Bgp Error Handling And Attribute Filtering Syslog Messages; Bgp-Rib Feedback Mechanism For Update Generation - Cisco NCS 5500 Series Configuration Manual

Implementing BGP
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 Error Handling and Attribute Filtering Syslog Messages

When a router receives a malformed update packet, an ios_msg of type
ROUTING-BGP-3-MALFORM_UPDATE is printed on the console. This is rate limited to 1 message per
minute across all neighbors. For malformed packets that result in actions "Discard Attribute" (A5) or "Local
Repair" (A6), the ios_msg is printed only once per neighbor per action. This is irrespective of the number of
malformed updates received since the neighbor last reached an "Established" state.
This is a sample BGP error handling syslog message:
%ROUTING-BGP-3-MALFORM_UPDATE : Malformed UPDATE message received from neighbor 13.0.3.50
- message length 90 bytes,
error flags 0x00000840, action taken "TreatAsWithdraw".
Error details: "Error 0x00000800, Field "Attr-missing", Attribute 1 (Flags 0x00, Length 0),
Data []"
This is a sample BGP attribute filtering syslog message for the "discard attribute" action:
[4843.46]RP/0/0/CPU0:Aug 21 17:06:17.919 : bgp[1037]: %ROUTING-BGP-5-UPDATE_FILTERED :
One or more attributes were filtered from UPDATE message received from neighbor 40.0.101.1
- message length 173 bytes,
action taken "DiscardAttr".
Filtering details: "Attribute 16 (Flags 0xc0): Action "DiscardAttr"". NLRIs: [IPv4 Unicast]
88.2.0.0/17
This is a sample BGP attribute filtering syslog message for the "treat-as-withdraw" action:
[391.01]RP/0/0/CPU0:Aug 20 19:41:29.243 : bgp[1037]: %ROUTING-BGP-5-UPDATE_FILTERED :
One or more attributes were filtered from UPDATE message received from neighbor 40.0.101.1
- message length 166 bytes,
action taken "TreatAsWdr".
Filtering details: "Attribute 4 (Flags 0xc0): Action "TreatAsWdr"". NLRIs: [IPv4 Unicast]
88.2.0.0/17

BGP-RIB Feedback Mechanism for Update Generation

The Border Gateway Protocol-Routing Information Base (BGP-RIB) feedback mechanism for update generation
feature avoids premature route advertisements and subsequent packet loss in a network. This mechanism
ensures that routes are installed locally, before they are advertised to a neighbor.
BGP waits for feedback from RIB indicating that the routes that BGP installed in RIB are installed in forwarding
information base (FIB) before BGP sends out updates to the neighbors. RIB uses the the BCDL feedback
mechanism to determine which version of the routes have been consumed by FIB, and updates the BGP with
that version. BGP will send out updates of only those routes that have versions up to the version that FIB has
installed. This selective update ensures that BGP does not send out premature updates resulting in attracting
traffic even before the data plane is programmed after router reload, LC OIR, or flap of a link where an
alternate path is made available.
To configure BGP to wait for feedback from RIB indicating that the routes that BGP installed in RIB are
installed in FIB, before BGP sends out updates to neighbors, use the update wait-install command in router
address-family IPv4 or router address-family VPNv4 configuration mode. The show bgp, show bgp neighbors,
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
Information about Implementing BGP
145