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352-001 · Question #616

When is it required to leak routes into an IS-IS level 1 area?

The correct answer is A. When MPLS L3VPN PE devices are configured in the level 1 areas. Route leaking from IS-IS Level 2 into Level 1 is required when MPLS L3VPN PE devices reside in L1 areas, because L1 routers only receive a default route and lack the specific prefix reachability needed for LDP and BGP PE-to-PE sessions.

Layer 3 Control Plane

Question

When is it required to leak routes into an IS-IS level 1 area?

Options

  • AWhen MPLS L3VPN PE devices are configured in the level 1 areas
  • BWhen unequal cost load balancing is required between the backbone and nonbackbone areas
  • CWhen a multicast RP is configured in the nonbackbone area
  • DWhen equal cost load balancing is required between the backbone and nonbackbone areas

How the community answered

(32 responses)
  • A
    78% (25)
  • B
    3% (1)
  • C
    13% (4)
  • D
    6% (2)

Why each option

Route leaking from IS-IS Level 2 into Level 1 is required when MPLS L3VPN PE devices reside in L1 areas, because L1 routers only receive a default route and lack the specific prefix reachability needed for LDP and BGP PE-to-PE sessions.

AWhen MPLS L3VPN PE devices are configured in the level 1 areasCorrect

IS-IS Level 1 routers only receive a default route pointing toward the L1/L2 border router and do not have specific inter-area prefix knowledge by default. MPLS L3VPN PE devices require specific loopback reachability to remote PE routers to form LDP label distribution sessions and iBGP VPNv4 peerings. Without leaking those L2 loopback prefixes into L1, the PE devices in the nonbackbone area cannot establish the necessary control-plane sessions for MPLS forwarding.

BWhen unequal cost load balancing is required between the backbone and nonbackbone areas

IS-IS does not support unequal cost load balancing natively, making this scenario inapplicable as a driver for route leaking from L2 into L1.

CWhen a multicast RP is configured in the nonbackbone area

Multicast RP placement does not inherently require IS-IS route leaking - PIM uses its own RP discovery mechanisms (BSR, Auto-RP, or static) that are independent of IS-IS level boundaries.

DWhen equal cost load balancing is required between the backbone and nonbackbone areas

Equal cost load balancing between backbone and nonbackbone areas can function using the default routes generated by L1/L2 border routers without requiring explicit prefix leaking into L1 areas.

Concept tested: IS-IS L2-to-L1 route leaking for MPLS PE reachability

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-16/irs-xe-16-book/irs-route-leak.html

Topics

#IS-IS#route leaking#L1 area#MPLS L3VPN

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