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How do BGP communities work for customer routing preferences?

Question

How do BGP communities work for customer routing preferences?

 

Environment

  • Silverline DDoS: Routed
  • BGP

 

Answer

Customers may choose to send Silverline-approved BGP communities with their advertised prefixes in an effort to control how and where traffic is returned to them. These communities will adjust local preference on our routers and determine the "best path" in the customer route table for returning traffic down a GRE tunnel (or backbone to GRE tunnel).

  • Higher local preference is "more preferred", lower local preference is "less preferred".
  • Silverline's default local preference for customer advertisements is 180.
  • Backbone traffic (site-to-site within Silverline) is local preference 160.
  • Local preference is a non-transitive attribute (it is local only to that router).
  • BGP communities in this article are referring to affecting Silverline local preference used to route traffic to customer backend, and does not affect Silverline local preference set to our carriers for attracting client traffic.

Preference

   

Community

Assignment

Comment

no community

default

Default local preference is 180

no-export

no-export

Prevents routes from being adverised to eBGP neighbor

55002:0

no-advertise, within site

Prevents routes from being re-advertised to any peers; for testing

55002:70

backup route across sites*

Sets local preference 70

* backbone site-to-site default is 160

55002:75

de-pref route across sites

Sets local preference 150

55002:80

de-pref route within site

Sets local preference 170

55002:90

prefer route within site

Sets local preference 190

55002:165

backup route across sites

Sets local preference to 180 to prefer a route to a specific site or specific prefix /29 thru /32 

Must be used with another BGP community, such as 55002:80 or 55002:75

55002:6666

src-blackhole

source-based blackhole

55002:9999

dst-blackhole

destination-based blackhole

 

Example of Route-maps with BGP Communities:

route-map customer-primary permit 10
match ip address prefix pl-route-to-f5
set community 55002:90 additive

route-map customer-backup permit 10
match ip address prefix pl-route-to-f5
set community 55002:70 additive

Then, apply the 'customer-backup' route-map to the F5 BGP neighbor statements on the routers at your backup site and apply the ‘customer-primary’ route-map on the routers at your primary site.

 

To see communities in the ASN:MM format you need to set in the global config:

ip bgp-community new-format

 

It is important to mention that Silverline recommends to have <= 100 local preference on advertisements from your Internet Service Provider (ISP) to the internet if you want to always prefer Silverline over your ISP.

 

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