ip community-list in BGP

[cisc0.ameer]

hello all

can please clarify for me little bit confused with :

ip community-list 85 permit 10:20
!
route-map RMAP permit 10
match community 85

------------------------

that community list tells labeled or TAG prefixes with community 10:20

what does exactly mean ?

does it mean when we take a output from "show ip bgp <ip add> command "

in the output we have to see

20 10

in the AS_PATH_SEQ

so it means anywhere we want to use (mostly match in route-map)

it says from AS 20 and 10 should be comes or passing from exact these 2 ASs

my question is 10:20   what is that 2 numbers ? are they AS numbers?

[Hector Gustavo Serrano Gutierrez]

Hi @cisc0.ameer,

route-map RMAP should match on any route received via BGP including a Standard Community value of 10:20 when configured as inbound route-map facing a BGP peer under the BGP configuration process.

The concept of Communities in BGP routes is analogous to the concept of TAGs in EIGRP, OSPF and even RIP routes.

In other words, you do not set TAGs on BGP routes, you set a Community or Communities (more than one) to BGP routes.

Standard Communities are attached as an additional Path Attribute in BGP UPDATES packets, the latter are the BGP packets used for the advertising of routes to BGP neighbors.

Yes, you should be able to see the community in the BGP route you received using show ip bgp <ip add>.

Hope this helps.

Cheers.

[cisc0.ameer]

so many thanks for your cool and beautiful answer

Yes do not set TAGs route in BGP

but number should indicates a meaning ? correct? e.g 20:10 means AS 20 and AS 10

i think i still in a confusing mode I AM :D

in this Link : Link

switch(config)# ip community-list standard test1 permit 65534:40 65412:60 no-export

In the above example, a standard community list is configured that permits routes from the following:

• Network 40 in autonomous system 65534 and from network 60 in autonomous system 65412.
• Peers in the same autonomous system or from subautonomous system peers in the same confederation.

:( Network 40 ...what is Network 40 ? how 40 comes and play in this place !!!!!

and in this Link : Link 2

under

How to Use BGP Named Community Lists

router bgp 45000

neighbor 192.168.3.2 route-map 2000 in

match community 1

set weight 30
exit

ip community-list 1 permit 100
and in the description of this command is come :

• In the first example, community list 1 permits routes with a COMMUNITIES attribute of 100. Router E routes all have a COMMUNITIES attribute of 100, so their weight will be set to 30.

still i am not understanding !!!match community 1 
and later ip community-list 1 permit 100 ....confused me ...

[Giuseppe Larosa · Accepted solution]

Hello cisc0.ameer,

As Hector has explained the BGP communities are a way to classify BGP prefixes.

They are an administrative tool that can be used to signal that a group of BGP prefixes should be treated in the same way.

BGP communities are not related to the AS path attribute, but the dotted notation usually have the first two octets that are the same of the BGP AS number where the BGP community is set. The second number after : simply is a way to indicate a group of routes.

>> but number should indicates a meaning ? correct? e.g 20:10 means AS 20 and AS 10

No, as I explained above a BGP community is a 32 bit integer number that with the BGP community new format can be represented as two 16 bits integers separated by the : character.

Usually with the 16 bit AS numbers (the original size of BGP AS number before introduction of 4 bytes AS numbers in BGP) the left field is set equal to the ASN of the router that is setting the BGP community, the right field is an administrative choice.

To make an example if an ISP customer wants to signal two different types of user routes it can use two different values like

64520:100

and

64520:201

Routes tagged with BGP community 64520:201 should be given a better treatment in terms of routing in the upstream service provider network.

The upstream provider can configure BGP routing policies based on BGP community and not on access-lists or prefix-lists.

This means that if in the future the customer wants to modify for example to add a route with BGP community 64520:201 it has to send it with the BGP community attached.

The upstream ISP does not need to change the configuration of its router. It simply processes the new update based on the BGP community settings.

So BGP communities provide a logical  layer to create configurations that are more easily managed without the need to refer to IP addresses.

Hope to help

Giuseppe

[cisc0.ameer]

thanks sir

i close to realizing it :)

regard