BGP Communities

 

Community	Meaning
no-export (65535:65281)	Don’t advertise this route to eBGP peers (stay within AS).
no-advertise (65535:65282)	Don’t advertise this route to any BGP peers.
local-AS (65535:65283)	Don’t advertise outside of local confederation.
internet (0:0)	Advertise to everyone (default).

🧩 The Topology Recap

            +--------+
            | ISP-A  |  AS 65001
            +--------+
                |
                |  (Link 1)
            +----------+
            | You (AS 65010) |
            +----------+
                |
                |  (Link 2)
            +--------+
            | ISP-B  |  AS 65002
            +--------+

Your enterprise AS 65010 is dual-homed to two ISPs (A and B).

You want:

• ISP-A → Primary

• ISP-B → Backup

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🧠 BGP Config Breakdown

1️⃣ BGP neighbor configuration

router bgp 65010
 neighbor 192.0.2.1 remote-as 65001   ! ISP-A
 neighbor 192.0.2.1 send-community
 neighbor 192.0.2.2 remote-as 65002   ! ISP-B
 neighbor 192.0.2.2 send-community

• You are establishing eBGP sessions with both ISPs.

• send-community is crucial — it ensures the community attributes you set are actually sent to each ISP.

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2️⃣ Route-maps definition

route-map TO_ISPA permit 10
 set community 65001:100 additive

route-map TO_ISPB permit 10
 set community 65002:200 additive

• TO_ISPA → tags routes with community 65001:100

  • Meaning: “Tell ISP-A this is my preferred path.”

• TO_ISPB → tags routes with community 65002:200

  • Meaning: “Tell ISP-B this is my backup path.”

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3️⃣ Applying route-maps to the correct neighbors

You must attach the correct route-map to the correct neighbor:

router bgp 65010
 neighbor 192.0.2.1 route-map TO_ISPA out
 neighbor 192.0.2.2 route-map TO_ISPB out

That line tells BGP:

When advertising routes outbound to ISP-A, apply TO_ISPA.
When advertising routes outbound to ISP-B, apply TO_ISPB.

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✅ Summary Table

ISP	Neighbor IP	Route-map applied	Direction	Effect
ISP-A (Primary)	192.0.2.1	TO_ISPA	outbound	Tags routes with 65001:100 (preferred)
ISP-B (Backup)	192.0.2.2	TO_ISPB	outbound	Tags routes with 65002:200 (less preferred)

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4️⃣ Why "out" direction?

Because:

• You are sending your prefixes to the ISPs.

• Communities are attached to outbound advertisements.

• Each ISP reads your community and applies their inbound policy (their side).

So the logic is:

You control inbound traffic (to you) by tagging outbound advertisements (from you).

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5️⃣ What the ISPs do with those tags

Each ISP has its own internal policy, for example:

• ISP-A:

  ip community-list 100 permit 65001:100
  route-map FROM_CUSTOMER permit 10
   match community 100
   set local-preference 200
  

  → Gives higher preference → Primary path.

• ISP-B:

  ip community-list 200 permit 65002:200
  route-map FROM_CUSTOMER permit 10
   match community 200
   set local-preference 80
  

  → Lower preference → Backup path.

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✅ In short:

TO_ISPA → applied to neighbor ISP-A (192.0.2.1)
TO_ISPB → applied to neighbor ISP-B (192.0.2.2)
Both are applied in the out direction, tagging routes you advertise.
The ISPs then interpret those tags to adjust their inbound preference for your routes.

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