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Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

A edge gateway router receives candidate routes from multiple routing sources for the destination network 172.16.10.0/24172.16.10.0/24. The available entries are:

- An OSPF route with an Administrative Distance of 110110 and a metric cost of 4545
- An iBGP route with an Administrative Distance of 200200 and a metric of 1010
- A floating static route configured with an Administrative Distance of 130130

Which route will the gateway router install into its active routing table for traffic destined to 172.16.10.0/24172.16.10.0/24?

  1. The OSPF route, because its Administrative Distance of 110 is lower than both the floating static route and the iBGP route.Answer
  2. B
    The iBGP route, because its metric value of 10 is lower than the OSPF cost metric of 45.
  3. C
    The floating static route, because static routes inherently override all dynamic routing protocol entries.
  4. D
    Both the OSPF and iBGP routes equally, load-balancing traffic between the two paths.

Answer

The OSPF route will be installed into the routing table because it has the lowest Administrative Distance (110) among the competing routes for the prefix 172.16.10.0/24.
When a router receives multiple routes for the exact same destination network prefix from different administrative sources, it evaluates Administrative Distance (AD) first. Lower AD values represent higher reliability. OSPF has an AD of 110, which is lower than the configured floating static route (130) and the iBGP route (200), so the router populates its routing table with the OSPF path.

Step-by-Step Solution

1
Evaluate the prefix lengths of all candidate routes.
All three candidate routes refer to the exact same prefix length (/24).
Longest prefix match does not filter out any route here since all prefixes are identical.
2
Compare the Administrative Distance (AD) values of the candidate routes.
OSPF AD = 110, Floating Static AD = 130, iBGP AD = 200.
When identical networks are learned from different sources, the route with the lowest Administrative Distance is preferred.
3
Select the route with the lowest Administrative Distance for insertion into the routing table.
The OSPF route (AD 110) is chosen.
110 is less than 130 and 200, so the OSPF route is installed into the active forwarding database.

Key Concept

Administrative Distance Precedence in Route Selection
Estimated Time:1m 15s
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Topics

Routing Concepts and Dynamic Routing Protocols

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