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350-501 · Question #342

An engineering team must implement Unified MPLS to scale an MPLS network. Devices in the core layer use different IGPs, so the team decided to split the network into different areas. The team plans…

The correct answer is C. Redistribute the IGP prefixes into another IGP to ensure end-to-end LSPs. To implement Unified MPLS with different IGPs in the core layer while maintaining existing MPLS services, engineers must redistribute IGP prefixes between the different IGP domains to ensure end-to-end reachability for PE router loopbacks, enabling LDP to establish transport…

Architecture

Question

An engineering team must implement Unified MPLS to scale an MPLS network. Devices in the core layer use different IGPs, so the team decided to split the network into different areas. The team plans to keep the MPLS services as they are and introduce greater scalability. Which additional action must the engineers take to implement the Unified MPLS?

Options

  • ARedistribute the IGP prefixes from one IGP into the other routers to ensure end-to-end LSPs.
  • BConfigure the ABR routers as route reflectors that redistribute IGP into BGP.
  • CRedistribute the IGP prefixes into another IGP to ensure end-to-end LSPs.
  • DMove the IGP prefixes into IS-IS as the loopback prefixes of the PE routers to distribute the

How the community answered

(42 responses)
  • A
    5% (2)
  • B
    12% (5)
  • C
    81% (34)
  • D
    2% (1)

Why each option

To implement Unified MPLS with different IGPs in the core layer while maintaining existing MPLS services, engineers must redistribute IGP prefixes between the different IGP domains to ensure end-to-end reachability for PE router loopbacks, enabling LDP to establish transport LSPs.

ARedistribute the IGP prefixes from one IGP into the other routers to ensure end-to-end LSPs.

This option describes the same action as C, which is the correct approach to ensure reachability across different IGP domains for Unified MPLS.

BConfigure the ABR routers as route reflectors that redistribute IGP into BGP.

Configuring ABRs as route reflectors that redistribute IGP into BGP is for scaling iBGP and distributing VPN routes, not for establishing end-to-end LDP LSPs by making PE loopbacks reachable across different IGP domains.

CRedistribute the IGP prefixes into another IGP to ensure end-to-end LSPs.Correct

In a Unified MPLS network with distinct IGP domains in the core, it is essential to redistribute IGP prefixes (specifically the loopback addresses of PE routers) from one IGP into another. This ensures that all PE routers have end-to-end IP reachability to each other, which is a prerequisite for establishing targeted LDP sessions and signaling the transport LSPs that carry MPLS VPN services across the disparate IGP domains.

DMove the IGP prefixes into IS-IS as the loopback prefixes of the PE routers to distribute the

Moving all IGP prefixes into IS-IS implies a standardization which might not be feasible or desired; the problem states 'different IGPs', implying they will coexist, making redistribution the appropriate method for interoperation.

Concept tested: Unified MPLS - IGP redistribution for reachability

Source: https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Data_Center/DCI/4-0/DCI4_0_DG.html

Topics

#Unified MPLS#MPLS Scaling#Inter-IGP Routing#PE Loopback Reachability

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