BGP Community String

Customers may set the following BGP communities on prefixes:

1290:27000 = Internal backup = paths to this prefix heard
on this session are de-preferred over ones heard
from another session with same customer.

1290:29000 = External backup = paths to this prefix heard
from customer are de-preferred over ones heard
from our peers or upstreams.

Prefixes tagged with these BGP communities will not be announced
to upstreams unnless they are only known via this one path.

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Applying BGP Community string with sample configuration

1. Get the latest BGP community string from your ISP/upstream provider or check new.CiscoNET.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

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.CiscoNET.com 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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