Some clients of MAN in one city complain that they cannot access network at some time segments in one evening. On the evening one core router changes the board. The complaint time is just upgrade time. Two routers are used in the city. When one router is upgraded, the traffic should pass another one. After the upgrade it is normal.
Change OSPF default route of two core routers as non-forcible advertisement. When uplink is ineffective, problematic router will use default route learnt by another router and not advertise default route. When two links are ineffective, default route is not advertised. The problem of loop and non-optimized path is solved.
Two routers are interconnected in core layer. The router in convergence layer sets up OSPF neighbor relation with two core routers through double-uplink. That day only one board is changed. The board connects with provincial router and another router of the city through GE link. The board connecting with router in convergence router is not changed. The router in convergence layer advertises default route through OSPF and guides traffic. Check configuration of core router. It advertises default route with forced way. Through the analysis of protocol behavior, when the link connecting provincial backbone and another core router is disconnected, the router cannot learn default route through other protocols, including the route advertised by provincial backbone and another core router through BGP. It is advertised with forced way, so the router still advertises default route to the router in convergence layer. The router learns one default route through the router in convergence layer. The router in convergence layer uses load balancing based on traffic. Some traffic flows to core router with the change of the board, but the router re-send the traffic to device in convergence layer. There is loop and parts of users cannot access network. If the link connecting core router and provincial backbone is problematic but the interconnection is normal, users can still access network. But some traffic will transit multi-hop and all traffic flows to provincial backbone through uplink. It is not the best traffic module.