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Complete Cisco Nexus BGP & OSPF Stub Area Lab Guide | Route Reflector Configuration Tutorial

Complete Cisco Nexus OSPF Stub Areas & BGP Route Reflector Lab | Part 7

Complete Cisco Nexus OSPF Stub Areas & BGP Route Reflector Lab | Part 7

Welcome to Part 7 of the Cisco Nexus enterprise networking series. This advanced lab combines OSPF Stub Areas, Totally Stubby Areas, OSPF Underlay design, iBGP, eBGP, Route Reflectors, and enterprise BGP architecture.

Topics Covered in This Lab:
  • OSPF Stub Areas
  • Totally Stubby Areas
  • OSPF Underlay Architecture
  • BGP Route Reflectors
  • Internal BGP (iBGP)
  • External BGP (eBGP)
  • Loopback-Based BGP Peering
  • Next-Hop Self
  • Route Reflection
  • Enterprise WAN Routing

Table of Contents

1. Enterprise Routing Overview

Modern enterprise networks use multiple routing protocols together. OSPF is commonly used as an Underlay IGP, while BGP provides scalable overlay routing.

This lab demonstrates:

  • OSPF Stub Area optimization
  • Totally Stubby Area design
  • BGP Route Reflection
  • Loopback-based BGP peering
  • Enterprise scalability concepts

BGP Administrative Distance

eBGP:

\[ AD = 20 \]

iBGP:

\[ AD = 200 \]

OSPF:

\[ AD = 110 \]

2. Task 1 - Configure OSPF Stub Area 10

Area 10 will become a Stub Area.

Stub Areas block external LSAs from entering the area.

Stub Area Benefits:
  • Reduced LSDB size
  • Lower CPU usage
  • Faster convergence
  • Simplified routing tables

NX-01 Configuration

router ospf 1
 area 10 stub

NX-03 Configuration

router ospf 1
 area 10 stub
Verification Output 
show ip ospf database external

No external LSAs should appear inside Area 10.

3. Task 2 - Configure Totally Stubby Area 20

Area 20 becomes a Totally Stubby Area.

Totally Stubby Areas block:

  • External Routes
  • Inter-Area Routes

Only a default route is injected.

NX-02 Configuration

router ospf 1
 area 20 stub no-summary

NX-04 Configuration

router ospf 1
 area 20 stub

LSA Reduction Mathematics

Suppose:

  • 100 Inter-Area Routes
  • 50 External Routes

Normal Area:

\[ 100 + 50 = 150 \text{ LSAs} \]

Totally Stubby Area:

\[ 1 \text{ Default Route} \]

Massive reduction in routing overhead.

4. Task 3 - Remove Previous OSPF Configuration

Before deploying the Underlay IGP architecture, remove previous OSPF and authentication configurations.

Example Cleanup Commands

no router ospf 1

no ip ospf authentication

no ip ospf authentication-key

no ip ospf message-digest-key
Best Practice: Always clean previous routing configurations before deploying a new enterprise routing design.

5. Task 4 - Configure OSPF Underlay for BGP

OSPF Area 0 becomes the Underlay IGP for BGP AS 123.

R1 OSPF Configuration

router ospf 1
 router-id 0.0.0.1
 network 192.1.10.0 0.0.0.255 area 0
 network 192.1.20.0 0.0.0.255 area 0
 network 10.1.1.1 0.0.0.0 area 0

NX-01 Configuration

feature ospf

router ospf 1
 router-id 0.0.0.21

interface vlan10
 ip router ospf 1 area 0

interface loopback10
 ip router ospf 1 area 0

NX-02 Configuration

feature ospf

router ospf 1
 router-id 0.0.0.22

interface vlan20
 ip router ospf 1 area 0

interface loopback10
 ip router ospf 1 area 0
Underlay Network: The Underlay provides IP reachability between loopbacks used for BGP peering.

6. Task 5 - Configure iBGP Route Reflector

R1 acts as a Route Reflector for NX-01 and NX-02.

Why Route Reflectors? Without Route Reflectors:

iBGP requires:

\[ \frac{n(n-1)}{2} \]

neighbor relationships.

Route Reflectors reduce complexity dramatically.

