352-001 · Question #563
In an OSPF network with 20 routers connected together with Ethernet cabling , which topology typically takes the longest to converge?
The correct answer is B. Ring. In an OSPF network of 20 Ethernet-connected routers, a ring topology takes the longest to converge because LSAs must propagate around the entire ring and only a single alternate path exists after a failure.
Question
In an OSPF network with 20 routers connected together with Ethernet cabling , which topology typically takes the longest to converge?
Options
- AFull mesh
- BRing
- CSquared
- DTriangulated
- EPartial mesh
How the community answered
(67 responses)- A1% (1)
- B84% (56)
- C1% (1)
- D4% (3)
- E9% (6)
Why each option
In an OSPF network of 20 Ethernet-connected routers, a ring topology takes the longest to converge because LSAs must propagate around the entire ring and only a single alternate path exists after a failure.
Full mesh provides direct adjacencies between all routers, so LSAs spread immediately to all neighbors and convergence is extremely fast.
A ring topology has exactly two paths between any two routers; when a link or node fails, LSAs must traverse the entire remaining ring segment to reach all routers, maximizing flood diameter and SPF recalculation time compared to any topology with more redundant links.
A squared (grid) topology is not standard OSPF terminology, but a grid provides multiple equal-cost paths that reduce convergence time compared to a ring.
A triangulated topology implies multiple triangular redundant paths, giving routers more next-hop options and faster SPF recalculation.
A partial mesh still provides more redundant links than a ring, reducing the flood diameter and convergence time.
Concept tested: OSPF convergence time vs. network topology
Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html
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