Configure and verify eBGP between directly connected neighbors (best path selection, neighbor relationships)

3.2 Layer 3

📘CCNP Encore (350-401-ENCORE-v1.1)

1. What is BGP?

BGP (Border Gateway Protocol) is a routing protocol used to exchange routing information between different organizations or networks that are controlled by different administrators.

BGP is called a path-vector routing protocol
It focuses on policy and control, not fastest speed
It is widely used on the Internet and large enterprise networks

2. What is eBGP?

eBGP (External BGP) is BGP used between different Autonomous Systems (AS).

Autonomous System (AS)

An AS is a group of routers managed by one organization
Each AS has a unique AS number (ASN)

Key Point

eBGP = BGP between different AS numbers
iBGP = BGP within the same AS

📌 Exam Rule
If AS numbers are different → eBGP

3. eBGP Between Directly Connected Neighbors

In this topic, we focus on directly connected eBGP neighbors, meaning:

Two routers
Connected with a direct link
Each router belongs to a different AS
Default TTL = 1 (important exam point)

4. Basic eBGP Neighbor Relationship

For eBGP to work, both routers must agree on:

Neighbor IP address
Remote AS number
Network connectivity between them
TCP port 179

If any of these fail, BGP will not form

5. BGP Neighbor States (Very Important for Exam)

BGP uses a state machine to form neighbor relationships.

Common BGP States

Idle – BGP is not active
Connect – TCP connection started
Active – Trying to establish connection
OpenSent – OPEN message sent
OpenConfirm – OPEN accepted
Established – Routes are exchanged ✅

📌 Exam Tip
Only in Established state does BGP exchange routes.

6. Configuring eBGP (Directly Connected)

Step 1: Enable BGP and Define Local AS

router bgp 65001

Step 2: Define the Neighbor and Remote AS

neighbor 192.0.2.2 remote-as 65002

192.0.2.2 → Neighbor IP address
65002 → Neighbor’s AS number

Step 3: Advertise Networks

network 10.10.10.0 mask 255.255.255.0

📌 Important Rule
The network must already exist in the routing table, or BGP will not advertise it.

7. Verifying eBGP Neighbor Relationship

Check Neighbor Status

show ip bgp summary

Key fields to look for:

Neighbor IP
AS number
State = Established
Prefixes received

Check Detailed Neighbor Info

show ip bgp neighbors

This shows:

Timers
Capabilities
Received routes
Session state

8. BGP Best Path Selection (Very Important Exam Topic)

When BGP learns multiple paths to the same destination, it selects one best path.

BGP does NOT use bandwidth or hop count.

9. BGP Best Path Selection Order (Exam-Relevant)

BGP compares attributes in a specific order.

Simplified Best Path Selection (Must Memorize)

Highest Weight (Cisco only, local to router)
Highest Local Preference
Locally originated routes
Shortest AS Path
Lowest Origin type
- IGP < EGP < Incomplete
Lowest MED
eBGP preferred over iBGP
Lowest IGP cost to next hop
Oldest route
Lowest Router ID

📌 Exam Tip
Weight and Local Preference are checked before AS Path.

10. Important BGP Attributes (Exam Must-Know)

1. Weight

Cisco proprietary
Highest value wins
Local to router only
Default = 0

2. Local Preference

Used inside an AS
Higher is better
Default = 100

3. AS Path

List of AS numbers the route passed through
Shorter path is preferred
Prevents routing loops

4. MED (Multi-Exit Discriminator)

Lower value is better
Suggests preferred entry point into an AS

11. eBGP Specific Behavior (Exam Focus)

TTL Behavior

Default TTL = 1
Requires direct connection

Next-Hop Rule

eBGP sets next-hop to itself
iBGP does not

Route Advertisement

eBGP advertises routes learned from anywhere
iBGP has more restrictions

12. Common eBGP Configuration Issues (Troubleshooting)

Neighbor Stuck in Active

IP connectivity problem
Wrong AS number
ACL blocking TCP 179

No Routes Advertised

Network not in routing table
Incorrect network mask
Route filtered

Session Flapping

Interface instability
Timer mismatch
CPU or memory issues

13. Useful Verification Commands (Exam Friendly)

show ip bgp
show ip bgp summary
show ip bgp neighbors
show ip route bgp

Know what each command shows, not just syntax.

14. Key Exam Points to Remember

✔ eBGP = different AS numbers
✔ Default TTL = 1
✔ Uses TCP port 179
✔ Must reach Established state
✔ Best path selection order is critical
✔ Weight > Local Preference > AS Path
✔ Network must exist before advertising

15. Summary

eBGP is used to exchange routes between different organizations or networks
Neighbor relationships must be correctly configured on both sides
Verification commands are essential for troubleshooting
BGP best path selection is policy-based, not speed-based
Understanding attributes and order is crucial for passing the exam