-Transit Peer

Cogent will send its internal routes without aggregating them, as well as the routes of other Cogent BGP customers. This is recommended for customers who are multi-homed within the Cogent Network and who want to use their connection to only access the Cogent Network.

 

Other BGP Communities

Routes announced to customers by Cogent will have one of the following communities associated with them:

 

BGP Community String Description
174:21000 Route is learned from NA (North America) non-customer.
174:21001 Route is NA internal or customer route.
174:21100 Route is learned from EU (Europe) non-customer.
174:21101 Route is a EU internal or customer route.

 

 

Applying BGP Community string with sample configuration

 

1. Get the latest BGP community string from your ISP/upstream provider or check www.ShowipBGP.com web site.

2. Pick the best BGP community string for your traffic shaping plan (mainly incoming traffic).
Most of ISPs are providing community string with local preference and AS prepending
option. Cannot tell which one is better than the other. It will depend on your global traffic shaping plan.

3. Follow the below commands ( Cisco only )

The below Sample configuration will tag the 10.0.0.0/24 route with [ISP AS]:120 or [ISP AS]:3 and will not tag any other routes.

router#config t
router(config)#ip bgp-community new-format
router(config)#access-list 10 permit 10.0.0.0 0.0.0.255
router(config)#access-list 10 deny any

router(config)#route-map [to-ISP] permit 10
router(config-route-map)#match ip address 10
router(config-route-map)#set community [ISP AS]:120 <—- using Local Preference

or

router(config-route-map)#set community [ISP AS]:3 <——- using AS prepending
router(config-route-map)#route-map [to-ISP] permit 20
router(config-route-map)#exit

router(config)#router bgp [xxxx] <——————————- xxxx = customer’s ASN
router(config-router)#neighbor x.x.x.x send-community
router(config-router)#neighbor x.x.x.x route-map [to-ISP] out
router(config-router)#exit
router(config)#exit
router#copy running-config startup-config


4. And then, go to www.routeserver.org and pick one of route server on the map to see your announcement. If you are using AS prepending option, you will see your AS prepends on route servers. Sometime you might not see your route with particular ISP path.
In most of case it might not be any routing problem, just the route path was dropped at somewhere by BGP best path selection scheme. Try Oregon route server, if you can see your route. The Oregon route server is providing many possible and available paths between BGP speakers and neighbors.
If you don’t see your route on there? check other route servers and also check your
BGP configuration. You might need to contact your upstream provider to check what they are learning BGP route from you.

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