R1 BGP Configuration

router bgp 123
 network 1.0.0.0
 network 100.1.1.0 mask 255.255.255.0

 neighbor 10.1.1.21 remote-as 123
 neighbor 10.1.1.21 update-source loopback10
 neighbor 10.1.1.21 next-hop-self
 neighbor 10.1.1.21 route-reflector-client

 neighbor 10.1.1.22 remote-as 123
 neighbor 10.1.1.22 update-source loopback10
 neighbor 10.1.1.22 next-hop-self
 neighbor 10.1.1.22 route-reflector-client

NX-01 BGP Configuration

feature bgp

router bgp 123
 address-family ipv4 unicast

 network 121.0.0.0 mask 255.0.0.0
 network 100.1.21.0 mask 255.255.255.0

 neighbor 10.1.1.1
 remote-as 123
 update-source loopback10

 address-family ipv4 unicast
 next-hop-self

NX-02 BGP Configuration

feature bgp

router bgp 123
 address-family ipv4 unicast

 network 122.0.0.0 mask 255.0.0.0
 network 100.1.22.0 mask 255.255.255.0

 neighbor 10.1.1.1
 remote-as 123
 update-source loopback10

 address-family ipv4 unicast
 next-hop-self

7. Task 6 - Configure eBGP Between R1 and R2

R1 Configuration

router bgp 123
 neighbor 192.1.12.2 remote-as 20

R2 Configuration

router bgp 20

 network 2.0.0.0
 network 100.1.2.0 mask 255.255.255.0

 neighbor 192.1.12.1 remote-as 123
Verify eBGP Neighbor 
show ip bgp summary

8. Task 7 - Configure eBGP Between NX-01 and NX-03

NX-01 Configuration

router bgp 123
 address-family ipv4 unicast

 neighbor 192.1.30.23
 remote-as 23

 address-family ipv4 unicast

NX-03 Configuration

feature bgp

router bgp 23
 address-family ipv4 unicast

 network 123.0.0.0 mask 255.0.0.0
 network 100.1.23.0 mask 255.255.255.0

 neighbor 192.1.30.21
 remote-as 123

 address-family ipv4 unicast

9. Task 8 - Configure eBGP Between NX-02 and NX-04

NX-02 Configuration

router bgp 123
 address-family ipv4 unicast

 neighbor 192.1.40.24
 remote-as 24

 address-family ipv4 unicast

NX-04 Configuration

feature bgp

router bgp 24
 address-family ipv4 unicast

 network 124.0.0.0 mask 255.0.0.0
 network 100.1.24.0 mask 255.255.255.0

 neighbor 192.1.40.22
 remote-as 123

 address-family ipv4 unicast

10. Verification Commands

Command Purpose
show ip bgp summary Verify BGP neighbors
show ip bgp View BGP table
show ip route bgp Verify BGP learned routes
show ip ospf neighbor Verify OSPF neighbors
ping Connectivity testing
BGP Verification Example 
show ip bgp summary

Neighbor        AS     State
10.1.1.21       123    Established
10.1.1.22       123    Established
192.1.12.2      20     Established

11. BGP Mathematics & Path Selection

iBGP Full Mesh Formula

\[ \frac{n(n-1)}{2} \]

Where:

  • \(n\) = Number of routers

Example:

\[ \frac{5(5-1)}{2} = 10 \]

10 iBGP peerings required.

Route Reflector Reduction

With Route Reflector:

\[ n-1 \]

For 5 routers:

\[ 5-1=4 \]

Only 4 peerings needed.

BGP Best Path Selection Priority

  1. Highest Weight
  2. Highest Local Preference
  3. Locally Originated
  4. Shortest AS Path
  5. Lowest MED
  6. eBGP over iBGP
  7. Lowest IGP Metric

12. OSPF Stub Area Theory

OSPF Area Types

Area Type Blocks External Routes Blocks Inter-Area Routes
Normal Area No No
Stub Area Yes No
Totally Stubby Area Yes Yes
Enterprise Design Insight: Stub Areas improve scalability in large enterprise OSPF deployments.

13. BGP Troubleshooting

Common BGP Problems

  • Incorrect AS number
  • Loopback reachability failure
  • Missing update-source
  • TTL issues
  • Neighbor authentication mismatch
  • Next-hop reachability failure

Troubleshooting Commands

show ip bgp summary
show ip bgp
show ip route
show ip ospf neighbor
debug ip bgp

15. Conclusion

This advanced Cisco Nexus enterprise lab demonstrated:

  • OSPF Stub Areas
  • Totally Stubby Areas
  • OSPF Underlay design
  • BGP Route Reflectors
  • iBGP scalability
  • eBGP peering
  • Loopback-based BGP peering
  • Enterprise routing optimization
Final Takeaway: Modern enterprise networks commonly use OSPF as the Underlay IGP and BGP as the Overlay routing protocol. Route Reflectors improve scalability, while Stub Areas optimize OSPF performance and reduce routing overhead.
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