BGP Outbound Route Filter Capability ORF

BGP Outbound Route Filter Capability ORF

BGP Outbound Route Filter Capability
This new feature, supported from IOS 12.0(5)S onwards, allows one BGP speaker to install its inbound locally configured
prefix-list filter on to the remote BGP speaking router. This is especially used for reducing the amount of unwanted routing
update from the remote peer.
The remote BGP speaker would apply the received prefix-list filter, in addition to its locally configured outbound filters (if
any), to constrain/filter its outbound routing updates to the neighbor. This mechanism can be used to avoid unwanted
routing updates and thus help reduce resources required for routing update generation and processing.For example, Prefix-List ORF can be used to address the issue of receiving (“unwanted”) full routes from multihomed
BGP customers. The customer can simply enable this feature on their router and thus allow their providers to manage the
filtering of their route announcements. This avoids unwanted routing updates coming from the customer to their upstream
ISP.
Currently the Prefix-List ORF is implemented for IPv4 unicast only. Some points to note about the implementation:
·  By default, the Prefix-List ORF Capability is not advertised to any neighbours
·  The capability can not be advertised to a neighbour that is a peergroup member.
·  The Prefix-List ORF is pushed over to the peer router immediately after the session is established if the local
router has received the ORF capability, and has configured inbound prefix-list filter for the neighbour.
Configuration
The router configuration command is included in the following example:
router bgp Y
neighbor x.x.x.x remote-as Z
neighbor x.x.x.x description Peer router R2
neighbor x.x.x.x capability prefix-filter
neighbor x.x.x.x prefix-list FilterZ-in in
This command can be used to enable the advertisement of the Prefix-List ORF Capability to a neighbour. Using the “no
neighbor x.x.x.x capability prefix-filter” command disables the Prefix-List ORF Capability.
When the BGP peering is established in this example, the above router (R1) will push its prefix-list “FilterZ-in” over to its
peer router x.x.x.x (R2). R2 will receive the prefix-list filter and apply it to its outbound update to R1 (in addition to its
local policy, if any is configured).
Pushing out a Prefix-list ORF
The command to push out a Prefix-list ORF and receive route refresh from a neighbour is:
clear ip bgp x.x.x.x in prefix-filter
When the inbound prefix-list changes (or is removed), this command can be used to push out the new prefix-list, and
consequently receive route refresh from the neighbour based on the new prefix-list. The keyword "prefix-filter" will be
ignored if the Prefix-list ORF Capability has not been received from the neighbour.
Without the keyword “prefix-filter”, the command:
clear ip bgp x.x.x.x in
would simply perform the normal route refresh from the neighbour. It does not push out the current inbound prefix-list
filter to the neighbour. The command is useful when inbound routing policies other than the prefix-list filter such as routemap
changes.
Displaying Prefix-list ORF
The command to display the prefix-list ORF received from a neighbour is:
show ip bgp neighbor x.x.x.x received prefix-filter
This will display the received prefix-list. Changes to the output of “show ip bgp neighbor x.x.x.x” are:
Prefixlist ORF
Capability advertised; received
Filter sent; received (25 entries)