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Understanding communites and BGP path selection

wilson_1234_2
Level 3
Level 3

We have two ISPs, connected to two ASR routers. Our configuration is set up Primary/Secondary.

We are using Level 3 as our primary ISP. Traffic outbound is controlled via HSRP and L3 has priority outbound.

L3 is also the primary path inbound to our Data Center for our customers who reach us via Internet.

We are adding another carrier who we want to be primary in/out to our Data Center.

When talking with Level 3, I am not sure if it is me, or them who is not understanding what I am asking for.

They are telling me to use the below community to prioritize, or reduce L3 as preferred route, but my understanding is that this will only set the local preference outbound for my AS route selection, and will have no affect on any upstream AS looking to get to my Data Center from Internet.

I am thinking AS path prepending is what I need to do. I have gone back and forth numerous times on this and not getting anywhere.

Am I wrong, and that by using a community they have provided, I can make L3 less desirable to my subnets being advertised to Internet?

Local PrefFor Customers:

3356:70 - Sets the customer prefix to a local pref of 70

3356:80 - Sets the customer prefix to a local pref of 80

3356:89 - Sets the customer prefix to a local pref of 89

3356:90 - Sets the customer prefix to a local pref of 90

3356:100 - Explicitly sets the customer prefix to the default local pref of 100

 

2 Replies 2

Philip D'Ath
VIP Alumni
VIP Alumni

Use AS path prepending.

Hello
I am assuming you are meaning BGP Community?
If so these attributes can be attached to routes you advertise to your isp, This can allow them either to filter on these prefixes or perform route path selection back towards your DC

res
Paul


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Paul
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