Cisco CCNA CCNP Certification Exam Lab Frame Relay Subinterfaces And Split Horizon
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Cisco CCNA/CCNP Certification Exam Lab: Understanding Frame Relay Subinterfaces and Split Horizon
Gain a deeper understanding of Frame Relay subinterfaces and split horizon through this comprehensive lab guide by Chris Bryant, CCIE
12933. Enhance your exam preparation and succeed in configuring Cisco routers effectively.
Understanding the Concepts
Pursuing your Cisco CCNA and CCNP certifications is challenging, especially since Cisco routers offer multiple ways to achieve configurations. Understanding every option is crucial for exam success and real-world networking tasks. Frame Relay subinterfaces and split horizon present one such scenario.
Frame Relay and Split Horizon
Frame Relay subinterfaces help overcome the split horizon rule. Remember from your CCNA studies, split horizon prevents a route from being advertised out the same interface it was initially learned on. Here's an example with R1 as the hub and R2 and R3 as spokes, all using RIPv2 and advertising loopbacks.
Network Configuration:
- R1 Configuration:
```plaintext
R1(config)
int s0
R1(config-if)ip address 172.12.123.1 255.255.255.0
R1(config-if)no frame inverse
R1(config-if)frame map ip 172.12.123.2 122 broadcast
R1(config-if)frame map ip 172.12.123.3 123 broadcast
R1(config-if)no shut
```- R2 Configuration:
```plaintext
R2(config)
int s0
R2(config-if)encap frame
R2(config-if)no frame inver
R2(config-if)frame map ip 172.12.123.1 221 broadcast
R2(config-if)frame map ip 172.12.123.3 221 broadcast
R2(config-if)ip address 172.12.123.2 255.255.255.0
```- R3 Configuration:
```plaintext
R3(config)
int s0
R3(config-if)encap frame
R3(config-if)no frame inver
R3(config-if)frame map ip 172.12.123.1 321 broadcast
R3(config-if)frame map ip 172.12.123.2 321 broadcast
R3(config-if)ip address 172.12.123.3 255.255.255.0
```R1 can route to both loopbacks, but R2 and R3 cannot see each other's loopbacks due to split horizon.
Solutions
Option 1: Disable Split Horizon
While disabling split horizon might solve this issue, it's not recommended due to potential downsides.
```plaintext
R1(config)
interface serial0
R1(config-if)no ip split-horizon
```Option 2: Use Subinterfaces
A more effective solution is configuring subinterfaces on R1. Updating IP addresses is necessary but straightforward.
- Updated R1 Configuration:
```plaintext
R1(config)
interface serial0
R1(config-if)encap frame
R1(config-if)no frame inverse-arp
R1(config-if)no ip address
```Subinterface Configurations:
```plaintext
R1(config-if)
interface serial0.12 multipoint
R1(config-subif)ip address 172.12.123.1 255.255.255.0
R1(config-subif)frame map ip 172.12.123.2 122 broadcast
``````plaintext
R1(config-if)
interface serial0.31 point-to-point
R1(config-subif)ip address 172.12.13.1 255.255.255.0
R1(config-subif)frame interface-dlci 123
```- Update R2 and R3 Configurations:
```plaintext
R2(config)
int s0
R2(config-if)ip address 172.12.123.2 255.255.255.0
R2(config-if)encap frame
R2(config-if)frame map ip 172.12.13.3 221 broadcast
R2(config-if)frame map ip 172.12.123.1 221 broadcast
``````plaintext
R3(config)
int s0
R3(config-if)ip address 172.12.13.3 255.255.255.0
R3(config-if)encap frame
R3(config-if)frame map ip 172.12.13.1 321 broadcast
R3(config-if)frame map ip 172.12.123.2 321 broadcast
```Verifying the Configuration
Use the `show frame map` command to check static mappings on R1. The subinterfaces configuration ensures R2 and R3 can now access each other's loopback networks via RIP.
Conclusion
While disabling split horizon offers a quick fix, using subinterfaces is a more effective and reliable method for achieving complete IP connectivity. This strategy ensures all spokes can access the hub's loopback network efficiently.
You can find the original non-AI version of this article here: Cisco CCNA CCNP Certification Exam Lab Frame Relay Subinterfaces And Split Horizon.
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