Check out this Cisco article about how IOS internally provides priority for certain control plane protocols.
What is PAK_PRIORITY?
While the IP precedence value specifies treatment of a datagram within its transmission through the network, the pak_priority mechanism specifies treatment of a packet during its transmission inside the router.
Which protocols are marked with high priority?
The RIP and OSPF routing processes that run on the core CPU of a router mark all traffic they originate with both IP precedence 6 and pak_priority. In contrast, the Border Gateway Protocol (BGP) instructs TCP to mark its traffic with IP precedence 6, but does not set pak_priority.
Cisco IOS must also ensure a low drop probability for several types of non-IP control packets. These packet types include these:
Intermediate System-to-Intermediate System (IS-IS) routing protocol messages
Enhanced Interior Gateway routing protocol (EIGRP) messages
Point-to-Point Protocol (PPP) and high-level data link control (HDLC) keepalives on serial and packet over SONET (POS) interfaces
Operations, administration, and maintenance (OAM) cells and address resolution protocol (ARP) messages on ATM interfaces
Since such traffic is not IP, Cisco IOS cannot match on the IP precedence value to provide prioritization. Instead, it uses only the internal pak_priority value in the packet buffer header.