BGP — community-list standard: tag outbound, filter on receive

• Routing
• Intermediate
• 14 min
• 3 steps
• Cisco IOS-style

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Free · no install · no account

Scenario

BGP communities are 32-bit tags carried as a path attribute. The printed form <asn>:<n> is convention; the wire form is just an integer. Communities don't *do* anything by themselves — they're read by route-maps that filter / set local-pref / set metric on match. This lab demonstrates the canonical pattern: 1. R1 tags outbound prefix with set community 65001:100. 2. R2 defines ip community-list standard PREFER permit 65001:100. 3. R2 binds an inbound route-map that matches that community and sets local-pref. After commit, R2 sees the community on R1's prefix and applies policy keyed on it. Standard community-lists match an exact community value (or list). Expanded community-lists (regex over the printed string) are not yet wired in netverdict — covered as a future engine slice.

Topology

2 × router · Cisco IOS-style

• R1 — router
• R2 — router

What you will do

1. On R1, define route-map TAG-OUT permit 10 with set community 65001:100 and bind outbound to R2.

2. On R2, define ip community-list standard PREFER permit 65001:100.

3. On R2, define route-map MATCH-PREFER permit 10 with match community PREFER and set local-preference 200. Bind inbound to R1.

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This lab in other languages

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