H12-831_V1.0 Exam Dumps : HCIP-Datacom-Advanced Routing & Switching Technology V1.0

To determine the correct command, we need to analyze BGP behavior in a Hub-Spoke topology and how routes are advertised in MPLS VPN environments.

1. Understanding the Hub-Spoke BGP/MPLS VPN Topology

The network consists of Hub-PE and Spoke-PE routers, which are MPLS VPN PEs.

Hub-PE is responsible for distributing routes between Spoke-PE1 and Spoke-PE2.

Spoke routers do not have direct IBGP peering with each other; instead, they rely on the Hub-PE to relay routes.

BGP next-hop handling is critical for proper route advertisement.

2. Why is the " next-hop-self " Command Required?

By default, when BGP advertises a route, the next-hop remains unchanged (i.e., the original next-hop is retained).

In Hub-Spoke topology, Spoke routers would receive routes with a next-hop IP that belongs to another Spoke-PE, which is not reachable directly.

To solve this issue, we use the next-hop-self command on the Hub-PE, so that:

Hub-PE updates the next-hop to its own address before advertising routes to the Spoke-PEs.

This ensures that Spoke-PE routers use the Hub-PE as the next-hop, making routes reachable and preventing blackholing.

3. Correct Command Usage

The administrator must run the following command under the BGP IPv4 address family of the BGP-VPN_out instance on Hub-PE:

shell

CopyEdit

peer 10.1.2.1 next-hop-self

This ensures that the next-hop for advertised routes is changed to the Hub-PE ' s IP, allowing proper route transmission in the Hub-Spoke MPLS VPN environment.

Step 1: Understanding OSPF Areas and Summarization

Area 1 (NSSA - Not So Stubby Area)

Allows external routes but converts Type-5 LSAs into Type-7 LSAs.

External routes must be translated by an ABR into Type-5 LSAs before being propagated into other areas.

Area 2 (Common Area - Regular OSPF Area)

Supports both inter-area and external routes.

asbr-summary 10.0.4.0 255.255.255.0 Command

This command summarizes all external routes within 10.0.4.0/24 into a single summarized route.

Since it is applied on R1 and R3, these routers will advertise 10.0.4.0/24 instead of 10.0.4.4/32 to their respective areas.

Step 2: Route Distribution Analysis

R4 (Originating Router in NSSA - Area 1)

R4 injects 10.0.4.4/32 as a Type-7 LSA into Area 1.

R1 (ABR between Area 1 & Area 0, Summarizing Route)

Since R1 is an ABR, it translates Type-7 LSAs from Area 1 into a summarized Type-5 LSA (10.0.4.0/24) for Area 0.

✅ R1 has 10.0.4.0/24 in its routing table.

R2 (Inside Backbone Area 0, No ASBR Summary Applied Here)

R2 only receives Type-5 LSAs from R1 but does not summarize them.

Since R2 is not explicitly summarizing, it will likely still see 10.0.4.4/32 rather than 10.0.4.0/24.

❌ R2 does not have 10.0.4.0/24.

R3 (ABR between Area 0 & Area 2, Summarizing Route)

R3 is also an ABR and has the asbr-summary 10.0.4.0/24 command configured.

It will advertise the summarized route 10.0.4.0/24 into Area 2.

✅ R3 has 10.0.4.0/24 in its routing table.

R5 (Inside Area 2, No Direct Connectivity to External Routes)

Since R3 summarizes 10.0.4.0/24, R5 (inside Area 2) will receive only summarized routes.

✅ R5 has 10.0.4.0/24.

Final Answer: ✅ R1, R3 (Option A, C)