Reverse Path Forwarding
Reverse Path Forwarding (RPF) is a fundamental networking mechanism that confirms that the source address of a packet is legitimate. In contrast to conventional routing, which looks at a packet’s destination to decide where to send it, RPF looks at the source address to determine whether the packet should have arrived on that particular interface.
The two main applications of RPF are Unicast Security (uRPF) to stop IP spoofing and Multicast Routing to prevent loops.
You can also read What Is A Metropolitan Area Network Architecture & Benefits
How RPF Works (Step by Step)
To summarize the basic reasoning behind an RPF check, ask yourself, “If I were replying to this source, would I use this interface?” These stages are taken in the process:
- A router receives a packet on a specific interface.
- It looks up the source IP address in its routing table (Unicast Routing Table or Forwarding Information Base).
- It identifies the best return path to reach that source.
- If the interface the packet arrived on matches the interface the router would use to send traffic back to that source, the check passes and the packet is forwarded.
- If the interfaces do not match, the packet is discarded to prevent loops or address spoofing.
Example
- Source network: 10.1.1.0
- Best route back to 10.1.1.0 is via Interface A
- Packet arrives on Interface A → ✅ accepted
- Packet arrives on Interface B → ❌ dropped
Multicast RPF (Loop Prevention)
Multicast routing replicates a single transmission to numerous recipients. Multicast packets could perpetually cycle in duplicate topologies without RPF, wasting network resources needlessly.
- Function: Multicast traffic is guaranteed to move down the distribution tree and away to RPF.
- Protocols: PIM (Protocol Independent Multicast), DVMRP, and MOSPF are among the protocols that depend on it.
- Critical Assumptions: The stability of the unicast routing table and the symmetry (the same in both directions) of the path from the sender to the router are essential for Multicast RPF to work well. RPF may reject valid traffic on all but the shortest path if paths are asymmetric, which would result in a distribution tree that is not optimal.
You can also read Types of Hubs, Features, Advantages And Disadvantages Of Hub
Unicast RPF (uRPF) Security
In order to counter Distributed Denial of Service (DDoS) attacks that use forged (spoofed) source IP addresses, uRPF is a security feature that was defined in RFC 3704. It stops known invalid networks from sending traffic to interfaces where it shouldn’t have come from.
Three main uRPF modes are as follows:
| Mode | Requirement for Success | Best Use Case |
|---|---|---|
| Strict Mode | The source IP must be in the routing table and reachable via the exact same interface the packet arrived on. | Symmetric routing environments, such as enterprise LANs. |
| Loose Mode | The source IP must exist in the routing table via any interface. It only fails if the route is unknown or points to Null0. | Asymmetric routing environments, such as ISP backbones. |
| Feasible Mode | The source must match any of the alternate routes maintained in the Forwarding Information Base (FIB) for that IP address. | Networks with multiple valid paths to a source. |
Advantages and Limitations
- Advantages: Benefits include enhanced multicast performance, protection against spoof traffic, and a straightforward and effective loop prevention technique offered by RPF.
- Limitations: Its efficacy is solely dependent on how accurate the routing tables are.
- Asymmetric Routing Warning: If you use Strict Mode on networks that have asymmetric paths traffic that leaves one way but returns another the router will discard valid traffic. The more adaptable Loose Mode may be necessary in some situations, otherwise the reverse-path strategy may not work.
You can also read Importance Of Computer Network Security Protect Your Data
Implementation Note
Generally, Cisco Express Forwarding (CEF) must be operational on Cisco equipment in order to enable uRPF. Using commands like ip verify unicast source reachable-via rx for Strict Mode or reachable-via any for Loose Mode, configuration is applied directly to the ingress interface.