Attribute Filtering

Scenario
The BGP attribute filter can be used, as the name suggests, to filter unsupported or undesired attributes attached to a received prefix. If prefix filtering is enabled for the appropriate attribute, it will be stripped/discarded from the prefix. A list of attributes can be found here. An alternative to discarding the attribute is to use treat-as-withdrawal option – in this instance, the router will treat the prefix advertisement as a withdrawal. This should be used with care as it could break fundamental BGP operations. The below example shows a basic eBGP neighborship implementing filtering/discarding on attribute 30 and treat-as-withdrawal on attribute 31 – both of which are depreciated. The main difference is that XR must uses an attribute-filter.

IOS Config

router bgp 100
 bgp router-id 172.16.1.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 10.1.1.2 remote-as 200
 neighbor 10.1.1.2 path-attribute discard 30 in
 neighbor 10.1.1.2 path-attribute treat-as-withdrawal 31 in
 address-family ipv4
  neighbor 10.1.1.2 activate
 exit-address-family
!

XR Config

router bgp 100
 bgp log neighbor changes detail
 bgp router-id 172.16.1.1
 !
 attribute-filter group FILTER-GROUP
  attribute 30 discard
  attribute 31 treat-as-withdraw
  !
 address-family ipv4 unicast
 !
 neighbor 10.1.1.2
  remote-as 200
  update in filtering
   attribute-filter group FILTER-GROUP
  !
  address-family ipv4 unicast
  !
 !
!

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