Making a WAN Edge the Less Preferred OSPF Exit

300-415 By islever Editorial Last updated

Configure Cisco Catalyst SD-WAN data plane Configure TLOCs

Short answer

C Advertise an OSPF summary route from the secondary WAN Edge so its less-specific prefix loses to the primary edge's more-specific routes but still provides failover.

Question

Which OSPF command makes the WAN Edge router a less preferred exit from a site with a dual WAN Edge design?
  1. [sampleXML/xmlfile-41_1.png]
  2. [sampleXML/xmlfile-41_2.png]
  3. [sampleXML/xmlfile-42_1.png] Correct Answer
  4. [sampleXML/xmlfile-42_2.png]

Key Takeaways

Tests how OSPF exit preference is engineered on the LAN/service side of a dual-edge design; the trap is picking the stub-router max-metric command, which in option A is missing its mandatory router-lsa keyword.

In a dual-WAN-Edge site, OSPF exit preference is set by route summarisation: option C injects a less-specific summary so the primary edge's specific prefixes win, while the bare max-metric in option A is syntactically incomplete without router-lsa.

Common mistake: Option A (`max-metric`) — conceptually it is the stub-router feature, but as written it lacks the required `router-lsa` keyword, so the router rejects it with an incomplete-command error.

Expert Analysis

Reading the scenario correctly The question is a dual-WAN-Edge site design problem: one edge must remain the preferred exit while the other still provides reachability if the primary fails. The technique must therefore leave a route installed on the underlay neighbours, but with lower preference, so the backup is only used after the primary's routes disappear.

Why option A fails on syntax, not on concept The stub-router (max-metric) feature is a legitimate OSPF design tool: advertising maximum link-state cost lets OSPF converge without attracting transit traffic. However, in Cisco Catalyst SD-WAN the CLI path is vpn <id> -> router -> ospf -> max-metric router-lsa. Option A shows bare max-metric, which the router rejects with an incomplete-command error, so as written it cannot be applied. The comments confirm this precisely: entering max-metric in (config-ospf) returns a syntax error with router-lsa as the only completion, and the same source also rejects option B (nssa no-summary), which is an NSSA area type command with no bearing on exit preference.

Why option C works: summarisation changes preference by specificity Option C advertises an OSPF summary (less-specific) route from the secondary edge into the site. OSPF installs both edges' prefixes, but longest-match forwarding selects the more-specific prefix from the primary edge, so it stays the preferred exit. Because the secondary still injects a summary rather than withdrawing, the route survives the primary's failure and takes over — exactly the dual-edge behaviour required. This is why the routing-based reasoning wins over the SD-WAN-only reading: summarisation makes the router less preferred without destroying backup capability.

Verdict The correct answer is C. It is the only listed command that is syntactically valid and achieves "less preferred exit" while preserving failover.

Exam Strategy

When an OSPF design question asks about exit preference, judge each option on two axes: command validity and routing outcome. Reject any option that is an incomplete or misplaced syntax fragment before weighing intent. Then ask whether the command still leaves a usable backup route after the preferred path is lost — options that zero out or invalidate routes fail the dual-edge design requirement.

Frequently Asked Questions

Why doesn't the bare `max-metric` OSPF command work on a WAN Edge router?

In Cisco Catalyst SD-WAN the OSPF stub-router command requires the router-lsa keyword: max-metric router-lsa. Entered without it, the CLI reports an incomplete command and the configuration is rejected.

How does summarisation make one WAN Edge the less preferred exit?

The secondary edge advertises a summary prefix that is less specific than the primary edge's routes, so longest-match forwarding always selects the primary. If the primary fails, the summary still provides reachability, keeping a working backup.

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Discussion 11 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 11

  1. konspali 👍 3
    Stop thinking only about SDWAN commands and think of the Routing portion. By doing summarization we instantly make the router less preferred, as the other one will have more specific routes. Correct answer should be C.
  2. Matt2727 👍 2
    whilst max-metric would work... it would need the syntax with area number & router-lsa at the end, both are missing. C looks like the best bet here.
  3. Stanleymahamadi 👍 2
    Correct answer C
  4. Jervv 👍 1
    A would be correct if there is "router-lsa" vEdge-INET-1# conf t Entering configuration mode terminal vEdge-INET-1(config)# vpn 1 vEdge-INET-1(config-vpn-1)# router vEdge-INET-1(config-router)# ospf vEdge-INET-1(config-ospf)# max-metric --------------------------------------^ syntax error: incomplete path vEdge-INET-1(config-ospf)# max-metric ? Possible completions: router-lsa Advertise own router LSA with infinite distance vEdge-INET-1(config-ospf)# max-metric
  5. Precission21 👍 1
    its not, doing "max-metric" will cause router to advertise it with infinite metric, even if primary path will be lost backup won't be installed since metric is infinite hence invalid to install. C is correct because you summarize prefix to make it less specific so it's not preferred when main route is installed but when the main is lost this summary will come into play
Show 6 more comments
  1. incog 👍 1
    A looks like the option: The OSPFv3 max-metric router LSA feature enables OSPFv3 to advertise its locally generated router LSAs with a maximum metric. The feature allows OSPFv3 processes to converge but not attract transit traffic through the device if there are better alternate paths.
  2. westh4m1234 👍 1
    Answer is A - How It Works • max-metric: This OSPF stub router / max-metric feature advertises maximum link-state cost for the router's routes. • Path Selection: Neighboring routers see the high metric and choose an alternative, more preferred path/exit out of the site.
  3. Outlaw_87 👍 1
    vpn vpn-id --> router --> ospf --> max metric not accepted, only accepts if you write router-lsa after max-metric vpn vpn-id --> router --> ospf --> area number --> nssa --> no-summary is ACCEPTED in vEdge
  4. Idro 👍 1
    A Is correct : https://sdwan-docs.cisco.com/Product_Documentation/vManage_Help/Release_18.2/Configuration/Templates/OSPF
  5. Jervv 👍 1
    If you enter A and B on a vEdge, these commands are not accepted. Go for C
  6. Roger95 👍 1
    try with "area number "

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