Hi what way is it configured , if replicate is enabled it will show cdp/stp packets , even if you didnt configure it make sure its not set in show run all
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_52_se/configuration/guide/3750scg/swspan.html The default configuration for local SPAN session ports is to send all packets untagged. SPAN also does not normally monitor bridge protocol data unit (BPDU) packets and Layer 2 protocols, such as Cisco Discovery Protocol (CDP), VLAN Trunk Protocol (VTP), Dynamic Trunking Protocol (DTP), Spanning Tree Protocol (STP), and Port Aggregation Protocol (PAgP). However, when you enter the encapsulation replicate keywords when configuring a destination port, these changes occur:
•Packets are sent on the destination port with the same encapsulation—untagged, Inter-Switch Link (ISL), or IEEE 802.1Q—that they had on the source port.
•Packets of all types, including BPDU and Layer 2 protocol packets, are monitored.
Therefore, a local SPAN session with encapsulation replicate enabled can have a mixture of untagged, ISL, and IEEE 802.1Q tagged packets appear on the destination port.
Switch congestion can cause packets to be dropped at ingress source ports, egress source ports, or SPAN destination ports. In general, these characteristics are independent of one another. For example:
•A packet might be forwarded normally but dropped from monitoring due to an oversubscribed SPAN destination port.
•An ingress packet might be dropped from normal forwarding, but still appear on the SPAN destination port.
•An egress packet dropped because of switch congestion is also dropped from egress SPAN.
