BGP Communities: Signaling Intent Across Networks

January 9, 2026 · 7 min read

BGP routes carry more than just prefix and next-hop. Communities are tags attached to routes, signaling intent to other networks. “This route is for backup only.” “Prepend this route 3 times to peers.” “Don’t announce this outside the region.”

Without communities, you’d need separate sessions, manual filters, or constant coordination. With communities, you tag once, everyone who understands acts accordingly.

Communities are the language networks speak to each other.

Community Types

Standard Communities

32-bit value, formatted as ASN:value:

65000:100  - AS 65000, value 100
65000:1000 - AS 65000, value 1000

Well-Known Communities

Predefined, universal meaning:

Community	Value	Meaning
no-export	65535:65281	Don’t export outside AS
no-advertise	65535:65282	Don’t advertise to any peer
local-as	65535:65283	Don’t export outside local confederation
no-peer	65535:65284	Don’t advertise to EBGP peers

Extended Communities

64-bit, more structure:

rt:65000:100     - Route Target
soo:65000:100    - Site of Origin

Large Communities

96-bit for 4-byte ASNs:

4200000000:1:100  - Global Admin : Local Data 1 : Local Data 2

Matching Communities

Define Community List

configure

# Match single community
set policy community-list BACKUP rule 10 regex "65000:100"

# Match any from set
set policy community-list CUSTOMER rule 10 regex "65000:1[0-9][0-9]"

# Match well-known
set policy community-list NO-EXPORT rule 10 regex "no-export"

commit

Use in Route Map

configure

# Match routes with community
set policy route-map FILTER-IN rule 10 match community community-list BACKUP
set policy route-map FILTER-IN rule 10 action permit
set policy route-map FILTER-IN rule 10 set local-preference 50

commit

Setting Communities

Add Community to Route

configure

# Set community when advertising
set policy route-map SET-COMMUNITY rule 10 action permit
set policy route-map SET-COMMUNITY rule 10 set community "65000:100"

# Add community (keep existing)
set policy route-map ADD-COMMUNITY rule 10 action permit
set policy route-map ADD-COMMUNITY rule 10 set community "65000:200 additive"

# Set multiple communities
set policy route-map MULTI-COMMUNITY rule 10 action permit
set policy route-map MULTI-COMMUNITY rule 10 set community "65000:100 65000:200"

commit

Apply to Neighbor

# Apply route-map to neighbor
set protocols bgp neighbor 10.0.0.2 address-family ipv4-unicast route-map export SET-COMMUNITY

Common Use Cases

Use Case 1: Traffic Engineering

Tell upstream how to prefer your routes:

# Community convention with ISP:
# 65000:90 = set local-pref 90 (less preferred)
# 65000:100 = set local-pref 100 (normal)
# 65000:110 = set local-pref 110 (more preferred)

configure

# Mark backup link routes as less preferred
set policy route-map TO-ISP-BACKUP rule 10 action permit
set policy route-map TO-ISP-BACKUP rule 10 set community "65000:90"

set protocols bgp neighbor 10.0.0.2 address-family ipv4-unicast route-map export TO-ISP-BACKUP

commit

Use Case 2: Prepending Control

Ask upstream to prepend your routes:

# Community convention:
# 65000:3001 = prepend 1x to all peers
# 65000:3002 = prepend 2x to all peers
# 65000:3003 = prepend 3x to all peers

configure

# Request 2x prepend on backup routes
set policy route-map PREPEND-REQUEST rule 10 action permit
set policy route-map PREPEND-REQUEST rule 10 set community "65000:3002"

commit

Use Case 3: Regional Filtering

Announce only within region:

# Community convention:
# 65000:1000 = US region
# 65000:2000 = EU region
# 65000:3000 = APAC region

configure

# Mark route as US-only
set policy route-map US-ONLY rule 10 action permit
set policy route-map US-ONLY rule 10 set community "65000:1000 no-export"

commit

Use Case 4: Customer vs Peer vs Transit

Tag routes by source:

# Internal convention:
# 65000:100 = customer route
# 65000:200 = peer route
# 65000:300 = transit route

configure

# Tag customer routes
set policy route-map FROM-CUSTOMER rule 10 action permit
set policy route-map FROM-CUSTOMER rule 10 set community "65000:100"

# Use for policy decisions
set policy route-map TO-PEER rule 10 match community community-list CUSTOMER
set policy route-map TO-PEER rule 10 action permit
# Only advertise customer routes to peers

set policy route-map TO-PEER rule 20 action deny
# Deny transit routes to peers (no transit)

commit

Use Case 5: Blackhole

Signal upstream to blackhole traffic:

