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

Comprehensive and Detailed In-Depth Explanation:

1. Understanding the Hub-Spoke Network Model in MPLS VPN

Hub-Spoke (or Route Reflector) topology is used in MPLS Layer 3 VPNs where multiple spoke sites communicate through a central hub.

The Hub-PE acts as an intermediate BGP speaker and redistributes routes between spokes.

The challenge is preventing routing loops when exchanging routes between spokes.

2. Understanding the Role of Site of Origin (SoO) in BGP/MPLS VPNs

BGP Site of Origin (SoO) is a route attribute used to prevent routing loops in scenarios where a hub redistributes routes between spokes.

If a route originating from a spoke is advertised back to the same site, SoO prevents it from being re-learned.

The SoO value (e.g., 200:1) ensures that a site does not accept its own previously advertised prefixes.

3. Analyzing the Answer Choices

❌ A. [Hub-PE-bgp-VPN_out] peer x.x.x.x allow-as-loop (Incorrect)

The allow-as-loop command allows BGP to accept routes with its own AS number in the AS path.

This is NOT needed in a Hub-Spoke VPN because BGP routes are not being looped through the same AS.

❌ This is unnecessary for preventing loops in a Hub-Spoke VPN.

❌ B. [Hub-PE-bgp-VPN_in] peer x.x.x.x allow-as-loop (Incorrect)

Again, allow-as-loop is used when a router needs to accept its own AS number in the BGP AS path.

This is NOT the mechanism used for preventing loops in Hub-Spoke VPNs; SoO is used instead.

❌ This is not required in this scenario.

✅ C. [Hub-PE-bgp-VPN_in] peer x.x.x.x soo 200:1 (Correct Answer)

The correct way to prevent spoke routes from looping through the hub is to apply the Site of Origin (SoO) attribute.

This command ensures that if a route with SoO 200:1 is received, it is not advertised back to the same spoke.

✅ This is the correct answer.

❌ D. [Hub-PE-bgp-VPN_out] peer x.x.x.x soo 200:1 (Incorrect)

Applying SoO on outgoing routes is NOT effective because the hub should mark received routes and prevent loops when re-advertising them.

SoO should be applied on the inbound (received) routes at the Hub-PE, not outbound.

❌ This is not the correct implementation of SoO.

Final Conclusion:

✅ Correct Answer: C. [Hub-PE-bgp-VPN_in] peer x.x.x.x soo 200:1❌ A. allow-as-loop is unnecessary in a Hub-Spoke MPLS VPN.❌ B. allow-as-loop is not relevant here.❌ D. SoO should be applied inbound, not outbound.

Thus, the correct answer is: C. [Hub-PE-bgp-VPN_in] peer x.x.x.x soo 200:1.

Comprehensive and Detailed In-Depth Explanation:

Step 1: Analyzing the Primary Path

The given network uses IS-IS Auto FRR, which ensures a backup route in case of failure.

The shortest path from R1 to 10.0.3.3/32 is calculated based on the OSPF link costs:

Path 1 (Primary Path): R1 → R2 → R3

Cost: 10 (R1→R2) + 5 (R2→R3) = 15 ✅

Step 2: Checking the Backup Path

The statement claims that the backup path is R1 → R4 → R2 → R3. Let's calculate the cost:

Path 2 (Backup Path via R4): R1 → R4 → R2 → R3

Cost: 10 (R1→R4) + 50 (R4→R2) + 5 (R2→R3) = 65 ❌ (Too high)

The backup path should be an alternative shortest path that is as close as possible to the primary path in cost.

R1 → R4 → R2 → R3 is not a feasible backup path because the cost (65) is significantly higher than the primary path (15).

Step 3: Correct Backup Path Selection

If IS-IS Auto FRR is enabled, a feasible backup path should have a cost close to the primary path.

In this scenario, no viable alternative path meets the Auto FRR requirements efficiently.

The given backup path is incorrect.

Final Answer: ✅ FALSE (Option B)

References:

HCIP-Datacom-Advanced Routing & Switching Technology V1.0 – IS-IS Auto FRR Mechanism

OSPF Route Selection and Fast Reroute Principles

OSPF and IS-IS Path Cost Calculation

Comprehensive and Detailed In-Depth Explanation:

To determine which routers are unable to import external routes, we need to analyze the behavior of each OSPF area type:

Stub Area (Area 1 - Contains R6)

Stub areas do not allow external routes (Type-5 LSAs).

Instead, the ABR (R2) generates a default route (0.0.0.0/0) for reachability outside the area.

R6 is in a stub area, so it cannot import external routes. ✅

Totally Stub Area (Area 2 - Contains R3, R4)

Totally stub areas block both external routes (Type-5 LSAs) and inter-area routes (Type-3 LSAs).

