What are BGP community strings?

Posted on by Muna Meyer

What are BGP community strings?

BGP can include more information with each prefix, in the form of Community Strings. A community string is a number value that the peer uses like a tag. Communities create routing policies between peers. By tagging prefixes with communities, we tell the peer to handle the prefixes in a special way.

How do you send a community on BGP?

To allow community values to be sent to a specific neighbor the command neighbor x.x.x.x send-community must be applied. BGP has default 4 well known communities that can be used to mark prefixes; listed as follows: Internet: advertise these routes to all neighbors.

What formats can I use to configure the BGP community attribute?

What formats can I use to configure the BGP community attribute? A. In Cisco IOS® Software release 12.0 and later, you can configure communities in three different formats called decimal, hexadecimal, and AA:NN.

How do I set up a community list?

Filtering Traffic Using Community Lists

  1. configure terminal.
  2. router bgp autonomous-system-number.
  3. neighbor {ip-address | peer-group-name} remote-as autonomous-system-number.
  4. address-family ipv4 [unicast | multicast | vrf vrf-name]
  5. neighbor {ip-address | peer-group-name} route-map route-map-name {in | out}
  6. exit.
  7. exit.

What is BGP send-community both?

BGP has two community attributes: the standard BGP communities (defined in RFC 1997) and extended BGP communities (defined in RFC 4360). You can control which communities are sent to a BGP neighbor with the (very obvious) standard, extended or both options of the neighbor send-community router configuration command.

How do I set BGP attributes?

BGP Table path selection

  1. Prefer the highest local-preference value.
  2. Prefer the shortest AS-path length.
  3. Prefer the lowest origin value.
  4. Prefer the lowest MED value.
  5. Prefer routes learned from an EBGP peer over an IBGP peer.
  6. Prefer best exit from AS.
  7. For EBGP-received routes, prefer the current active route.

How do you add a prefix in BGP?

Enter a network address to which you want to apply the prefix rule. Enter the subnet mask of the network. If you want to define a prefix length parameter and use it as a match criteria for applying rules, enter an appropriate value for the optional LE and GE operators. The allowed range of values is 1–32.

What is BGP community Cisco?

BGP communities are an optional transitive BGP attribute that can traverse from autonomous system to autonomous system. A BGP community is a 32-bit number that can be included with a route.

What is set community additive?

When setting community values under a route-map (set extcommunity xxxx) with the “additive” keyword, this prevents the new entry to overwrite the existing entries.

How do BGP communities work?

A BGP community is an optional, transitive BGP attribute that is recognized and passed to other BGP peers. You might see a BGP community as a tag attached to the BGP routes exchanged between two BGP peers. To get started, let’s look at what a BGP community set to a route looks like in an Update message.