Routing blackhole occurs when OSPF and IS-IS import routes from each other


When OSPF and IS-IS import routes to each other, packets cannot be forwarded based on routes due to the sequence of configuration operations and route convergence.
To solve the problem, perform the following operations:
1.When routes of routing protocols are distributed, filter imported routes to prevent loops.
2. When routes of routing protocols are imported, the route priority changes. Notice the route selection change due to imported routes and take measures to control routes.
For details, see Routing Blackhole Occurs When OSPF and IS-IS Import Routes From Each Other.

Other related questions:
Can OSPF import IBGP routes
In V200R001C00, open shortest path first (OSPF) cannot import Internal Border Gateway Protocol (IBGP) routes. Starting from V200R001C01, the OSPF protocol can import IBGP routes. By default, the OSPF protocol does not import IBGP routes. However, OSPF can import IBGP routes on Multi-VPN-Instance CE (MCE) devices. You can run the vpn-instance-capability simple command for OSPF processes to disable route loop detection and directly calculate routes.

How can I set the cost value when OSPF imports external routes
When OSPF imports external routes, the cost of original routes is not used. By default, the cost of the external routes imported by OSPF is 1. To change the default cost value of external routes imported by OSPF, run the default { cost { cost | inherit-metric } command. In the preceding command: -cost: specifies the cost of the imported external routes. The value is an integer ranging from 0 to 16777214. -inherit-metric: indicates that the cost of the imported external routes is the cost carried in the route.

Solution to route flapping when routes are imported to each othe
When static routes, OSPF routes, and IS-IS routes are imported to each other, the source route will be changed because the priority of each routing protocol is different. Solution: Frequent route flapping consumes much bandwidth and CPU resources and even seriously affects the normal operation of a network. When routing protocols import routes from each other, exercise caution. In particular, when multiple routing protocol import routes from each other, route flapping or even loop easily occurs. For details, see Route Flapping Occurs When Multiple Routing Protocols Import Routes from Each Other on AR Routers.

Why cannot OSPF import BGP routes
As defined in RFC 1364, OSPF cannot import IBGP routes. Routes learned via IBGP must not be imported into OSPF OSPF, however, can import IBGP routes on a PE. BGP routes exist in a VPNv4 routing table on a device functioning as a PE but cannot be imported to OSPF. This is because the role of the device changes from a PE to an MCE after the vpn-instance-capability simple command in run in the OSPF process. Solution: If the device needs to function as a PE, you need to run the undo vpn-instance-capability command on the device.

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