In complex enterprise networks, route redistribution becomes necessary when multiple routing protocols coexist. OSPF, being a link-state protocol, offers flexible methods to manage redistributed routes and control which prefixes enter the OSPF domain.
Scenario Overview
Imagine a network running OSPF internally but connected to an external EIGRP domain. The design goal is to redistribute EIGRP routes into OSPF, but only allow specific prefixes that match internal policy guidelines.
Configuration Walkthrough
router ospf 1
redistribute eigrp 100 subnets route-map EIGRP_TO_OSPF
!
route-map EIGRP_TO_OSPF permit 10
match ip address prefix-list ALLOWED_ROUTES
!
ip prefix-list ALLOWED_ROUTES seq 5 permit 10.10.0.0/16 le 24
Explanation and Rationale
The route-map EIGRP_TO_OSPF applies a prefix-list
filter that only allows 10.10.x.x/24 subnets to be redistributed. This
limits route noise and prevents unintended prefixes from being
advertised into OSPF. The use of le 24 enables inclusion of subnets within the permitted /16 range up to /24 granularity.
Verification and Testing
After implementing the configuration, validate the results using:
show ip ospf database external
show ip route ospf
You should see only the allowed prefixes from EIGRP appearing in the OSPF routing table as type 5 LSAs.
Conclusion
Prefix-based filtering during route redistribution provides control and minimizes routing instability. Using route-maps and prefix-lists together enables fine-tuned policies that align with design intents, reduce administrative overhead, and improve routing convergence stability in mixed-protocol networks.