Huawei H12-831_V1.0 Exam - Topic 1 Question 77 Discussion

Comprehensive and Detailed In-Depth

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.

HCIP-Datacom-Advanced Routing & Switching Technology V1.0 -- OSPF Route Redistribution and Route-Tagging

Huawei Official HCIP-Datacom Study Guide -- Preventing OSPF-BGP Loops in MPLS VPNs

Huawei Documentation on Using Route Tags to Prevent OSPF Routing Loops