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[Junos] What is the meaning of the PrefixLimitIdle flag shown in the "show bgp neighbor" command output?

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Article ID: KB30490 KB Last Updated: 23 Aug 2021Version: 2.0
Summary:

This article explains what the PrefixLimitIdle flag shown in the show bgp neighbor CLI command output means and the scenario in which it is set.

Symptoms:

What does the PrefixLimitIdle flag shown in the show bgp neighbor CLI command output mean, and in what scenario is it set?

Cause:

Whenever the number of prefixes received from the neighbor exceeds the configured allowed value, Junos OS will set the PrefixLimitIdle flag and the BGP session will be torn down.

The maximum number of allowed prefixes can be configured by using the following example:

set protocols bgp group BGP neighbor 10.0.12.2 family inet unicast prefix-limit maximum 5
set protocols bgp group BGP neighbor 10.0.12.2 family inet unicast prefix-limit teardown 100
Solution:

Topology


            10.0.12.1
 +------------+                +-------------+
 |            |                |             |
 |            |                |             |
 |   R1       +----------------|     R2      |
 |            |                |             |
 +------------+                +-------------+
                           10.0.12.2

R1 BGP config:

set protocols bgp group BGP type external
set protocols bgp group BGP peer-as 200
set protocols bgp group BGP neighbor 10.0.12.2 family inet unicast prefix-limit maximum 5
set protocols bgp group BGP neighbor 10.0.12.2 family inet unicast prefix-limit teardown 100

R2 BGP config:

set protocols bgp group BGP type external
set protocols bgp group BGP peer-as 100
set protocols bgp group BGP neighbor 10.0.12.1

During the normalized operation, R1 is receiving a total of four prefixes from R2.

R1> show bgp summary 
Groups: 1 Peers: 1 Down peers: 0
Table          Tot Paths  Act Paths Suppressed    History Damp State    Pending
inet.0               
                       4          4          0          0          0          0
Peer                     AS      InPkt     OutPkt    OutQ   Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
10.0.12.2               200          3          3       0       9          14 4/4/4/0              0/0/0/0

R1> show route protocol bgp 

inet.0: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.1.0/24     *[BGP/170] 00:00:48, localpref 100
                      AS path: 200 I, validation-state: unverified
                    > to 10.0.12.2 via ge-0/1/2.0
192.168.2.0/24     *[BGP/170] 00:00:48, localpref 100
                      AS path: 200 I, validation-state: unverified
                    > to 10.0.12.2 via ge-0/1/2.0
192.168.3.0/24     *[BGP/170] 00:00:48, localpref 100
                      AS path: 200 I, validation-state: unverified
                    > to 10.0.12.2 via ge-0/1/2.0
192.168.4.0/24     *[BGP/170] 00:00:48, localpref 100
                      AS path: 200 I, validation-state: unverified
                    > to 10.0.12.2 via ge-0/1/2.0

When the received number of prefixes from R2 exceeds five, the session will go into an idle state and the software will set the flag as PrefixLimitIdle.

R1> show bgp summary 
Groups: 1 Peers: 1 Down peers: 1
Table          Tot Paths  Act Paths Suppressed    History Damp State    Pending
inet.0               
                       0          0          0          0          0          0
Peer                     AS      InPkt     OutPkt    OutQ   Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
10.0.12.2               200          8          8       0       9          15 Idle  

R1> show bgp neighbor 10.0.12.2 
Peer: 10.0.12.2 AS 200         Local: 10.0.12.1 AS 100  
  Type: External    State: Idle           Flags: <PrefixLimitIdle>
  Last State: Established   Last Event: RecvUpdate
  Last Error: Cease
  Options: <Preference AddressFamily PeerAS PrefixLimit Refresh>
  Address families configured: inet-unicast
  Holdtime: 90 Preference: 170
  Number of flaps: 9
  Last flap event: RecvUpdate
  Error: 'Cease' Sent: 10 Recv: 1
  Trace options:  all
  Trace file: /var/log/R1/DEBUG_BGP size 131072 files 10

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