1.11 Troubleshoot BGP (Internal and External; unicast and VRF-Lite)

📘CCNP Enterprise – ENARSI (300-410)

1. What Are BGP Policies?

In BGP, a policy is a set of rules that controls:

Which routes are accepted
Which routes are advertised
Which path is preferred
Which path is avoided

Without policies, BGP would:

Accept all routes
Advertise all routes
Choose paths automatically (sometimes not what we want)

👉 BGP policies allow network administrators to control routing behavior.

2. Why BGP Policies Are Important (Exam Perspective)

For the ENARSI exam, you must understand:

How BGP policies affect route selection
How routing problems occur due to wrong policies
How to troubleshoot when routes are:
- Missing
- Preferred incorrectly
- Advertised unexpectedly

3. Two Main Types of BGP Policies

BGP policies fall into two major categories:

A. Route Filtering

(Control which routes are allowed or blocked)

Inbound filtering
Outbound filtering

B. Path Manipulation

(Control which route/path is preferred)

PART A – ROUTE FILTERING

4. What Is Route Filtering?

Route filtering means controlling which routes BGP accepts or advertises.

Think of it as a security and control mechanism.

Filtering is done using:

Prefix-lists
Route-maps
AS-path filters (basic idea only)

5. Inbound Filtering (Incoming Routes)

What Is Inbound Filtering?

Inbound filtering controls which routes are accepted FROM a BGP neighbor.

➡ Applied when routes are received
➡ Happens before routes enter the BGP table

Why Use Inbound Filtering?

Inbound filtering is used to:

Prevent unwanted routes
Reduce routing table size
Improve security
Avoid incorrect routing decisions

How Inbound Filtering Works (Simple Flow)

Neighbor sends routes
Router checks inbound policy
Allowed routes are installed
Denied routes are discarded

Common Inbound Filtering Tools

1. Prefix-Lists (Most Common)

Allow or deny specific networks
Very efficient
Exam favorite

Example logic (not configuration):

Allow 10.0.0.0/8
Deny everything else

2. Route-Maps

More advanced
Can filter and modify attributes
Used when prefix-lists alone are not enough

Exam Tip – Inbound Filtering

⚠ Important:

Inbound filtering does NOT affect what you advertise
It only affects what you accept

6. Outbound Filtering (Outgoing Routes)

What Is Outbound Filtering?

Outbound filtering controls which routes are advertised TO a BGP neighbor.

➡ Applied before sending routes
➡ Happens after BGP selects best path

Why Use Outbound Filtering?

Outbound filtering is used to:

Prevent leaking internal routes
Control routing visibility
Enforce routing policies
Improve security

How Outbound Filtering Works

Router selects best paths
Outbound policy is applied
Allowed routes are advertised
Denied routes are not sent

Common Outbound Filtering Tools

Prefix-lists
Route-maps

Exam Tip – Outbound Filtering

⚠ Important:

Outbound filtering does NOT affect local routing decisions
It only affects what neighbors see

7. Inbound vs Outbound Filtering (Very Important for Exam)

Feature	Inbound Filtering	Outbound Filtering
Affects local routing	Yes	No
Affects neighbor routing	No	Yes
Applied when	Receiving routes	Sending routes
Common use	Block bad routes	Control advertisements

PART B – PATH MANIPULATION

8. What Is Path Manipulation?

Path manipulation means changing BGP attributes to influence:

➡ Which path is selected as best

BGP uses many attributes to choose the best path.
Policies allow us to modify these attributes.

9. Why Path Manipulation Is Needed

Without path manipulation:

BGP might choose a path that is:
- Slower
- Less preferred
- Against company policy

Path manipulation ensures:

Predictable routing
Controlled traffic flow
Stable networks

10. Common BGP Attributes Used for Path Manipulation

For ENARSI, you MUST understand these:

1. Weight (Cisco-Specific)

Highest value wins
Local to router only
Not advertised

Use case:

Prefer one path on a single router

2. Local Preference (Very Important)

Higher value is preferred
Advertised within iBGP
Used inside an AS

Use case:

Control outbound traffic from AS

3. AS-Path

Shorter path is preferred
Can be modified using AS-path prepending

Use case:

Influence incoming traffic

4. MED (Multi-Exit Discriminator)

Lower value is preferred
Used between neighboring ASes
Optional and not always honored

Use case:

Suggest preferred entry point into an AS

5. Next-Hop Attribute

Determines where traffic is sent next
Incorrect next-hop causes route failures

Used often with:

Route-maps
iBGP and eBGP policies

11. Path Manipulation Using Route-Maps

Why Route-Maps?

Route-maps can:

Match routes
Set attributes
Filter routes

They are the most powerful BGP policy tool.

Route-Maps Can Be Applied:

Inbound
Outbound

Example Policy Logic (Conceptual)

If prefix is 10.1.0.0/16
- Set Local Preference to 200
If prefix is 10.2.0.0/16
- Prepend AS-Path

12. AS-Path Prepending (Exam Favorite)

What Is AS-Path Prepending?

AS-Path prepending means:
➡ Artificially increasing AS-Path length

This makes a route less preferred.

Why Use AS-Path Prepending?

Influence incoming traffic
Make one path look worse
Encourage use of another path

Key Exam Points

Prepending affects other ASes
It does NOT affect local routing directly
Overuse can cause instability

13. Common Policy-Related BGP Problems (Troubleshooting)

For ENARSI, recognize these symptoms:

Problem 1: Routes Not Appearing

Possible causes:

Inbound filter blocking routes
Prefix-list too strict
Route-map deny statement

Problem 2: Routes Advertised Incorrectly

Possible causes:

Missing outbound filter
Wrong prefix-list applied
Route-map sequence order issue

Problem 3: Wrong Path Selected

Possible causes:

Incorrect local preference
Weight applied unexpectedly
AS-path prepending mistake

14. Important Troubleshooting Concepts (Exam)

Know these commands conceptually:

show ip bgp
show ip bgp neighbors
show ip bgp neighbors advertised-routes
show ip bgp neighbors received-routes
show route-map
show ip prefix-list

15. Key Exam Summary (Must Remember)

Filtering

Inbound = controls accepted routes
Outbound = controls advertised routes

Path Manipulation

Weight → router-local
Local Preference → inside AS
AS-Path → incoming traffic
MED → external suggestion

Tools

Prefix-lists → simple filtering
Route-maps → filtering + attribute control

16. Final Exam Tip

📌 ENARSI focuses on troubleshooting, not memorizing commands.

You must be able to:

Identify which policy causes a problem
Understand where it is applied
Know which attribute affects which traffic direction