Selected H12-821_V1.0 HCIP-Datacom Questions Answers

 BGP Route Flapping and Mitigation

While route flapping is a concern in large-scale networks, BGP employs mechanisms such as Route Dampening to mitigate its impact. Route dampening suppresses frequently flapping routes for a period of time to reduce resource consumption and network instability.

Therefore, it is incorrect to state that the issue cannot be resolved in BGP.

 HCIP-Datacom-Core Reference

The mechanism of route dampening is detailed in the BGP optimization chapters​​.

Comprehensive and Detailed Step-by-Step Explanation:

1. Overview of AS_Path Attribute Modification:

The apply as-path command in a route-policy is used to modify the AS_Path attribute of routes. This is often required to influence the selection of routes or to adjust routing policies for incoming or outgoing BGP updates.

2. Parameters of the apply as-path Command:

Option A: Delete

Correct .

The delete parameter removes specific AS numbers from the AS_Path attribute. This is used to simplify or manipulate the AS_Path for specific routing policies.

Option B: Additive

Correct .

The additive parameter appends an AS number to the existing AS_Path attribute, instead of overwriting it. This is useful when you want to prepend AS numbers to influence route selection without removing the original AS_Path.

Option C: Copy

Incorrect .

The copy parameter is not supported in the apply as-path command for modifying AS_Path attributes.

Option D: Overwrite

Correct .

The overwrite parameter replaces the existing AS_Path with a new one. This provides complete control over the AS_Path attribute but removes the original AS_Path entirely.

3. Summary:

The valid parameters for the apply as-path command are delete, additive, and overwrite.

Explanation of Root Port Election in STP:

In a Spanning Tree Protocol (STP) network, the Root Port is selected on each switch (except the Root Bridge). The Root Port is the port with the lowest path cost to the Root Bridge. If there is a tie in path cost, the following criteria are used in sequence to break the tie:

Steps for Root Port Election in Sequence:

RPC Comparison (Root Path Cost):

The port with the lowest Root Path Cost is preferred.

Root Path Cost is the cumulative cost of the path to the Root Bridge.

Peer BID (Bridge ID) Comparison:

If the Root Path Costs are equal, the Bridge ID of the connected switch (peer) is compared.

The port connecting to the switch with the lower Bridge ID is preferred.

Peer PID (Port ID) Comparison:

If the Bridge IDs are equal, the Port ID of the peer ' s sending port is compared.

The port connecting to the lower Port ID is preferred.

Local PID (Port ID) Comparison:

If all else is equal, the Local Port ID of the switch is used.

The port with the lower Local Port ID is selected.

Option B is false . By default, when the aggregate command is used without any options , BGP generates a summary route , and suppresses the specific routes that fall under that summary. To advertise both specific and summary routes, you must explicitly configure the detail-suppressed option to control whether specifics are suppressed or not.

Explanation of the other options:

A is correct: When suppress-policy is specified, only specific routes matching the policy are advertised; others are suppressed.

C is correct: The summary automatic command causes summarization based on classful boundaries, which is a legacy behavior.

D is correct: With detail-suppressed, only the summary route is advertised, not specific prefixes.

Reference from Huawei HCIP-Datacom-Core Technology Study Guide:

“The aggregate command suppresses specific routes by default unless explicitly configured otherwise. Use of suppress-policy or detail-suppressed allows control over which routes are advertised.”

(Chapter: BGP Advanced Features – Section: Route Aggregation)