350-401 Question #257: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. interface Ethernet 0/0 ip policy routemap PBR routemap PBR match ip address 144 set ip nexthop 172.16.12.5 set ip nexthop recursive 192.168.3.2 Which statement describes how a router with this configuration treats packets if the devices at 172.16.12.5 and 192.168.3.2 are unreachable?

Options

• AIt routes the packet using the default routing table.
• BIt routes the packet into a loop and drops it when the TTL reaches zero.
• CIt drops the packet immediately.
• DIt sends an ICMP source quench message.

Explanation

When Policy-Based Routing (PBR) next-hops become unreachable, the router reverts to its normal forwarding behavior, using the standard IP routing table to route the packet.

Common mistakes.

• B. PBR failing to find a next-hop does not inherently lead to routing loops or dropping packets due to TTL expiry; it means the PBR rule is not applied, and normal routing takes over.
• C. The router does not immediately drop the packet if PBR next-hops are unreachable; instead, it attempts to route the packet using the standard routing table.
• D. ICMP source quench messages are used for congestion control, not as a response when PBR next-hops become unreachable.

Concept tested. Policy-Based Routing (PBR) fallback

Reference. https://www.cisco.com/c/en/us/td/docs/ios/12_2/qos/configuration/guide/qcfipbr.html

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