Only a default route (0.0.0.0/0) is injected by the ABR (R2).

R3 is in a totally stub area, so it cannot import external routes. ✅

R4, however, is just inside the totally stub area but is not restricted from generating external routes itself. ❌ (R4 can import external routes)

NSSA (Area 3 - Contains R5)

NSSAs allow external routes (Type-7 LSAs) but do not accept standard external routes (Type-5 LSAs) from other areas.

R5, being in an NSSA, can import external routes but only as Type-7 LSAs. ❌ (R5 can import external routes)

Final Answer: ✅ R3, R6 (Option A, C)

References:

HCIP-Datacom-Advanced Routing & Switching Technology V1.0 – OSPF Stub, Totally Stub, and NSSA Areas

OSPF LSA Types and External Route Restrictions

Comprehensive and Detailed In-Depth Explanation:

Understanding the IS-IS Topology in the Question:

R1 (Level-1 Router) is connected to R2 (Level-1-2 Router).

R2 connects Level-1 and Level-2 areas, making it an inter-area router.

R3 (Level-2 Router) connects to R2.

The loopback addresses are:

R1: 10.0.1.1/32

R2: 10.0.2.2/32

R3: 10.0.3.3/32

Key Command Used: default-route-advertise always level-1

This command forces R1 to advertise a default route (0.0.0.0/0) to Level-1 routers.

Level-1 routers (like R1) do not have visibility into Level-2 routes unless a default route is provided by an L1-L2 router (R2 in this case).

Analyzing Each Option:

Option A (TRUE):

If R1 pings an unknown address like 10.0.4.4, it will use the default route if available. However, if no such route exists, it will not send any packets.

This behavior is expected, so this statement is true.

Option B (TRUE):

10.0.2.2 (R2) is a Level-1-2 router, and it can communicate with 10.0.3.3 (R3), which is a Level-2 router.

Since R2 and R3 are both in Level-2, they can exchange routes.

So, 10.0.2.2 can ping 10.0.3.3. This statement is true.

Option C (TRUE):

10.0.1.1 (R1) is a Level-1 router, and 10.0.2.2 (R2) is a Level-1-2 router.

Since R2 is Level-1-2, it shares Level-1 routes with R1, allowing communication between R1 (10.0.1.1) and R2 (10.0.2.2).

Therefore, this statement is true.

Option D (FALSE - Correct Answer):

10.0.1.1 (R1) is a Level-1 router, which means it only knows about Level-1 routes.

10.0.3.3 (R3) is a Level-2 router, and its route is only known by Level-2 routers (R2 & R3).

Since Level-1 routers do not learn Level-2 routes directly (unless specific redistribution is done), R1 does not have a route to 10.0.3.3.

Therefore, R1 (10.0.1.1) cannot ping R3 (10.0.3.3), making this statement false.

Conclusion:

The false statement is Option D (10.0.1.1 can ping 10.0.3.3).

Comprehensive and Detailed In-Depth Explanation:

1. Understanding the Issue of OSPF Type 5 LSAs in a Dual-Homed CE Scenario

OSPF Type 5 LSAs (External LSAs) are used in OSPF to advertise external routes, such as those learned via BGP or redistributed from another protocol.

In a dual-homed CE setup, like the one in the figure, CE2 connects to both PE2 and PE3 using OSPF.

PE2 and PE3 both redistribute BGP routes into OSPF and advertise them to CE2.

CE2 then forwards these LSAs between PE2 and PE3, potentially causing a routing loop because each PE may re-learn its own previously advertised routes.

2. How Route Tags Prevent Loops in OSPF-BGP Redistribution

The route-tag command assigns a tag to redistributed routes.

When a router receives an external OSPF LSA (Type 5), it checks the route tag.

If the route tag matches the locally configured tag, the router ignores the route to prevent loops.

Example Configuration on PE Routers:

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PE2:

router ospf 1

import-route bgp 100 route-tag 65001

PE3:

router ospf 1

import-route bgp 100 route-tag 65001

Both PE2 and PE3 tag their redistributed BGP routes with 65001.

If PE3 receives an LSA from CE2 with the tag 65001, it ignores it (because it knows it originally advertised it).

✅ This prevents OSPF from incorrectly preferring a route that was originally sourced from itself, avoiding routing loops.

3. Evaluating the Answer Choices

✅ TRUE: If the route tag of the received route matches the locally configured tag, the route is ignored, preventing loops.❌ FALSE: The statement is correct, so False is incorrect.

Thus, the correct answer is: A. TRUE

Final Conclusion:

✅ The statement is TRUE because route tagging is a valid method to prevent OSPF Type 5 LSA loops in dual-homed scenarios.