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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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OSPF-BGP Association

OSPF-BGP Association

Definition

When a new device is deployed in the network or a device is restarted, network traffic may be lost during BGP convergence. This is because IGP convergence is faster than BGP convergence.

This problem can be solved through the synchronization between OSPF and BGP.

Purpose

If a backup link exists, during traffic switchback, BGP traffic is lost because BGP route convergence is slower than OSPF route convergence.

As shown in Figure 5-17, RouterA, RouterB, RouterC, and RouterD run OSPF and establish IBGP connections. RouterC functions as the backup of RouterB. When the network is stable, BGP and OSPF routes converge completely on the device.

Normally, traffic from RouterA to 10.3.1.0/30 passes through RouterB. When RouterB becomes faulty, traffic is switched to RouterC. After RouterB recovers, traffic is switched back to RouterB. During this process, packet loss occurs.

This is because when traffic is switched back to RouterB, IGP route convergence is faster than BGP route convergence. Consequently, convergence of OSPF routes is already complete when BGP route convergence is still going on. As a result, RouterB does not know the route to 10.3.1.0/30.

Therefore, when packets from RouterA to 10.3.1.0/30 arrive at RouterB, they are discarded because RouterB does not have the route to 10.3.1.0/30.

Figure 5-17 OSPF-BGP synchronization

Principle

The device enabled with OSPF-BGP synchronization remains as a stub router within the set synchronization period. That is, the link metric in the LSA advertised by the device is the maximum value 65535. Therefore, the device instructs other OSPF devices not to use it for data forwarding.

As shown in Figure 5-17, OSPF-BGP synchronization is enabled on RouterB. In this situation, before BGP route convergence is complete, RouterA continues to use the backup link RouterC rather than forward traffic to RouterB until BGP route convergence on RouterB is complete.

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Updated: 2019-05-17

Document ID: EDOC1000174069

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