Implementing Shared Services Across VRFs Without Firewalls

300-610 By islever Editorial Last updated

Evaluate options for Layer 3 connectivity Evaluate network management and redundancy strategies

Short answer

A BGP is needed to implement route leaking between VRFs to share the DNS service.

Question

An engineer must design a DNS service available to multiple network zones as a shared service. The network zones are deployed as VRFs within the data center network and no firewall is available for communication between VRFs.
Which protocol is needed to implement the shared services?
  1. BGP Correct Answer
  2. OSPF
  3. ISIS
  4. VXLAN

Key Takeaways

The trap is choosing VXLAN for segmentation instead of BGP for route leaking; VXLAN handles tenant overlay, while BGP handles inter-VRF route distribution.

Learn how to use BGP route leaking to provide shared DNS services across multiple VRFs in a data center when firewalls are unavailable.

Common mistake: Choosing OSPF because it is a common routing protocol, but it lacks the native VRF route-target import/export flexibility that BGP offers for shared services.

Expert Analysis

VRF Isolation and Route Leaking Constraints

The scenario presents a classic data center networking challenge: providing connectivity to a shared service (DNS) across multiple isolated VRFs without a firewall. The critical constraint is the absence of a firewall, which eliminates Layer 3 security policies but leaves routing mechanisms available for traffic steering.

Why BGP is Correct In Cisco IOS-XR and modern enterprise routers, BGP is the standard protocol for implementing route leaking between VRFs in a shared services context. While OSPF and static routes can be replicated using specific route-map techniques, BGP provides a scalable, policy-driven mechanism via Route Distinguishers (RDs) and Route Targets (RTs). This allows the DNS server's routes to be imported into all relevant VRFs while maintaining isolation for other traffic. Comments referencing "route replication" often mistakenly cite OSPF; however, OSPF route redistribution into VRFs is typically limited or requires complex manual redistribution maps that do not scale well for dynamic shared services compared to BGP's native VRF-aware capabilities.

Why Other Options Fail

  • OSPF (B): OSPF is an IGP designed for single-domain routing. It does not natively support multi-VRF route leaking in the same flexible, policy-based manner as BGP. While possible with heavy configuration, it is not the primary design choice for shared services across VRFs.
  • ISIS (C): Similar to OSPF, ISIS is an IGP and lacks the built-in VRF import/export policy framework required for shared services.
  • VXLAN (D): VXLAN is an overlay technology used for tenant segmentation and scalability, primarily at Layer 2/Layer 3 boundaries in cloud environments. It does not solve the intra-data-center VRF route leaking problem directly. As noted by some commenters, VXLAN adds complexity without addressing the fundamental routing isolation issue between VRFs on the underlay network.

Exam Strategy

When you see 'shared service' and 'VRFs' together, immediately think about route leaking mechanisms. BGP is the most robust tool for this due to its Route Target (RT) community attributes. Ignore overlay technologies like VXLAN if the question focuses on internal VRF communication rather than tenant sprawl.

Frequently Asked Questions

Can OSPF be used for VRF route leaking?

While technically possible through manual redistribution, OSPF is not ideal for scalable shared services compared to BGP, which uses Route Targets for precise import/export control.

Does VXLAN replace VRFs?

No, VXLAN is an overlay encapsulation. VRFs still exist within the VXLAN environment to provide logical isolation. VXLAN does not inherently solve inter-VRF routing issues.

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Community Votes

D
50%
A
25%
B
25%

50% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 6 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 6

  1. Fcpoultry 👍 2
    VXLAN is not an appropriate solution, as you'll allow the traffic between all VRF as there is no control access via VXLAN, so there is no restriction among all VRFs as one routing table.
  2. Fcpoultry 👍 1 Answered B
    Route replication is supported for static, EIGRP, and OSPF routes. It is not possible to replicate routes to and from BGP, but that is not an issue because the BGP import and export method of copying routes between VRFs is available in a virtual network. https://www.cisco.com/en/US/docs/ios-xml/ios/evn/configuration/xe-3sg/evn-shared-svcs.html
  3. zeppie 👍 1 Answered D
    The key here is that the question refers to data center network and shared services. Within a router you would be able to use for instance BGP or OSPF to perform the route leaking, but VXLAN is the answer for the data center since it supports the feature natively.
  4. garza_javi 👍 1 Answered A
    When DNS must be shared across multiple network zones implemented as separate VRFs, BGP (Border Gateway Protocol) can be used to enable route leaking between VRFs, allowing each zone to reach the shared DNS service.
  5. lurker8000 👍 1 Answered D
    VXLAN is correct, there was no VRF isolation requirement. The requirement was a shared service that had DNS that should be reachable to everything in the network.
Show 1 more comment
  1. b44c199 👍 1
    As you mentioned, it refers to BGP？

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