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Cisco ENARSI 300-410 study materials
Cisco ENARSI 300-410 study resources

Cisco ENARSI 300-410: Layer 3 Technologies

Cisco ENARSI 300-410 · Work through your exam, one topic at a time.

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Layer 3 Technologies

Layer 3 Technologies is the largest ENARSI domain and tests implementation and troubleshooting of EIGRP, OSPF, BGP, redistribution, policy-based routing, and VRF-lite. Questions typically give a topology and ask which control-plane setting produces the required path or why a prefix is missing.

Core control-plane rules

  • EIGRP uses a composite metric based on bandwidth and delay by default. A feasible successor needs a reported distance lower than the current feasible distance, not merely a lower total metric. Variance only load-balances across feasible successors. Stub routing suppresses queries; summarization is configured per interface.
  • OSPF ABRs generate type 3 summaries; ASBRs originate type 5 externals, and type 7 in NSSAs. Virtual links transit a normal area, not a stub. Summarization is supported at ABRs for inter-area routes and at ASBRs for externals.
  • BGP best-path order starts with weight, local preference, locally originated, AS-path length, origin, MED, eBGP over iBGP, and lowest router ID. iBGP requires a full mesh, route reflectors, or confederations.

Worked example: controlling mutual redistribution

R1 runs OSPF 10 and EIGRP 100. OSPF subnet 10.10.0.0/16 must enter EIGRP with a seed metric, but EIGRP routes must not feed back into OSPF. Create a prefix list for 10.10.0.0/16, permit it in a route map that sets tag 100, and apply that route map to redistribute ospf 10 under EIGRP with metric 100000 100 255 1 1500. Under OSPF, deny tag 100 and permit other routes in a second route map, then use redistribute eigrp 100 subnets route-map .... The seed metric is required because EIGRP cannot derive a metric from OSPF cost. The tag prevents the redistributed prefix from being re-originated into OSPF as an external route. Verify with show ip route, show ip eigrp topology, and show ip ospf database external.

Traps

  • Advertising into EIGRP without a metric silently produces no route.
  • default-information originate needs an existing default route unless always is added.
  • OSPF stub and nssa are incompatible on the same area; type 7 is translated at the NSSA ABR.
  • iBGP-learned routes keep the eBGP next hop unless next-hop-self is configured.
  • Administrative distance is a tiebreaker between sources, not a substitute for longest-prefix matching.