Route Map question with BGP

[benolyndav]

Hi

I am confused here what does the set commuity command do please, we call the same acl in the route map so am a bit confused.?

route-map RM_NCRS_TO_VM1 permit 10

match ip address ACL_NCRS_TO_VM_01

set community 28

!

route-map RM_NCRS_TO_VM2 permit 10

match ip address ACL_NCRS_TO_VM_01

set community 29

Thanks

[balaji.bandi]

depends on the config and what you looking to replace with community here

Community Attribute

The community attribute is a transitive, optional attribute in the range of 0 to 4,294,967,200. The community attribute is a way to group destinations in a certain community and apply routing decisions according to those communities. The routing decisions are accept, prefer, and redistribute, among others.

more explaination can be find here :

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.html

[MHM Cisco World]

set community add attribute to IPv4 or VPNv4 prefix which we will use it later to match it and permit or deny or modify the prefix.

[paul driver]

Hello
Its a bgp attribute that can be used to mark specific or all traffic which can then be advertised to other bgp peers who then can match upon this advertised community marking and traffic engineer (filter/readvertise etc) with it.

In your OP route-map is it matching on an access-list called ACL_NCRS_TO_VM_01 and  ACL_NCRS_TO_VM_02 then setting a community value of 28/29 respectively against those access-lists.

[benolyndav]

Hi Paul

We are calling the same acl with both route-maps which is why I'm confused ??

[paul driver]

Hello

Yes you are, i missed that...then it must be a typo
If you call the same acl on different route-maps then it would be applicable if but this isnt the case.

[benolyndav]

Hello
We only have the one acl and as mentioned both route-maps are calling the same acl, but when I look at the routing table BGP is sending traffic across both links and I have tested with traces etc, how are certain prefixes advertised across the different linkks
is this something our ISP is doing.??

P.s its the same ISP for both links
Thanks

[paul driver]

Hello

---

@benolyndav wrote:

 how are certain prefixes advertised across the different linkks
is this something our ISP is doing.??

---

We do not know this as you have not posted how the rout-map is being used, however i assume its egress towards your ebgp peers?
As to what the receiving ebgp peer is doing with the prefixes that have an attach community I also cannot tell you.

But the route-map as it stands is calling the same acl but appending the two community’s and by default the community value 28 will be advertised

[MHM Cisco World]

Yes but in other router you will get different community for same prefix  and you can match it.

[benolyndav]

Hi

Can you explain please I'm not sure what you mean.?

Thanks

[MHM Cisco World]

draw the topolgy and I will explain which case I need it

[benolyndav]

Hi

I will send asap

Thanks

[benolyndav]

 

[MHM Cisco World]

as I mention before even if same prefix the Comm is differ and I run small Lab 
R1 advertise same prefix 1.1.1.1/32 but with 28 and 29 comm for each neighbor 
R4 receive same prefix but we need to control the Inbound to prefix 1.1.1.1 and hence the R4 can use Comm to modify the LP for prefix and select one path. 

so we can use Comm like we use AS-PATH prepend to control the Inbound traffic

[paul driver]

Hello @MHM Cisco World 
Can you elaborate as i am not sure on what you are trying to explain 

Pertaining to the OP the rm stanza has two clauses setting different community values but its calling the same acl as such based on rm logic(logical AND) a match will be made on clause 10 acl and the set community value 28 will be appended as such so would be the only value that should be advertised - community value 29 in clause 20 shouldn’t even be reached as that has the exact same acl .