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352-001 · Question #309

The Customer X network consists of an MPLS core, IS-IS running as IGP, a pair of BGP route reflectors for route propagation, and a few dozens of MPLS-TE tunnels for specific tactical traffic engineeri

The correct answer is B. It only affects the use of the respective node as a transit node, and networks advertised within the C. It can be set on a router during the startup process for a fixed period of time. E. It can be set on a router until other interacting protocols have signaled convergence.. The IS-IS Overload Bit signals other routers to exclude an overloaded node from transit SPF calculations while still keeping directly attached prefixes reachable, and can be set on a timer or until convergence is confirmed.

Layer 3 Control Plane

Question

The Customer X network consists of an MPLS core, IS-IS running as IGP, a pair of BGP route reflectors for route propagation, and a few dozens of MPLS-TE tunnels for specific tactical traffic engineering requirements. The customer engineering department has some questions about the use of the Overload Bit in the IS-IS networks and how it could be used to improve their current network design. Which three options about the Overload Bit are true? (Choose three.)

Options

  • AIt is not recommended on BGP route reflectors.
  • BIt only affects the use of the respective node as a transit node, and networks advertised within the
  • CIt can be set on a router during the startup process for a fixed period of time.
  • DIt forces the midpoint MPLS-TE node to reoptimize the primary tunnels going through the OL
  • EIt can be set on a router until other interacting protocols have signaled convergence.

How the community answered

(28 responses)
  • A
    11% (3)
  • B
    82% (23)
  • D
    7% (2)

Why each option

The IS-IS Overload Bit signals other routers to exclude an overloaded node from transit SPF calculations while still keeping directly attached prefixes reachable, and can be set on a timer or until convergence is confirmed.

AIt is not recommended on BGP route reflectors.

Setting the overload bit on BGP route reflectors during startup is a well-established and recommended practice to prevent them from carrying transit traffic before their BGP tables are fully synchronized.

BIt only affects the use of the respective node as a transit node, and networks advertised within theCorrect

When the overload bit is set, other IS-IS routers exclude that node from their SPF shortest-path tree for transit traffic, but prefixes directly advertised by that node (connected and redistributed networks) remain reachable so local connectivity is preserved.

CIt can be set on a router during the startup process for a fixed period of time.Correct

IOS supports the 'set-overload-bit on-startup [seconds]' command, which sets the overload bit for a fixed configurable time period after a router restarts, preventing it from attracting transit traffic before its routing table is fully populated.

DIt forces the midpoint MPLS-TE node to reoptimize the primary tunnels going through the OL

When a midpoint node sets the overload bit, MPLS-TE head-end routers re-run CSPF to find alternate paths that avoid the overloaded node - it is the head-end that initiates reoptimization, not the midpoint node itself.

EIt can be set on a router until other interacting protocols have signaled convergence.Correct

The 'set-overload-bit on-startup wait-for-bgp' command keeps the overload bit set until BGP has converged and signaled readiness, ensuring the router does not become a transit hop before it has a complete BGP forwarding table.

Concept tested: IS-IS Overload Bit behavior and startup use cases

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-16/iri-xe-16-book/iri-overload-bit.html

Topics

#IS-IS#overload bit#BGP route reflector#MPLS-TE

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