06-17-2024 03:49 AM
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
Solved! Go to Solution.
06-17-2024 04:25 AM - edited 06-17-2024 04:37 AM
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.
06-17-2024 03:56 AM
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
06-17-2024 03:59 AM - edited 06-17-2024 03:59 AM
Hi
So are you saying that those 2 lines will be doing nothing in the BGP process ??
06-17-2024 04:07 AM
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
06-17-2024 04:17 AM
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.??
06-17-2024 04:22 AM
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
06-18-2024 06:49 AM
Nice Doc
Thanks
06-18-2024 06:55 AM
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
06-18-2024 08:21 AM
"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".
06-18-2024 08:34 AM
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
06-18-2024 09:43 AM
"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:
06-17-2024 04:25 AM - edited 06-17-2024 04:37 AM
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.
06-18-2024 07:07 AM - edited 06-18-2024 09:33 AM
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
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide