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352-001 · Question #552

You must make IGP redesign recommendations for a client that has old equipment, with low CPU power and memory, that they do not have budget replace. They are very concerned about CPU load on…

The correct answer is A. Use mesh groups to limit flooding of LSAs. In a fully meshed point-to-point IS-IS network, mesh groups prevent redundant LSP re-flooding across multiple interfaces, directly reducing the CPU and processing overhead on low-resource routers.

Layer 3 Control Plane

Question

You must make IGP redesign recommendations for a client that has old equipment, with low CPU power and memory, that they do not have budget replace. They are very concerned about CPU load on routers. They are using IS-IS as the IGP in a single l1 area and all routers are connected to each other with point-to-point links. Which method do you recommend to reduce or limit CPU overhead caused by IS-IS?

Options

  • AUse mesh groups to limit flooding of LSAs
  • BImplement wide style metrics for IS-IS on all routers
  • CSelect a router to act as a pseudowire to limit topology synchronization
  • DDivide the router into multiple areas and implement address summarization

How the community answered

(56 responses)
  • A
    71% (40)
  • B
    9% (5)
  • C
    4% (2)
  • D
    16% (9)

Why each option

In a fully meshed point-to-point IS-IS network, mesh groups prevent redundant LSP re-flooding across multiple interfaces, directly reducing the CPU and processing overhead on low-resource routers.

AUse mesh groups to limit flooding of LSAsCorrect

IS-IS mesh groups allow an administrator to group point-to-point interfaces so that an LSP received on one interface in the group is not re-flooded out the other interfaces in the same group. In a fully meshed topology, every router would otherwise receive duplicate copies of the same LSP from multiple neighbors, causing unnecessary CPU processing. By assigning all interfaces to a mesh group, flooding is suppressed for redundant copies, significantly reducing CPU load without requiring hardware upgrades.

BImplement wide style metrics for IS-IS on all routers

Wide-style metrics extend the IS-IS metric range from 6-bit narrow to 24-bit wide to support traffic engineering, but this change does not reduce CPU overhead from LSP flooding or SPF calculations.

CSelect a router to act as a pseudowire to limit topology synchronization

A pseudowire is a Layer 2 emulation technology unrelated to IS-IS topology synchronization - the concept of a Designated Intermediate System (DIS) applies only to broadcast multi-access links, not point-to-point links, making this option technically inapplicable.

DDivide the router into multiple areas and implement address summarization

Dividing into multiple areas introduces L1/L2 boundary routers and requires significant redesign effort - while it could reduce LSDB size, it does not address the core issue of redundant LSP flooding in a fully meshed single-area topology and is a more invasive change.

Concept tested: IS-IS mesh groups to reduce LSP flooding CPU overhead

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

Topics

#IS-IS#mesh groups#flooding#CPU optimization

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