Bgp Update Message Error Handling; Bgp Attribute Filtering; Bgp Attribute Filter Actions - Cisco ASR 9000 Series Configuration Manual

Implementing BGP

BGP Update Message Error Handling

The BGP UPDATE message error handling changes BGP behavior in handling error UPDATE messages to
avoid session reset. Based on the approach described in IETF IDR I-D:draft-ietf-idr-error-handling, the Cisco
IOS XR BGP UPDATE Message Error handling implementation classifies BGP update errors into various
categories based on factors such as, severity, likelihood of occurrence of UPDATE errors, or type of attributes.
Errors encountered in each category are handled according to the draft. Session reset will be avoided as much
as possible during the error handling process. Error handling for some of the categories are controlled by
configuration commands to enable or disable the default behavior.
According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a
malformed attribute is required to reset the session over which the offending attribute was received. This
behavior is undesirable as a session reset would impact not only routes with the offending attribute, but also
other valid routes exchanged over the session.

BGP Attribute Filtering

The BGP Attribute Filter feature checks integrity of BGP updates in BGP update messages and optimizes
reaction when detecting invalid attributes. BGP Update message contains a list of mandatory and optional
attributes. These attributes in the update message include MED, LOCAL_PREF, COMMUNITY etc. 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.

BGP Attribute Filter Actions

The Attribute-filtering is configured by specifying a single or a range of attribute codes and an associated
action. The allowed actions are:
• " Treat-as-withdraw"— The associated IPv4-unicast or MP_REACH NLRIs, if present, are withdrawn
• "Discard Attribute"—The matching attributes alone are discarded and the rest of the Update message
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.
OL-30423-03
from the neighbor's Adj-RIB-In.
is processed normally.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
BGP Update Message Error Handling
71