What is BGP community additive?

BGP communities are extra attributes you can attach to an IP route carried by BGP. Important: The set community command erases existing communities attached to a route and replaces them with the new set of communities unless you specify the additive option.

What is 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.

What is BGP community list?

A BGP community list is used to create groups of communities to use in a match clause of a route map. A community list can be used to control which routes are accepted, preferred, distributed, or advertised, for example. You can also use a community list to set, append, or modify the communities of a route.

Why is BGP a community?

BGP communities provide additional capability for tagging routes and for modifying BGP routing policy on upstream and downstream routers. BGP communities can be appended, removed, or modified selectively on each attribute as the route travels from router to router.

What is BGP standard community?

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.

What is BGP community value?

The BGP community attribute is a numerical value that can be assigned to a specific prefix and advertised to other neighbors. When the neighbor receives the prefix it will examine the community value and take proper action whether it is filtering or modifying other attributes.

What is BGP community string?

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 I get rid of BGP community?

Step-by-Step Procedure

  1. Configure the interfaces. [edit interfaces]
  2. Configure BGP. Apply the import policy to the BGP peering session with Device R2.
  3. Configure the routing policy that deletes communities.
  4. Configure the autonomous system (AS) number and the router ID.

Why we use no synchronization in BGP?

BGP synchronization is an old rule from the days where we didn’t run IBGP on all routers within a transit AS. In short, BGP will not advertise something that it learns from an IBGP neighbor to an EBGP neighbor if the prefix can’t be validated in its IGP.

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

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

What is the value of no advertise community?

The value of No-Advertise Community is FFFF:FF02. This Community prevents advertising the routes to ouside the Local Autonomous System (AS) or other Confederation peers. It is similar to No export except one point. With this Well Known community, you can advertise the routes only to the routers in the same BGP Confederation.

What’s the difference between a community and a local Pref?

The idea is that customers with specific policies that require the modification of LOCAL_PREF in the provider network set the corresponding community values in their routing updates. A community is a group of prefixes that share some common property and can be configured with the BGP community attribute.

Can a prefix have more than one community attribute?

A prefix can have more than one community attribute. A BGP speaker that sees multiple community attributes in a prefix can act based on one, some or all the attributes. A router has the option to add or modify a community attribute before the router passes the attribute on to other peers.

How is BGP community attribute assigned to prefixes?

BGP Community Attributes are assigned to specific prefixes and advertised to the neighbors. Neighbor which receives this advertisements recognize this value and behave according to this Community Attribute. (Advertise again or not). With BGP Community Attribute, we can apply special policies to special customers.