Cisco directors and implementers come throughout queries associated to Cisco BGP neighbor output. One such typical question is distinction between “Implicit Withdraw” and “Express Withdraw” when one points the command “present ip bgp neighbors” and output shows each the phrases i.e. Express and Implicit. Beneath is one pattern output –
R2#sh ip bgp neighb | embody Withdraw
Express Withdraw: 0 2
In easy phrases, “Implicit Withdraw” counter will increase when prefix has been withdrawn and re marketed. Most of occasions, prefix is up to date with new attribute (Like metric change, AS path attribute change and so on) and obtained by BGP talking machine.
“Express Withdraw” counter will increase when prefix has been withdrawn as a result of it’s not possible.
Situation: Cisco BGP Implicit Withdraw and Express Withdraw
The above distinction between each the phrases might be illustrated in higher method via beneath state of affairs –
R1 config
hostname R1
!
interface Loopback0
ip tackle 1.1.1.1 255.255.255.0
!
interface Loopback11
ip tackle 11.11.11.1 255.255.255.0
!
interface FastEthernet0/0
ip tackle 192.168.12.1 255.255.255.0
!
router bgp 1
community 1.1.1.0 masks 255.255.255.0
community 11.11.11.0 masks 255.255.255.0
community 192.168.12.0
neighbor 192.168.12.2 remote-as 2
neighbor 192.168.12.2 soft-reconfiguration inbound
R2 config
hostname R2
!
interface Loopback0
ip tackle 2.2.2.2 255.255.255.0
!
interface FastEthernet0/0
ip tackle 192.168.12.2 255.255.255.0
!
interface FastEthernet0/1
ip tackle 192.168.23.2 255.255.255.0
!
router bgp 2
community 2.2.2.0 masks 255.255.255.0
community 192.168.12.0
community 192.168.23.0
neighbor 192.168.12.1 remote-as 1
neighbor 192.168.12.1 soft-reconfiguration inbound
neighbor 192.168.23.3 remote-as 3
neighbor 192.168.23.3 soft-reconfiguration inbound
R3 Config
hostname R3
!
interface Loopback0
ip tackle 3.3.3.3 255.255.255.0
!
interface FastEthernet0/1
ip tackle 192.168.23.3 255.255.255.0
!
router bgp 3
community 3.3.3.0 masks 255.255.255.0
community 192.168.23.0
neighbor 192.168.23.2 remote-as 2
neighbor 192.168.23.2 soft-reconfiguration inbound
As soon as the neighborship is shaped between eBGP talking neighbors, the output on R3 shall be –
R3#sh ip bgp
Standing codes: s suppressed, d damped, h historical past, * legitimate, > finest, i – inner,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete
Community Subsequent Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 192.168.23.2 0 2 1 i
*> 2.2.2.0/24 192.168.23.2 0 0 2 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 11.11.11.0/24 192.168.23.2 0 2 1 i
*> 192.168.12.0 192.168.23.2 0 0 2 i
* 192.168.23.0 192.168.23.2 0 0 2 i
*> 0.0.0.0 0 32768 i
Additional, output for Implicit and Express Withdraw is accessible through beneath command –
R3#sh ip bgp neighb | incl With
Express Withdraw: 0 0
The output exhibits Withdraw sort in 1st column, Withdrawals despatched in 2nd column and Withdrawals obtained in 3rd column. Our focus shall be on 3rd column ie withdrawals obtained.
Now, 1st we are going to change the AS-Path attribute for Community 11.11.11.0/24 on R1 by including 5 AS path (AS 1) as beneath –
R1(config)#route-map NET11 allow 10
match ip tackle 11
set as-path prepend 1 1 1 1 1
!
route-map NET11 allow 20
R1(config)#router bgp 1
R1(config-router)#neighbor 192.168.12.2 route-map NET11 out
Beneath is the output obtained on R3 –
R3#sh ip bgp
Standing codes: s suppressed, d damped, h historical past, * legitimate, > finest, i – inner,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete
Community Subsequent Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 192.168.23.2 0 2 1 i
*> 2.2.2.0/24 192.168.23.2 0 0 2 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 11.11.11.0/24 192.168.23.2 0 2 1 1 1 1 1 1 i
*> 192.168.12.0 192.168.23.2 0 0 2 i
* 192.168.23.0 192.168.23.2 0 0 2 i
*> 0.0.0.0 0 32768 i
As proven in above output, the AS Path attribute is modified for community 11.11.11.0/24 from “2 1 i” to “2 1 1 1 1 1 1 i”.
This corresponding change in path attribute ends in increment of Implcit withdraw counter by 1 as proven beneath (worth is now 7) –
R3#sh ip bgp neighb | incl With
Implicit Withdraw: 0 7
Express Withdraw: 0 0
Subsequent, we are going to cease promoting community 1.1.1.0/24 from R1 a bellow –
R1(config-router)#router bgp 1
R1(config-router)#no community 1.1.1.0 masks 255.255.255.0
The corresponding change shall be mirrored in “present ip bgp” command output on R3 –
R3#sh ip bgp
Standing codes: s suppressed, d damped, h historical past, * legitimate, > finest, i – inner,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete
Community Subsequent Hop Metric LocPrf Weight Path
*> 2.2.2.0/24 192.168.23.2 0 0 2 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 11.11.11.0/24 192.168.23.2 0 2 1 1 1 1 1 1 i
*> 192.168.12.0 192.168.23.2 0 0 2 i
* 192.168.23.0 192.168.23.2 0 0 2 i
*> 0.0.0.0 0 32768 i
Since community 1.1.1.0/24 has been withdrawn, a corresponding increment is seen in Express Withdraw counter as beneath –
R3#sh ip bgp neighb | incl With
Implicit Withdraw: 0 7
Express Withdraw: 0 1
Hope the general lab examined state of affairs is ready to make clear the variations between “Implicit Withdraw” and “Express Withdraw” in easy phrases.
