Understanding Obtaining Packet Headers

Network engineers use the obtaining packet headers feature to obtain packet headers to be sent to central processing units (CPUs) or forwarded. Then they analyze the obtained headers to locate network faults. Figure 1 shows how to implement this feature.

A network engineer logs in to Device A and configures a packet header obtain policy. This policy defines the interface on which packets will be obtained, access control list (ACL) rule, packet length, and timeout time.
When Device A receives packets and sends the packets to its CPU or forwards packets, it replicates the packets based on the configured packet obtaining policy. It then generates an obtained packet file and stores this file on its compact flash (CF) card.
The network engineer obtains this packet header obtain file to locate network faults.

Figure 1 Obtaining packet headers

The following table lists the differences between obtaining packet headers to be sent to CPUs and obtaining packet headers to be forwarded.

**Table 1** Difference between obtaining packet headers to be sent to CPUs and obtaining packet headers to be forwarded
Item	Obtaining Packet Headers Sent to CPUs	Obtaining Forwarded Packet Headers
Object	Packets destined to CPUs	Forwarded packets
ACL Rule	Supported ACLs: Basic ACL: Values range from 2000 to 2999. Advanced ACL: Values range from 3000 to 3999. Layer 2 ACL: Values range from 4000 to 4999.	Supported ACLs: Basic ACL: Values range from 2000 to 2999. Advanced ACL: Values range from 3000 to 3999. Layer 2 ACL: Values range from 4000 to 4999. Multiprotocol Label Switching (MPLS)-based ACL: Values range from 10000 to 10999.
Impact on Performance	Has no impact on system performance.	Affects system forwarding performance.