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400-007 · Question #141

Company XYZ has implemented policy-based routing in their network. Which potential problem must be kept in mind about network reconvergence and PBR?

The correct answer is B. It can create microloops during reconvergence. Policy-based routing operates independently of the routing protocol, so during reconvergence the mismatch between PBR decisions and the updating routing table can create temporary forwarding loops called microloops.

Designing Network Infrastructure

Question

Company XYZ has implemented policy-based routing in their network. Which potential problem must be kept in mind about network reconvergence and PBR?

Options

  • AIt can limit network scalability
  • BIt can create microloops during reconvergence
  • CIt increases convergence time.
  • DIt reduces convergence time.

How the community answered

(25 responses)
  • A
    12% (3)
  • B
    76% (19)
  • C
    8% (2)
  • D
    4% (1)

Why each option

Policy-based routing operates independently of the routing protocol, so during reconvergence the mismatch between PBR decisions and the updating routing table can create temporary forwarding loops called microloops.

AIt can limit network scalability

PBR does not inherently limit network scalability; it adds configuration complexity but does not constrain the number of routes or devices the network can support.

BIt can create microloops during reconvergenceCorrect

During network reconvergence after a topology change, the routing protocol updates the RIB incrementally across routers, but PBR route maps continue forwarding based on potentially stale next-hop or interface information. This inconsistency between PBR-forwarded traffic and the converging routing table can cause packets to loop between two routers before the routing table fully stabilizes - the definition of a microloop.

CIt increases convergence time.

PBR does not increase convergence time because convergence is driven entirely by the routing protocol (e.g., OSPF, EIGRP), not by PBR policies.

DIt reduces convergence time.

PBR has no interaction with the routing protocol convergence mechanism and therefore cannot reduce convergence time.

Concept tested: PBR microloops during network reconvergence

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_pi/configuration/xe-16/iri-xe-16-book/iri-policy-route-map.html

Topics

#policy-based routing#microloops#network reconvergence#routing design

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