At which layer of the OSI model would you assume the problem is if you type show interface serial 1 and receive the following message?

Serial 1 is down, line protocol is down

OSI layer 1 defines the standards for cabling and connectors.

Different Memories used in a CISCO router are as follows :

NVRAM stores the startup configuration file.

DRAM stores the configuration file that is being executed.

Flash Memory – stores the Cisco IOS.

By default, all Windows 2000 trusts within a domain tree or forest are transitive trusts.

The following are the different types of VPNs :

PPTP ( Point to Point Tunnel protocol ) VPN : Most commonly used VPN. PPTP is a method for implementing virtual private networks. PPTP uses a control channel over TCP and a GRE tunnel operating to encapsulate PPP packets.

Site-to-Site VPN : Site-to-site VPN is a type of VPN connection that is created between two separate locations. Site-to-site VPN can be intranet based or extranet based. 

Intranet-based site-to-site VPN is created between an organization's propriety networks, while extranet-based site-to-site VPN is used for connecting with external partner networks or an intranet.

L2TP ( Layer Two Tunneling Protoco) VPN : L2TP is an extension of the Point-to-Point Tunneling Protocol (PPTP) used by an Internet service provider (ISP) to enable the operation of a virtual private network over the Internet.

IPsec (Internet Protocol Security) : IPsec is a framework for a set of protocols for security at the network or packet processing layer of network communication.

SSL (Secure Sockets Layer) VPN : SSL  is a form of VPN that can be used with a standard Web browser. In contrast to the traditional Internet Protocol Security (IPsec) VPN, an SSL VPN does not require the installation of specialized client software on the end user's computer. 

MPLS (Multiprotocol Label Switching) VPN : MPLS is a protocol for speeding up and shaping network traffic flows. 

Hybrid VPN : A hybrid VPN combines Multiprotocol Label Switching (MPLS) and Internet protocol security (IPsec)-based VPNs. 

PPPoE  or  PPPoA

A VPN allows or describes the creation of private networks across the Internet, enabling privacy and tunneling of non-TCP/IP protocols. A VPN can be set up across any type of link.

IPSec is an industry-wide standard suite of protocols and algorithms that allows for secure data transmission over an IP-based network that functions at the layer3 Network layer of the OSI model.

If you have a serial port configured with multiple DLCIs connected to multiple remote sites, split horizon rules stop route updates received on an interface from being sent out the same interface. By creating subinterfaces for each PVC, you can avoid the split horizon issues when using Frame Relay.