iBGP with using loopback interface keeps flapping

Valeriehsh · ‎09-09-2021

Just testing out simple iBGP with 2 routers using a loopback address each. Before peering the loopback addresses and configuring "update-source", the loopback interfaces are able to ping each other using iBGP. After peering the loopback addresses, the link keeps flapping.

- show ip route does show the route to each loopback address

- show ip bgp topo * also show the network as "valid" and "best"

Any idea why it is flapping up and down?

*Sep  9 22:14:26.963: %BGP-3-NOTIFICATION: sent to neighbor 1.1.1.1 4/0 (hold time expired) 0 bytes
SG01-RT02(config-router)#
*Sep  9 22:14:26.967: %BGP-5-NBR_RESET: Neighbor 1.1.1.1 reset (BGP Notification sent)
*Sep  9 22:14:26.975: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Down BGP Notification sent
*Sep  9 22:14:26.979: %BGP_SESSION-5-ADJCHANGE: neighbor 1.1.1.1 IPv4 Unicast topology base removed from session  BGP Notification sent
*Sep  9 22:14:27.151: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Up
SG01-RT02(config-router)#
*Sep  9 22:17:38.467: %BGP-3-NOTIFICATION: sent to neighbor 1.1.1.1 4/0 (hold time expired) 0 bytes
SG01-RT02(config-router)#
*Sep  9 22:17:38.471: %BGP-5-NBR_RESET: Neighbor 1.1.1.1 reset (BGP Notification sent)
*Sep  9 22:17:38.475: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Down BGP Notification sent
*Sep  9 22:17:38.475: %BGP_SESSION-5-ADJCHANGE: neighbor 1.1.1.1 IPv4 Unicast topology base removed from session  BGP Notification sent
*Sep  9 22:17:38.671: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Up

Below are the "show run".

Thanks.

SG01-RT01#sh run
Building configuration...

Current configuration : 2238 bytes
!
! Last configuration change at 19:52:03 UTC Thu Sep 9 2021
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
!
hostname SG01-RT01
!
boot-start-marker
boot-end-marker
!
!
!
no aaa new-model
no ip icmp rate-limit unreachable
ip cef
!
!
!
!
!
!
no ip domain lookup
no ipv6 cef
!
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
ip tcp synwait-time 5
!
!
!
!
!
!
!
!
!
!
!
!
interface Loopback1
 ip address 1.1.1.1 255.255.255.0
!
interface Ethernet0/0
 no ip address
 shutdown
 duplex auto
!
interface GigabitEthernet0/0
 no ip address
 shutdown
 media-type gbic
 speed 1000
 duplex full
 negotiation auto
!
interface FastEthernet1/0
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet1/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet2/0
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet2/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet3/0
 ip address 192.168.1.1 255.255.255.0
 speed auto
 duplex auto
!
interface FastEthernet3/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface Serial4/0
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial4/1
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial4/2
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial4/3
 no ip address
 shutdown
 serial restart-delay 0
!
interface FastEthernet5/0
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet5/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet6/0
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet6/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
router bgp 65200
 bgp log-neighbor-changes
 network 1.1.1.0 mask 255.255.255.0
 network 192.168.1.0
 neighbor 1.2.0.1 remote-as 65200
 neighbor 1.2.0.1 update-source Loopback1
 neighbor 192.168.1.2 remote-as 65200
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
!
!
control-plane
!
!
line con 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
 stopbits 1
line aux 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
 stopbits 1
line vty 0 4
 login
!
!
end

SG01-RT02#sh run
Building configuration...

*Sep  9 22:24:00.431: %BGP-3-NOTIFICATION: sent to neighbor 1.1.1.1 4/0 (hold time expired) 0 bytes
Current configuration : 1802 bytes
!
! Last configuration change at 22:19:56 UTC Thu Sep 9 2021
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
!
hostname SG01-RT02
!
boot-start-marker
boot-end-marker
!
!
!
no aaa new-model
no ip icmp rate-limit unreachable
ip cef
!
!
!
!
 --More--
*Sep  9 22:24:00.435: %BGP-5-NBR_RESET: Neighbor 1.1.1.1 reset (BGP Notification sent)
*Sep  9 22:24:00.443: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Down BGP Notification sent
*Sep  9 22:24:00.447: %BGP_SESSION-5-ADJCHANGE: neighbor 1.1.1.1 IPv4 Unicast topology base removed from session  BGP Notification sent
*Sep  9 22:24:00.647: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Up
!
!
no ip domain lookup
no ipv6 cef
!
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
ip tcp synwait-time 5
!
!
!
!
!
!
!
!
!
!
!
!
interface Loopback1
 ip address 1.2.0.1 255.255.255.255
!
interface Ethernet0/0
 no ip address
 shutdown
 duplex auto
!
interface GigabitEthernet0/0
 no ip address
 shutdown
 media-type gbic
 speed 1000
 duplex full
 negotiation auto
!
interface FastEthernet1/0
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet1/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet2/0
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet2/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet3/0
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet3/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
interface FastEthernet4/0
 ip address 192.168.1.2 255.255.255.0
 speed auto
 duplex auto
!
interface FastEthernet4/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
router bgp 65200
 bgp log-neighbor-changes
 network 1.2.0.1 mask 255.255.255.255
 network 192.168.1.0
 neighbor 1.1.1.1 remote-as 65200
 neighbor 1.1.1.1 update-source Loopback1
 neighbor 192.168.1.1 remote-as 65200
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
!
!
control-plane
!
!
line con 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
 stopbits 1
line aux 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
 stopbits 1
line vty 0 4
 login
!
!
end

Martin L · ‎09-09-2021

BGP often needs or runs over the IGP like OSPF, EIGRP, etc (static routes will do too). I do not see any of those, so you have a case when BGP puts route(s) into RIB but BGP itself needs those route(s) to reach other guy. Think about BGP as a TCP/IP application protocol not as protocol itself. You can run BGP over directly connected interface w/o any other protocols (try it) but once you use loopbacks to establish BGP, it needs help. Add static routes telling us how to reach other guy's loopback and let us know if that helped.

Regards, ML
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Valeriehsh · ‎09-09-2021

Thanks. I'd seen examples on the Internet of how static route and other IGPs are used and was wondering why I couldn't find any using iBGP as THE IGP so I thought I'd give it a try. Was wondering if it's some misconfiguration or some command I'd missed, since I actually could use iBGP and ping between the loopbacks. Problem only starts when i tried to form neighborship between the 2 loopback interfaces.

So I guess it's really not possible at all. Thanks!

Martin L · ‎09-09-2021

Just noticed that you do have extra commands! you have 2 peerings: 1 directly and 1 via loopback, check it using show ip bgp status, which one is UP and not going down? you need only neighbor statement; BGP is slow but big; it is not recommended as IGP although could work. Note that BGP keep alive time out is 180 seconds.

 neighbor 1.1.1.1 remote-as 65200
 ..
 neighbor 192.168.1.1 remote-as 65200

Regards, ML
**Please Rate All Helpful Responses **