BGP · Advanced BGP Features

Synchronization

Understand the legacy rule intended to protect transit through routers that did not run BGP.

A BGP-free transit router

R2 and R4 are iBGP border routers in AS 65024; R3 runs OSPF only. R1 originates the /24 and R5 receives it through R4. OSPF carries infrastructure addresses, not the customer /24.

Lab topologyPhysical links

Solid lines are physical links. Endpoint labels are the last octets of 192.0.2.x/30, first router then second router in the address table. Loopback0 is 10.255.0.N/32 on RN. Logical BGP peers are listed in the configurations.

Link	First endpoint	Second endpoint
R1–R2	GigabitEthernet1 192.0.2.1/30	GigabitEthernet1 192.0.2.2/30
R2–R3	GigabitEthernet2 192.0.2.5/30	GigabitEthernet1 192.0.2.6/30
R3–R4	GigabitEthernet2 192.0.2.9/30	GigabitEthernet1 192.0.2.10/30
R4–R5	GigabitEthernet2 192.0.2.13/30	GigabitEthernet1 192.0.2.14/30

Complete baseline configurations

R1

hostname R1
ip cef
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
exit
interface Loopback10
 ip address 198.51.100.1 255.255.255.0
exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
exit
router bgp 65001
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 192.0.2.2 remote-as 65024
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.2 activate
  network 198.51.100.0 mask 255.255.255.0
 exit-address-family
end

R2

hostname R2
ip cef
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 no shutdown
exit
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.2
 network 10.255.0.2 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.5 0.0.0.0 area 0
exit
router bgp 65024
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 192.0.2.1 remote-as 65001
 neighbor 10.255.0.4 remote-as 65024
 neighbor 10.255.0.4 update-source Loopback0
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.1 activate
  neighbor 10.255.0.4 activate
  neighbor 10.255.0.4 next-hop-self
 exit-address-family
end

R3

hostname R3
ip cef
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 no shutdown
exit
interface GigabitEthernet2
 ip address 192.0.2.9 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.3
 network 10.255.0.3 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.6 0.0.0.0 area 0
 network 192.0.2.9 0.0.0.0 area 0
end

R4

hostname R4
ip cef
interface Loopback0
 ip address 10.255.0.4 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.10 255.255.255.252
 no shutdown
exit
interface GigabitEthernet2
 ip address 192.0.2.13 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.4
 network 10.255.0.4 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.10 0.0.0.0 area 0
exit
router bgp 65024
 bgp router-id 10.255.0.4
 no bgp default ipv4-unicast
 neighbor 10.255.0.2 remote-as 65024
 neighbor 10.255.0.2 update-source Loopback0
 neighbor 192.0.2.14 remote-as 65005
 address-family ipv4 unicast
  no auto-summary
  neighbor 10.255.0.2 activate
  neighbor 10.255.0.2 next-hop-self
  neighbor 192.0.2.14 activate
 exit-address-family
end

R5

hostname R5
ip cef
interface Loopback0
 ip address 10.255.0.5 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.14 255.255.255.252
 no shutdown
exit
router bgp 65005
 bgp router-id 10.255.0.5
 no bgp default ipv4-unicast
 neighbor 192.0.2.13 remote-as 65024
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.13 activate
 exit-address-family
end

Open plain-text configurations

Outputs are illustrative, not recorded router results. Each lab starts from its own baseline. Configuration and feature support vary by image and release; platform-specific examples identify their prerequisites.

Observe the forwarding gap

R5# show ip bgp
     Network          Next Hop        Metric LocPrf Weight Path
*>   198.51.100.0/24  192.0.2.13           0             0 65024 65001 i

R3# show ip route 198.51.100.0
% Network not in table

With synchronization disabled, R4 can advertise a usable iBGP route externally. But traffic crossing R3 toward R1’s /24 has no matching customer route there. The control plane can look healthy while transit forwarding fails.

Apply the historical rule

On an older image that implements the command, enabling synchronization makes R4 require IGP knowledge of the iBGP-learned destination before using/exporting it. Current images may omit this legacy command; do not assume the experiment is supported everywhere.

router bgp 65024
 address-family ipv4 unicast
  synchronization
 exit-address-family

R4# show ip bgp 198.51.100.0/24
BGP routing table entry for 198.51.100.0/24
  65001
    10.255.0.2 from 10.255.0.2
      Origin IGP, metric 0, localpref 100, valid, internal
      not synchronized

R5# show ip bgp 198.51.100.0/24
% Network not in table

Fix forwarding rather than restoring Internet routes into an IGP

Historical deployments redistributed destinations into the IGP to satisfy synchronization. Carrying a large external table in an IGP is not a modern scaling solution. Instead, make transit routers BGP-aware where appropriate, or use a correctly designed labeled/tunneled core whose edge and transport reachability are verified. Simply disabling the check does not repair R3’s missing forwarding information.

Restore the lab setting

router bgp 65024
 address-family ipv4 unicast
  no synchronization
 exit-address-family

Apply on R4 where the legacy command is supported, then restore the saved baseline. The route can return to R5 while R3’s data-plane gap remains intentionally visible. This lesson explains old configurations; synchronization is not a recommendation for a new design.

Key Takeaways

Synchronization is a legacy IGP-validation rule for iBGP destinations.
An established session does not prove working transit forwarding.
Next-hop reachability and destination reachability are different.
A BGP-free core requires a suitable forwarding design.
Current software may not implement the historical synchronization command.

Protocol and command references

BGP specification · Router-symbol attribution.

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