CCNA Virtual Routing and Forwarding (VRF) 💻

Overview

• VRF: Virtual Routing and Forwarding
  • Divides one physical router into multiple virtual routers, similar to how VLANs (Virtual LANs) divide a switch into multiple virtual switches.
  • Each virtual router within a VRF has its routing table.
• Purpose: Understanding VRF and its basic configurations (although VRF configuration isn't on the CCNA exam).
• Lab: VRF isn't supported in Packet Tracer; use Cisco Modeling Labs (CML) or real Cisco devices.

Concepts and Capabilities

• Default Routing Domain: Without VRF, traffic received on one router interface can be forwarded out of any other interface based on the routing table.
• VRF Isolation: With VRF, traffic cannot be forwarded between interfaces in different VRFs.
  • Example: Traffic on R1’s G0/0 (VRF1) cannot be forwarded to G1/2 (VRF3).
• Routing Tables: VRF allows creation of multiple routing tables (one per VRF).
• Layer 3 Only: VRF applies to router interfaces, SVIs, and routed ports on multilayer switches, not Layer 2 interfaces.
• VRF Leaking: Allows traffic to pass between VRFs, but it's a more advanced topic.

VRF vs. VRF-lite

• VRF: Commonly used with MPLS (Multiprotocol Label Switching).
• VRF-lite: VRF without MPLS, the focus of this lecture.
  • Usage: Service providers use VRF-lite to allow one device to carry traffic from multiple customers.
  • Benefits:
    • Traffic isolation: Each customer’s virtual router within a physical router is isolated from others.
    • Overlapping IP addresses: Customers can use the same IP subnets without conflict due to VRF.

Configuration Example

1. Initial Setup Without VRF
   • Attempting to configure overlapping subnets without VRF results in errors.
   • Interface G0/2 on SPR1 cannot use 192.168.1.1/30 because it overlaps with G0/0’s subnet.
2. Configuring VRF
   • Step 1: Create VRFs in global config mode: IP VRF CUSTOMER1 and IP VRF CUSTOMER2.
   • Step 2: Assign interfaces to VRFs. E.g., IP VRF FORWARDING CUSTOMER1 on G0/0.
   • Note: Reconfigure IP address after assigning an interface to a VRF as the IP is removed.
   • Verification: SHOW IP VRF and SHOW IP ROUTE VRF [VRF_NAME] to check VRF routing tables.
3. Ping Tests
   • Pinging without specifying a VRF will fail as the global routing table has no routes.
   • Specify VRF in ping command: PING VRF CUSTOMER1 [IP_ADDRESS].

Key Points

• Isolation: Hosts within the same VRF can communicate, while those in different VRFs cannot.
• Usage: VRFs allow service providers to deliver multiple isolated networks to customers.
• Configuration Steps: Simple - creating VRFs and assigning interfaces.
• Further Study: For CCNP and beyond, deeper study into VRF is required.

Quiz Recap

1. Question: Why doesn’t G0/0 have an IP address after ‘SHOW IP INTERFACE BRIEF’?
   • Answer: The IP address was removed by the IP VRF FORWARDING command.
2. Question: Which device responds to PING 192.168.1.10 on R1?
   • Answer: No device, as pings without a specified VRF use the empty global routing table.
3. Statements About VLANs and VRFs
   • Correct Answers: C, D, F

Conclusion

• VRF provides the capability to split a router into multiple virtual routers, each with its own routing table, facilitating traffic isolation and overlapping IP addresses for service providers.
• Understanding VRF is essential for higher-level network certifications like CCNP.