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BGP prefix-based outbound route filtering uses the BGP ORF send and receive capabilities to minimize the number of BGP updates that are sent between BGP peers. Configuring BGP ORF can help reduce the amount of system resources required for generating and processing routing updates by filtering out unwanted routing updates at the source. For example, BGP ORF can be used to reduce the amount of processing required on a router that is not accepting full routes from a service provider network.

The BGP prefix-based outbound route filtering is enabled through the advertisement of ORF capabilities to peer routers. The advertisement of the ORF capability indicates that a BGP peer will accept a prefix list from a neighbor and apply the prefix list to locally configured ORFs (if any exist). When this capability is enabled, the BGP speaker can install the inbound prefix list filter to the remote peer as an outbound filter, which reduces unwanted routing updates.

The BGP prefix-based outbound route filtering can be configured with send or receive ORF capabilities. The local peer advertises the ORF capability in send mode. The remote peer receives the ORF capability in receive mode and applies the filter as an outbound policy. The local and remote peers exchange updates to maintain the ORF on each router. Updates are exchanged between peer routers by address family depending on the ORF prefix list capability that is advertised. The remote peer starts sending updates to the local peer after a route refresh has been requested with the clear ip bgp in prefix-filter command or after an ORF prefix list with immediate status is processed. The BGP peer will continue to apply the inbound prefix list to received updates after the local peer pushes the inbound prefix list to the remote peer.

Before You Begin

BGP peering sessions must be established, and BGP ORF capabilities must be enabled on each participating router before prefix-based ORF announcements can be received.

Note

BGP prefix-based outbound route filtering does not support multicast. IP addresses that are used for outbound route filtering must be defined in an IP prefix list. BGP distribute lists and IP access lists are not supported. Outbound route filtering is configured on only a per-address family basis and cannot be configured under the general session or BGP routing process. Outbound route filtering is configured for external peering sessions only.

1.    enable


2.    configure terminal


3.    ip prefix-list list-name [seq seq-value] {deny network / length | permit network / length} [ge ge-value] [le le-value]


4.    router bgp autonomous-system-number


5.    address-family ipv4 [unicast | multicast | vrf vrf-name]


6.    neighbor {ip-address | peer-group-name} remote-as autonomous-system-number


7.    neighbor ip-address ebgp-multihop [hop-count]


8.    neighbor ip-address capability orf prefix-list [send | receive | both]


9.    neighbor {ip-address | peer-group-name} prefix-list prefix-list-name {in | out}


10.    end


11.    clear ip bgp {ip-address | *} in prefix-filter

Device> enable

Device# configure terminal

Device(config)# ip prefix-list FILTER seq 10 permit 192.168.1.0/24 

Device(config)# router bgp 100

Device(config-router)# address-family ipv4 unicast

Device(config-router-af)# neighbor 10.1.1.1 remote-as 200

Device(config-router-af)# neighbor 10.1.1.1 ebgp-multihop

Device(config-router-af)# neighbor 10.1.1.1 capability orf prefix-list both 

Device(config-router-af)# neighbor 10.1.1.1 prefix-list FILTER in

Device(config-router-af)# end

Device# clear ip bgp 10.1.1.1 in prefix-filter

The following example creates an outbound route filter and configures Router A (10.1.1.1) to advertise the filter to Router-B (172.16.1.2). An IP prefix list named FILTER is created to specify the 192.168.1.0/24 subnet for outbound route filtering. The ORF send capability is configured on Router A so that Router A can advertise the outbound route filter to Router B.

ip prefix-list FILTER seq 10 permit 192.168.1.0/24 
!
router bgp 65100
 address-family ipv4 unicast 
 neighbor 172.16.1.2 remote-as 65200
 neighbor 172.16.1.2 ebgp-multihop 
 neighbor 172.16.1.2 capability orf prefix-list send
 neighbor 172.16.1.2 prefix-list FILTER in
 end 

router bgp 65200
 address-family ipv4 unicast 
 neighbor 10.1.1.1 remote-as 65100
 neighbor 10.1.1.1 ebgp-multihop 255
 neighbor 10.1.1.1 capability orf prefix-list receive
 end 
clear ip bgp 10.1.1.1 in prefix-filter

router bgp 65100
 network 172.16.0.0
 network 172.17.0.0
 neighbor 10.69.232.70 remote-as 65200
 neighbor 10.69.232.70 route-map set-as-path out
!
route-map set-as-path 10 permit
 match address 1
 set as-path prepend 65100 65100
!
route-map set-as-path 20 permit
 match address 2
!
access-list 1 permit 172.16.0.0 0.0.255.255
access-list 1 permit 172.17.0.0 0.0.255.255
!
access-list 2 permit 0.0.0.0 255.255.255.255