BGP Communities Explained

Lessons Discussion
85

Hi
Well know has 4 types and all clear

There is another type called private community and has 2 type
Standard and extended

I am talking about this one which you can use it to manipulate as path prepend
Which type this? Is it standard?

86

Hello Ali

I see, you’re talking about the two different types of BGP communities, the standard and extended communities. These are two different ways of representing communities. The standard BGP community is defined by a 32-bit number that can be included with a route within the BGP update. A community is actually an optional transitive attribute.

Now an extended BGP community is simply a redefining of the BGP header that allows a 64-bit number to represent the community rather than a 32-bit number. This simply gives more granularity to the configuration of various attributes. In particular, extended BGP communities provide structure for various classes of information and are very often used for VPN services. The extended communities format is explained in detail in RFC 4360.

Now this should not be confused with a Private BGP community. Private BGP communities follow the convention that the first 16-bits represent the AS of the community origination, and the second 16-bits represent a pattern defined by the originating AS. The private BGP community pattern could vary from organization to organization, do not need to be registered or published. That’s the meaning of the word “private” in the title.

You can find more info about all of these at the following links:

I hope this has been helpful!

Laz

87

@ReneMolenaar : Thanks for this amazing BGP community explanation which I have had used for troubleshooting. I have a question here.
Is there a possibility for any customer to present with their communities like : and ask to add this is within t he BGP communities.
I have had checked the community examples that you have shared with specific like Prepending/Regional/Local pref
but not able to wrap my head around the customer ASN number for specific customer.

88

Hello Rinat

From what I understand, you’re asking if you as a customer can create your own community and share it with the ISP? I’m not sure I understand your question. Can you share a little bit more and clarify so that we can help you further?

Thanks!

Laz

89

the link is broken BGP Community No Advertise

90

Hello Konstantinos

I think it’s OK now:

Thanks!

Laz

91

Hello!

From the examples Rene mentioned

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.

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.

So I understand what Communities are but the question is, when exactly would I want to use them in the real world? From the examples that Rene mentioned, couldn’t all of this be simply achieved by creating a route-map and changing the path attributes for our prefixes there ourselves? Therefore we can do all this traffic engineering ourselves.

Thank you in advance for your help.

David

92

Hello David

BGP communities allow us to group routes that share common properties. There are situations in which you’d prefer the use of communities over other mechanisms such as route maps.

Communities deliver more granularity than route maps and other attributes. You are able to group prefixes by simply adding the community value. Think of communities as labels that are attached to each prefix, labels that are contained within the BGP updates that can then be used to manipulate and modify the management of prefixes based on those labels. If you had to do that using route maps that identify specific prefixes, your route maps may become hundreds or even thousands of lines long.

Communities are more advantageous for large and complex networks since you can categorize routes based on policies, and allow administrators to apply consistent policies to a group of routes. It also delivers easier management of routing policies since you can tag and sort routes much more flexibly. Communities also have no impact on the route selection processes since they don’t influence the BGP best path selection algorithm. They are outside of the BGP attributes, which means you can affect route propagation without affecting how BGP routers choose the best path. And finally, communities offer better scalability, especially in larger networks.

The use of communities does require a higher level of planning and organization to make it useful and workable, so there is a bit more work that needs to be done beforehand, however, ultimately it does deliver more granularity and flexibility in the end.

I hope this has been helpful!

Laz

93

What exactly is an extended community in plain english?

94

Hello Cameron

Take a look at this NetworkLessons note on the topic with its related links. If you have further questions, feel free to let us know!

I hope this has been helpful!

Laz

95

I read in the Cisco CCNP text that the entire “Community” attribute in BGP is an optional transitive attribute, this is different from a question in the practice exam, just thought to mention that here since the answer referred me to this topic

96

Hello Amogu

Indeed, BGP communities are optional transitive attributes. I went through the exam questions but was unable to find the one that stated that it was different. Do you remember the specific question or they way it was worded? Let me know so I can make the correction.

Thanks for catching that and I look forward to hearing back from you…

Laz

97

Hi there. What happens if you have two ISP links via eBGP from your enterprise (both run iBGP internally) and you only set the community on one of the links to get to the enterprise network and it goes down. Does the other path continue to function correctly?

98

Hello Paul

Yes, assuming your second ISP eBGP connection is actively exchanging routes (and it is properly advertising your prefixes), the other path should continue to function correctly when the first link goes down. This should be the case even if you have set a specific community only on the first link. BGP’s best path selection process is designed to automatically prefer any available, valid route when the primary path is lost.

BGP failover does not inherently depend on a specific community being set on both links. As long as the second eBGP connection is advertising and receiving the necessary routes, and you haven’t inadvertently filtered or messed with the attributes in a way that would invalidate the path, traffic will shift to the remaining link as soon as the primary link goes down.

This is not to say that such a situation will not cause problems or unpredictable behavior. Depending upon what you want to achieve, your BGP community configurations should be consistent across both ISPs. Otherwise, when the primary link disappears, your second connection might not have the same policy applied, potentially leading to suboptimal or unexpected routing.

I hope this has been helpful!

Laz