Which IP precedence value does BFD use to prioritize traffic within an infrastructure device?

Bidirectional Forwarding Detection (BFD) is a crucial protocol for rapid network failure detection. To ensure BFD control packets receive priority and are not dropped during periods of congestion, they are marked with a high Quality of Service (QoS) value. In Cisco IOS, IOS XE, and IOS XR implementations, BFD packets are, by default, marked with IP Precedence 6. This value corresponds to the Differentiated Services Code Point (DSCP) value of CS6 (Class Selector 6), which is designated for "Internetwork Control" traffic. This classification places BFD in the same high-priority queue as essential routing protocols like OSPF, EIGRP, and BGP, ensuring network stability and fast convergence.

A. 4: IP Precedence 4 (DSCP CS4) is typically reserved for streaming video or other high-throughput, real-time applications, not for network control plane traffic.

B. 5: IP Precedence 5 (DSCP CS5) is standard for voice and video signaling traffic (e.g., SIP, H.323), which is critical but distinct from network infrastructure control.

D. 7: IP Precedence 7 (DSCP CS7) is the highest value, designated for "Network Control." While BFD is a control protocol, this value is typically reserved by the network device itself for its most critical internal processes, and routing protocols are standardized at precedence 6.

1. Cisco Systems

IP Routing: BFD Configuration Guide

Cisco IOS XE Fuji 16.9.x. Under the "BFD Packet Priority" section

it is stated: "BFD packets are sent with IP precedence 6." This document directly confirms the value used in a major Cisco operating system.

2. Cisco Systems

BFD Configuration Guide for Cisco NCS 5500 Series Routers

IOS XR Release 7.3.x. In the "Implementation of BFD" chapter

the documentation specifies: "BFD control packets are marked with IP precedence 6 (DSCP CS6) by default." This confirms the same default behavior on the IOS XR platform.

3. Cisco Systems

Enterprise QoS Solution Reference Network Design Guide

Version 3.3. In Chapter 3

"QoS Baseline

" Table 3-2 ("Per-Hop Behavior (PHB) Recommendations") maps "Routing" traffic (which includes protocols like OSPF

EIGRP

BGP

and their adjuncts like BFD) to the "Internetwork Control" class

which is marked with DSCP CS6 and IP Precedence 6.