Hi all One of Control Plane Host subinterface's purpose is to control routing protocol packets incoming rate. EBGP directly connected peers and OSPF packets uses TTL of 1. Similarly all packets to 224.0.0.1 (all system multicast address) is sent with TTL with 1.
Hence it seems these packets will go to CEF Exception sub-interface not to the Host Sub-interfaces. I observed OSPF falling into CEF Exception sub-interface. Just wondering why Cisco has decided to push packets of TTL = 1 to CEF-exception sub-interface. Snippet from http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/ctrl_plane_prot_ps6441_TSD_Products_Configuration_Guide_Chapter.html *Control-plane host subinterface*. This interface receives all control-plane IP traffic that is directly destined for one of the router interfaces. Examples of control-plane host IP traffic include tunnel termination traffic, management traffic or routing protocols such as SSH, SNMP, BGP, OSPF, and EIGRP. All host traffic terminates on and is processed by the router. Most control plane protection features and policies operate strictly on the control-plane host subinterface. Since most critical router control plane services, such as routing protocols and management traffic, is received on the control-plane host subinterface, it is critical to protect this traffic through policing and protection policies. CoPP, port-filtering and per-protocol queue thresholding protection features can be applied on the control-plane host subinterface. The control-plane host subinterface only supports TCP/UDP-based host traffic. All IP packets entering the control-plane matching any of the following conditions are not classified any further and are redirected to the cef-exception subinterface: •IP Packets with IP options. *•**IP Packets with TTL less than or equal to 1.* With regards Kings
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