BGP Question

[benolyndav]

HI

If I have dual connections to the same ISP, and im my route maps I say set community 28 for one connection and set community 29 for the other connection, what is this telling the ISP side and how does it influence any routing desicions please .

Thanks

[MHM Cisco World]

set community is not have effect as bgp as path prepend in best path select 
i.e.
the community is not one of criteria that bgp use for best path selection 

But 
it use as attribute use by ISP or other site to modify LP, MED or AS-Path or other criteria. 
in simple words
R1 send prefix with community 
ISP use this community to modify the LP, MED or AS-Path 
OR 
R2 (other sites) use this community to modify the LP, MED or AS-Path

MHM

[benolyndav]

Hi

So are you saying that those 2 lines will be doing nothing  in the BGP process ??

[MHM Cisco World]

No friend 
R1-ISP-R2 (other sites)
you admin R1 and set community 
to see for which these community will effect we need to see the config of ISP or R2 
you need to see route-map 
match community list 
Set <<- set here is different it depend on how ISP or R2 is config it 

MHM

[benolyndav]

Hi

So what I am trying to understand from anyone is what doe ISP do their side if we set the community as i mentioned in original message, any ideas what they might do and how affects routing.??

[MHM Cisco World]

https://www.noction.com/knowledge-base/bgp-inbound-traffic-engineering

ISP use it mostly to control Inbound traffic to your router 

Check this link how ISP use bgp community and LP to control inbound traffic.

MHM

[benolyndav]

Nice Doc 

Thanks

[MHM Cisco World]

Thanks 

Hope you get idea now.

There are many way we can control inbound traffic one of them is using community.

But you need to understand that community not use in best path select but it use with route-map to set attribute use for best path selection

Goodluck 

Have a nice summer 

MHM

[Joseph W. Doherty]

"There are many way we can control inbound traffic one of them is using community."

BTW, possibly "influence" might be a better word than "control".

[MHM Cisco World]

If you check my link you see it clearly mention control.

Also your example of 4 prefix' the use of community to Control inbound is use even if we have one prefix not need multi.

When we have one prefix one link to ISP will use as primary and other use as backup.

And again it not tag it attribute.

MHM

[Joseph W. Doherty]

"There are many way we can control inbound traffic one of them is using community."

Then I would disagree with your reference because "control" means:

control/kən-trōl′/

transitive verb

1. To exercise authoritative or dominating influence over; direct: synonym: conduct.
   "The majority party controls the legislative agenda."
   Similar: conduct
2. To adjust to a requirement; regulate.
   "rules that control trading on the stock market; valves that control the flow of water."
3. To hold in restraint; check.
   "struggled to control my temper."

The American Heritage® Dictionary of the English Language, 5th Edition • More at Wordnik

Which recipient AS doesn't truly have.  However, perhaps "influence" is too weak, maybe "direct" would be even better because is can be used both for requiring or suggesting.

"Also your example of 4 prefix' the use of community to Control inbound is use even if we have one prefix not need multi."

Agreed, but you miss the point.  I used multiple prefixes so you mix them across links however desired.

Correct, it's an "attribute", not a "tag" but conceptionally basically the same.  Much as BGP has "peers" and OSPF "neighbors" or L2 has "CoS" and L3 "ToS" or "byte" vs. "octet", etc.

In this case I intentionally used "tag" because I thought "tag" better conveys the concept of a BGP community purpose.  Also it works as a verb, i.e. "tag routes" vs. explaining setting an "attribute" to be analyzed.

For example, assuming you have head hair an attribute would be its color, but I could write you bleached your hair rather than you had black hair that has been changed to white.

BTW, years ago I asked if you're a native English speaker, to which you didn't reply.  So, again, is English your native language?

[Joseph W. Doherty · Accepted solution]

What it does is tag routes sent to the ISP.  This allows routing, coming toward you, to "know" the same destination route is different and treat it differently (from default BGP routing).

For example, assume you had 4 public /24s.  You asked your ISP to route community 28 and 29 on the different links (as preferred).  Now, you can control which link will be used for each /24, which you can change without needing to contact the ISP again.

Community tags can be used for other purposes, but nothing automatically happens.  You need to agree with your peers what particular community tags are to be used for.

[MHM Cisco World]

FYI
it is not tag it community BGP attribute the tag is different it use in IGP and BGP but community only use in BGP and tag not send with prefix it local

router reject route-map is it have set tag, the tag not attribute so it not support in bgp route-map

MHM