Route Preference and Longest Match

Two different questions the router must answer

Students often mash selection rules into one vague idea of “best route.” HCIA scores higher when you split the problem:

1. Which prefix matches this destination IP? → Longest-prefix match
2. Which source’s route for that prefix should we trust? → Preference (Pre)
3. Which of the remaining equal candidates is best? → Cost/metric (and platform tie-breakers)

This section drills preference tables and the interaction with longest match until the order is automatic under exam pressure.

Longest-prefix match (again, because items keep testing it)

Given destination IP D, collect all routes whose network address and mask cover D. Choose the route with the largest mask length (most specific). That route’s next hop is used regardless of whether another covering prefix has a “better” preference number.

Summarization intentionally installs shorter prefixes. That is useful for scaling, but it also means a stray more-specific route elsewhere can steal traffic for a subset of the summary. Operators use that on purpose (traffic engineering) or hit it by accident (asymmetric leaks).

Huawei preference: lower wins

Preference on Huawei VRP is the analog of Cisco administrative distance. It expresses how much the router trusts the protocol source of a route to a given prefix.

If two routes both offer exactly 192.168.1.0/24, Pre 10 beats Pre 60. If one offers /24 and one offers /16, longest match already picked the /24 before preference compared them as peers.

Default preference values to memorize for HCIA

Huawei training materials emphasize this common set (values you should know cold):

Notes that keep you out of trouble:

• Direct 0 explains why you cannot casually “override” a connected network with a normal static without unusual design—connected wins on the same prefix length at Pre 0.
• OSPF 10 vs Static 60 means that if OSPF and static both install the same prefix, OSPF wins by default. To force a static to win, raise OSPF’s effective competitiveness (not typical at HCIA) or lower the static’s preference below 10 (for example preference 5)—a deliberate design choice.
• OSPF ASE 150 reminds you that not all OSPF-looking routes share preference 10. Externals are different.
• IBGP 255 is nearly “last resort” among the listed sources; enterprise exit selection often depends on policy beyond HCIA depth, but the number appears in preference tables.

Different Huawei documents may list additional protocol preferences (EBGP, other IGPs). For H12-811, nail the common table above first.

Seeing preference in the CLI

<Huawei> display ip routing-table

Locate the Pre column. Compare only among rows that share the same Destination/Mask. Then look at Cost for metric within that selection context.

<Huawei> display ip routing-table protocol static
<Huawei> display ip routing-table protocol ospf

Protocol filters help you see candidates that lost selection (depending on platform display behavior for inactive routes). Even when only active routes show, knowing defaults lets you predict winners before lab verification.

Cost is not preference

Two OSPF routes to the same /24 both have Pre 10; the lower OSPF cost wins (or ECMP if costs equal and multipath allowed). A static with Pre 60 does not “use cost 0 to beat OSPF”—preference already eliminated it if OSPF offers the same prefix at Pre 10.

Exam trap wording: “lowest cost always wins.” False if preference differs or if longest match already chose another prefix.

Customizing static preference

Static routes accept an explicit preference:

[Huawei] ip route-static 192.168.80.0 255.255.255.0 10.1.1.2 preference 40
[Huawei] ip route-static 192.168.80.0 255.255.255.0 10.1.1.6 preference 80

Both lines can exist in configuration; only the best preference for that prefix is selected for active use (assuming next hops resolve). When the primary next hop fails, the backup can install. That is the core of floating static routes detailed in the next section.

To make a static beat OSPF for the same prefix (rare but taught for understanding):

[Huawei] ip route-static 192.168.80.0 255.255.255.0 10.1.1.2 preference 5

Pre 5 is better than OSPF’s 10 → static wins for that identical prefix. Production designs must consider what happens when you hide OSPF’s path this way—often you only do it for carefully controlled exceptions.

Worked scenarios (train like the exam)

Scenario A — Same prefix, different sources

Table offers:

• 10.10.10.0/24 OSPF Pre 10 Cost 15 NextHop A
• 10.10.10.0/24 Static Pre 60 Cost 0 NextHop B

Active path: OSPF via A. Static remains configuration backup only if it cannot win (or may show as inactive depending on display). Traffic uses A.

Scenario B — Different prefixes

• 10.10.10.0/24 Static Pre 60 via B
• 10.10.0.0/16 OSPF Pre 10 via A

Packet to 10.10.10.5 → static /24 via B (longest match). Packet to 10.10.20.5 → OSPF /16 via A.

Scenario C — Default vs specific

• 0.0.0.0/0 Static Pre 60 via edge
• 172.16.0.0/16 OSPF Pre 10 via core

Packet to 172.16.5.5 → OSPF /16. Packet to 8.8.8.8 → default static (if no more specific ISP routes).

Scenario D — OSPF internal vs ASE (conceptual)

If an internal OSPF route (Pre 10) and an OSPF external (ASE Pre 150) both somehow target the same prefix length, internal preference wins. Exact external origination is deeper OSPF content; the preference numbers still matter when reading tables.

Decision flowchart you can redraw from memory

Packet destination D
        |
        v
Find all covering prefixes in routing table
        |
        v
Pick longest mask  ---------------------> use that prefix
        |
        v
Multiple routes for THAT prefix?
   no -> use the only route
   yes -> lowest Preference wins
              |
              v
        still tied? -> lowest Cost / multipath rules
              |
              v
        Resolve next hop / ARP / forward

If you can reproduce this flowchart, preference trick questions lose their power.

Preference direction: Huawei vs “higher is better” myths

Some tools and human conversations say “higher priority.” On Huawei preference, lower numeric value wins. Floating static backups use higher preference values (worse trust) so they stay standby. If you reverse this—setting backup to preference 10 and primary to 100—you invert production traffic. The next section builds entirely on that directionality.

Compare with cost: lower cost is also better, but cost is not preference. Two numbers, two meanings, same “lower is better” direction for both—yet they apply at different steps.

Practical verification checklist

1. Identify destination IP in the question.
2. List matching prefixes and masks.
3. Circle the longest mask.
4. If multiple sources for that mask, compare Pre (defaults if not stated).
5. If still tied, compare Cost.
6. Read next hop/interface; sanity-check reachability.

Exam traps

• Reversing preference direction (thinking 60 beats 10).
• Using preference to override a longer prefix.
• Mixing OSPF 10 with OSPF ASE 150 as if identical.
• Assuming static always wins because “admin configured it.”
• Forgetting Direct 0 beats essentially everything on the same prefix.
• Comparing cost across different protocols as if preference did not exist.

What mastery looks like

You glance at a three-row routing table excerpt and immediately say which row forwards a given packet—and you can explain the losing rows without hand-waving. That skill transfers directly to OSPF troubleshooting and to floating static design next.