Cisco NCS 5500 Series Configuration Manual page 58

Configure BGP Attribute Filtering
These attributes in the update message include MED, LOCAL_PREF, COMMUNITY, and so on. In some
cases, if the attributes are malformed, there is a need to filter these attributes at the receiving end of the router.
The BGP Attribute Filter functionality filters the attributes received in the incoming update message. The
attribute filter can also be used to filter any attributes that may potentially cause undesirable behavior on the
receiving router. Some of the BGP updates are malformed due to wrong formatting of attributes such as the
network layer reachability information (NLRI) or other fields in the update message. These malformed updates,
when received, causes undesirable behavior on the receiving routers. Such undesirable behavior may be
encountered during update message parsing or during re-advertisement of received NLRIs. In such scenarios,
its better to filter these corrupted attributes at the receiving end.
The Attribute-filtering is configured by specifying a single or a range of attribute codes and an associated
action. When a received Update message contains one or more filtered attributes, the configured action is
applied on the message. Optionally, the Update message is also stored to facilitate further debugging and a
syslog message is generated on the console. When an attribute matches the filter, further processing of the
attribute is stopped and the corresponding action is taken. Perform the following tasks to configure BGP
attribute filtering:
SUMMARY STEPS
1. configure
2. router bgp as-number
3. attribute-filter group attribute-filter group name
4. attribute attribute code { discard | treat-as-withdraw }
DETAILED STEPS
Step 1 configure
Step 2 router bgp as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 100
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP
routing process.
Step 3 attribute-filter group attribute-filter group name
Example:
RP/0/RP0/CPU0:router(config-bgp)# attribute-filter group ag_discard_med
Specifies the attribute-filter group name and enters the attribute-filter group configuration mode, allowing you to configure
a specific attribute filter group for a BGP neighbor.
Step 4 attribute attribute code { discard | treat-as-withdraw }
Example:
RP/0/RP0/CPU0:router(config-bgp-attrfg)# attribute 24 discard
Specifies a single or a range of attribute codes and an associated action. The allowed actions are:
• Treat-as-withdraw— Considers the update message for withdrawal. The associated IPv4-unicast or MP_REACH
NLRIs, if present, are withdrawn from the neighbor's Adj-RIB-In.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
48
Implementing BGP