# Standard blackhole community (check with provider)
# Many ISPs use: ISP_ASN:666

configure

# Mark route for blackholing
set policy route-map BLACKHOLE rule 10 action permit
set policy route-map BLACKHOLE rule 10 match ip address prefix-list ATTACK-PREFIX
set policy route-map BLACKHOLE rule 10 set community "65000:666"

commit

Extended Communities

Route Targets (for VRF/VPN)

configure

# Import routes with specific RT
set protocols bgp address-family ipv4-vpn
set vrf name CUSTOMER-A rd 65000:1
set vrf name CUSTOMER-A route-target import 65000:1
set vrf name CUSTOMER-A route-target export 65000:1

commit

Site of Origin

# Prevent routing loops in multi-homed sites
# Routes from site won't be sent back to same site

set policy route-map SET-SOO rule 10 action permit
set policy route-map SET-SOO rule 10 set extcommunity soo "65000:100"

Large Communities

For networks with 4-byte ASNs or needing more structure:

configure

# Define large community list
set policy large-community-list CUSTOMER rule 10 regex "4200000000:1:.*"

# Set large community
set policy route-map SET-LARGE rule 10 action permit
set policy route-map SET-LARGE rule 10 set large-community "4200000000:1:100"

# Match large community
set policy route-map MATCH-LARGE rule 10 match large-community large-community-list CUSTOMER

commit

Viewing Communities

Show Communities on Routes

# Show BGP routes with communities
show bgp ipv4 unicast community

# Show specific prefix with communities
show bgp ipv4 unicast 203.0.113.0/24

# Output includes:
# Community: 65000:100 65000:200

# Filter by community
show bgp ipv4 unicast community 65000:100

Show Community Lists

# Show defined community lists
show policy community-list

Stripping Communities

Remove Specific Communities

configure

# Delete specific community
set policy route-map STRIP-INTERNAL rule 10 action permit
set policy route-map STRIP-INTERNAL rule 10 set community delete community-list INTERNAL

commit

Remove All Communities

configure

# Remove all communities (nuclear option)
set policy route-map STRIP-ALL rule 10 action permit
set policy route-map STRIP-ALL rule 10 set community none

commit

Community Design Principles

1. Document Your Scheme

# Community Scheme for AS65000

## Route Type (65000:1xx)
- 65000:100 = Customer route
- 65000:110 = Peer route
- 65000:120 = Transit route

## Traffic Engineering (65000:2xx)
- 65000:200 = Normal preference
- 65000:210 = Higher preference
- 65000:220 = Lower preference

## Regional (65000:3xx)
- 65000:300 = All regions
- 65000:310 = US only
- 65000:320 = EU only

## Action Requests (65000:4xx)
- 65000:410 = Prepend 1x
- 65000:420 = Prepend 2x
- 65000:430 = Prepend 3x
- 65000:499 = Blackhole

2. Use Consistent Patterns

# Good: Predictable scheme
# 65000:1xxx = route type
# 65000:2xxx = preference
# 65000:3xxx = regional
# 65000:4xxx = actions

# Bad: Random assignment
# 65000:42 = customer
# 65000:7 = blackhole
# 65000:1234 = US

3. Don’t Trust External Communities

# Strip customer communities on ingress
set policy route-map FROM-CUSTOMER rule 1 action permit
set policy route-map FROM-CUSTOMER rule 1 set community delete community-list ALL-INTERNAL
set policy route-map FROM-CUSTOMER rule 1 set community "65000:100 additive"

# Then apply customer tag

The Lesson

Communities are the language networks speak to each other.

Without communities:

Manual coordination for traffic engineering
Separate sessions for different policies
No way to signal intent across ASes

With communities:

Tag routes with meaning
Upstream acts on tags automatically
Complex policies become simple

Design your community scheme before you need it. Document it. Use consistent numbering. Make it extensible.

Communities scale your network’s communication without scaling your operational overhead.

Community Types

Standard Communities

Well-Known Communities

Extended Communities

Large Communities

Matching Communities

Define Community List

Use in Route Map

Setting Communities

Add Community to Route

Apply to Neighbor

Common Use Cases

Use Case 1: Traffic Engineering

Use Case 2: Prepending Control

Use Case 3: Regional Filtering

Use Case 4: Customer vs Peer vs Transit

Use Case 5: Blackhole

Extended Communities

Route Targets (for VRF/VPN)

Site of Origin

Large Communities

Viewing Communities

Show Communities on Routes

Show Community Lists

Stripping Communities

Remove Specific Communities

Remove All Communities

Community Design Principles

1. Document Your Scheme

2. Use Consistent Patterns

3. Don’t Trust External Communities

The Lesson

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