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BGP Communities
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Problem: Scale Routing PolicySolution: COMMUNITY
NOT in decision algorithm
BGP route can be a member of manycommunities
Typical communities:
Destinations learned from customers
Destinations learned from ISPs or peers
Destinations in VPNBGP community isfundamental to the operation
of BGP VPNs
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ISP 4
Problem: Scale Routing PolicySolution: COMMUNITY
ISP 1ISP 1
Customer 1(No Default,
Wants Full Routes)
ISP 2
Communities:
1:100Customer Routes
1:80ISP Routes
Communities:
1:100Customer Routes
1:80ISP Routes
Customer 2(Uses Default,
Wants Your Routes)
0.0.0.0
ISP 3
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RST-20038109_05_2003_c1
Problem: Scale Routing PolicySolution: COMMUNITY
ISP 1ISP 1
ISP 2
Communities:
1:100Customer Routes
1:80ISP Routes
Communities:
1:100Customer Routes
1:80ISP Routes
Match Community1:100
Match Community1:100 1:80
Match Community1:100
Set Community1:80
Set Community
1:100
ISP 4
Customer 1(No Default,
Wants Full Routes)
Customer 2(Uses Default,
Wants Your Routes)
0.0.0.0
ISP 3
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BGP Attributes: COMMUNITY
Activated per neighbor/peer-group:
neighbor {peer-address | peer-group-name}
send-community
Carried across AS boundaries
Common convention is string
of four bytes: :[0-65536]
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BGP Attributes: COMMUNITY (Cont.)
Each destination can be a member of multiple communities
Using a route-map: set community
community number
aa:nn community number in aa:nn format
additive Add to the existing community
none No community attribute
local-ASDo not send to EBGP peers (well-known community)
no-advertise Do not advertise to any peer (well-known
community)
no-export Do not export outside AS/confed (well-known
community)
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Community Filters
Filter based on Community Strings
ip community-list [permit|deny] comm
ip community-list [permit|deny] regexp
Per neighbor
Inbound or outbound route-maps
Match community [exact-match]
Exact match only for standard lists
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Community Filters
Example 1:Mark some prefixes as part of the 1:120 community
(+removeexisting community!)
Configuration:
router bgp 1
neighbor 10.0.0.1 remote-as 2neighbor 10.0.0.1
send-community
neighbor 10.0.0.1 route-map set_community out
!
route-map set_community 10 permit
match ip address 1set community 1:120
!
access-list 1 permit 10.10.0.0 0.0.255.255
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Community Filters
Example 2:
Set LOCAL_PREF depending on the community that theprefix belongs
to
Configuration:
router bgp 1
neighbor 10.0.0.1 remote-as 2
neighbor 10.0.0.1 route-map filter_on_community in
!
route-map filter_on_community 10 permit
match community 1
set local-preference 150
!
ip community-list 1 permit 2:150
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