N10-009 Question 142
Single answerDynamic routing: Border Gateway Protocol (BGP), Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF)A midsize company has two internet connections: one to ISP-A and one to ISP-B. The company also runs OSPF internally between its core and branch routers. The network administrator wants the company to advertise its public prefixes to both ISPs and automatically reroute inbound and outbound internet traffic if one provider fails, while keeping OSPF limited to internal route sharing. Which routing solution best meets these requirements?
- A
Configure BGP with both ISPs for external routing, and continue using OSPF only for the internal network
- B
Replace OSPF with EIGRP everywhere, including peering with both ISPs, because EIGRP converges faster than BGP
- C
Use OSPF with both ISPs so the company can exchange internet routes using link-state advertisements
- D
Use static default routes to both ISPs and remove OSPF, because dynamic routing is unnecessary with only two providers
Show answer and explanation
Correct answer: A
Explanation
The best answer is to use BGP for external routing with the ISPs and OSPF for internal routing. In real-world enterprise networks, OSPF commonly operates as the interior gateway protocol (IGP), while BGP is used as the exterior gateway protocol (EGP) to exchange routes between autonomous systems. A multihomed organization that advertises its own public prefixes to multiple ISPs typically uses external BGP sessions with each provider. This allows controlled route advertisement, failover, and path selection. By contrast, OSPF and EIGRP are designed primarily for internal routing within a single organization. This aligns with widely accepted networking best practices and vendor documentation from sources such as Cisco and Juniper, which describe BGP as the standard protocol for interdomain routing and OSPF/EIGRP as IGPs for enterprise internetworks.
- A. Correct.
Correct. BGP is the standard exterior gateway protocol used between autonomous systems, such as a company and its ISPs. In this scenario, the company needs multihoming to two providers, public prefix advertisement, and automatic path changes when an ISP fails. Those are classic BGP use cases. OSPF should remain the interior gateway protocol for exchanging routes inside the organization's own network.
- B. Incorrect.
Incorrect. EIGRP is an interior gateway protocol designed for use within a single administrative domain, not for standard internet routing between autonomous systems. Although EIGRP can converge quickly in enterprise networks, ISPs do not typically use EIGRP to exchange customer internet routes. Choosing EIGRP here reflects the misconception that faster convergence alone makes a protocol suitable for external peering.
- C. Incorrect.
Incorrect. OSPF is also an interior gateway protocol and is intended for routing within an organization, not for exchanging internet routing information with public providers. OSPF uses areas and LSAs internally, but it is not the standard protocol for advertising public routes to multiple ISPs. This option confuses internal dynamic routing with external interdomain routing.
- D. Incorrect.
Incorrect. Static default routes could provide basic outbound connectivity, but they do not meet the requirement to advertise the company's public prefixes to both ISPs or support robust multihoming policy control. Removing OSPF would also break the stated need to keep internal dynamic route sharing. This option is tempting because static routing can work in small environments, but it is not the best fit for dual-provider internet routing.