BGP Communities Explained

A BGP community is bit of “extra information” that you can add to one of more prefixes which is advertised to BGP neighbors. This extra information can be used for things like traffic engineering or dynamic routing policies. There are 4 well known BGP communities that you can use or you can pick a numeric value that you can use for your own policies.

Here are the 4 well known BGP communities:

  • Internet: advertise the prefix to all BGP neighbors.
  • No-Advertise: don’t advertise the prefix to any BGP neighbors.
  • No-Export: don’t advertise the prefix to any eBGP neighbors.
  • Local-AS: don’t advertise the prefix outside of the sub-AS (this one is used for BGP confederations).

Once you finish reading this lesson, click on one of the links above to learn more about these well known BGP communities. I explained each of them in a separate lesson.

Why do we call them communities? A community is a group of prefixes that should be treated the same way. For example maybe you have 100 prefixes that require the same local preference or weight. You could match all prefixes using an access-list or prefix-list but using BGP communities is more convenient.

Instead of manually selecting the prefixes, an ISP could instruct its customers to tag prefixes with a certain BGP community. When the customer does this, their prefixes get a certain treatment.

To give you an idea, here are some examples that I found from Level 3 (large ISP in the US):

--------------------------------------------------------
customer traffic engineering communities - Prepending
--------------------------------------------------------
65001:0   - prepend once  to all peers
65001:XXX - prepend once  at peerings to AS XXX
65002:0   - prepend twice to all peers
65002:XXX - prepend twice at peerings to AS XXX
65003:0   - prepend 3x to all peers
65003:XXX - prepend 3x    at peerings to AS XXX
65004:0   - prepend 4x to all peers
65004:XXX - prepend 4x    at peerings to AS XXX
--------------------------------------------------------
customer traffic engineering communities - Regional
--------------------------------------------------------
Will only work for regional peers
64980:0   - announce to customers but not to EU peers
64981:0   - prepend once  to all EU peers
64982:0   - prepend twice to all EU peers
64983:0   - prepend 3x    to all EU peers
64984:0   - prepend 4x    to all EU peers
--------------------------------------------------------
customer traffic engineering communities - LocalPref
--------------------------------------------------------
3356:70   - set local preference to 70
3356:80   - set local preference to 80
3356:90   - set local preference to 90

This list might not be up-to-date anymore but it gives you an impression of how BGP communities are used. If a customer of Level 3 tags their prefixes with 3356:90 then they will set the local preference to 90. If you tag them with 64983:0 then they will prepend the AS number three times to all their BGP neighbors in Europe.

These BGP communities are 32-bit values that are divided in two sections. For labs you can pick whatever values you like but normally the first 16 bits are used to indicate the AS number that originates the community, the next 16 bits are assigned by the AS. For example, Level 3 uses these communities:

--------------------------------------------------------
customer traffic engineering communities - LocalPref
--------------------------------------------------------
3356:70   - set local preference to 70
3356:80   - set local preference to 80
3356:90   - set local preference to 90

The first 16 bits is their AS number (3356) and the next 16 bits (70, 80 and 90) corresponds with the local preference value. On their routers they configured a policy that sets the local preference to these values if they receive prefixes with these BGP communities.

Nowadays we also use extended communities which are 8 octets. These are used often for MPLS VPN which we will discuss in another lesson. Let’s take a look at a configuration example so you can see how to implement BGP communities.

Configuration

For this example I will use the following topology:

BGP Communities AS Path Prepend Example

On the left side we have a customer router that is connected to ISP1. This ISP is connected to ISP2 and ISP3. Let’s imagine that ISP2 is somewhere in Europe and that ISP1 has a policy that they will prepend their AS number four times to BGP neighbors in Europe whenever a customer adds BGP community value 64984:0 to their prefixes.

Let’s see how we can configure this on the ISP1 and customer router.

BGP Configuration

Here is the BGP configuration, it’s straight-forward eBGP:

Customer#show running-config | section bgp
router bgp 10
 no synchronization
 bgp log-neighbor-changes
 network 10.10.10.10 mask 255.255.255.255
 neighbor 192.168.10.1 remote-as 1
 no auto-summary
ISP1#show running-config | section bgp
router bgp 1
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.168.10.10 remote-as 10
 neighbor 192.168.12.2 remote-as 2
 neighbor 192.168.13.3 remote-as 3
 no auto-summary
ISP2#show running-config | section bgp
router bgp 2
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.168.12.1 remote-as 1
 no auto-summary
ISP3#show running-config | section bgp
router bgp 3
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.168.13.1 remote-as 1
 no auto-summary

Let’s see if ISP1 has learned any prefixes from the customer router:

ISP1#show ip bgp 10.10.10.10
BGP routing table entry for 10.10.10.10/32, version 2
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
  Advertised to update-groups:
        1
  10
    192.168.10.10 from 192.168.10.10 (192.168.10.10)
      Origin IGP, metric 0, localpref 100, valid, external, best

ISP1 has learned the network on the loopback interface of the customer router. Right now we don’t have any BGP communities. Let’s start with the configuration of ISP1…

ISP1 AS Path Prepend Configuration

First we will create a community list that matches the community- value:

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Forum Replies

  1. Hi Hamood,

    Like Mauro explained below, AS path prepending is used to make the path less preferable. BGP uses AS path length in its selection for the best path.

    The community values are defined by an ISP, there are no fixed values or anything. Basically it’s just a “tag”, if you tag your prefixes with a certain value then the ISP will do something with it…prepend it’s AS path, set the local preference, etc.

    Rene

  2. Hi Rene,

    Now I have a problem that makes me want to pull my hairs out. We’re a global company that has 5 different branches in 3 different countries. The current situation is every branch has its own private AS# and we use eBGP to connect all the branches. We have all the devices run OSPF and IBGP inside of each branch. As I said before, we have 5 different branches in 3 different countries, which means actually 3 branches we’ll need to use the same ISP. In order to help with the understanding, please refer to the diagram attached. let’s see 65521 R3 is running

    ... Continue reading in our forum

  3. Hello,

    Is the prefix list defined on the Customer router in the example correct? I had to change it to 10.10.10.0/24 to get it to work in my testing.

    Thanks

    Mark

  4. Hi Rene,

    very nice introduction to BGP communities. However, I’ve a little improvement: A hint that communities were only forwarded from second router to a third one if send-community is activated on that neighborhood were be helpful.

    Br,
    Sebastian

  5. Hello Staut

    You make a good point, and this is an opportunity for clarification.

    The Internet community is actually a Cisco defined community. It is kind of like a “catch all” that defines “normal” behaviour in the event that no other communities are defined. So in a sense, it is the “default” community that allows all prefixes to be advertised. Where the other three say “don’t advertise such and such” the Internet community says “advertise everything!”

    I hope this has been helpful!

    Laz

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