A context is a logical grouping or mapping of configuration parameters that pertain to various physical ports, logical IP interfaces, and services. A context can be thought of as a virtual private network (VPN).

The system supports the configuration of multiple contexts. Each context is configured and operates independently of the others. Once a context has been created, administrative users can configure services, logical IP interfaces, and subscribers for that context and then bind the logical interfaces to physical ports.

You can also assign a domain alias to a context; if a subscriber's domain name matches one of the configured alias names for a context, that context is used.

Ports are the physical connectors on line cards that support remote access and subscriber traffic. Port configuration includes traffic profiles, data encapsulation methods, media type, and other information for physical connectivity between the system and the rest of the network.

Ports are identified by the chassis slot number for the Management Input/Output (MIO/UMIO) card, followed by the physical connector number. For example, Port 5/10 identifies connector number 10 on the MIO/UMIO card in slot 5.

Associate ports with contexts through bindings. For additional information on bindings, refer to the Bindings section below. You can configure each physical port to support multiple logical IP interfaces, each with up to 17 IP addresses (one primary and up to 16 secondaries).

For complete information on line cards and port assignments, refer to the ASR 5500 Installation Guide.

You must associate a port with a StarOS virtual circuit or tunnel called a logical interface before the port can allow the flow of user data.Within StarOS, a logical interface is a named interface associated with a virtual router instance that provides higher-layer protocol transport, such as Layer 3 IP addressing. Interfaces are configured as part of VPN contexts and are independent from the physical ports that will be used to bridge the virtual interfaces to the network.

Logical interfaces are associated with ethernet+ppp+tunnel addresses and are bound to a specific port during the configuration process. Logical interfaces are also associated with services through bindings. Services are bound to an IP address that is configured for a particular logical interface. When associated, the interface takes on the characteristics of the functions enabled by the service.

There are several types of logical interfaces to configure to support Simple and Mobile IP data applications. These are briefly defined below.

Subscribers are the end-users of the service; they gain access to the Internet, their home network, or a public network through the system.

The system comes configured with a context called local that you use specifically for management purposes. The context selection process for context-level administrative users (those configured within a context) is simplified because the management ports on the MIO are associated only with the Local context. Therefore, the source and destination contexts for a context-level administrative user responsible for managing the entire system should always be the local context.

A context-level administrative user can also connect through other interfaces on the system and still have full system management privileges.

A context-level administrative user can be created in a non-local context. These management accounts have privileges only in the context in which they are created. This type of management account can connect directly to a port in the context in which they belong, if local connectivity is enabled (SSHD, for example) in that context.

For all FTP or SFTP connections, you must connect through an MIO management interface. If you SFTP or FTP as a non-local context account, you must use the username syntax of username@contextname.

The context selection process becomes more involved if you are configuring the system to provide local authentication or work with a AAA server to authenticate the context-level administrative user.

The system gives you the flexibility to configure context-level administrative users locally (meaning that their profile will be configured and stored in its own memory), or remotely on an AAA server. If a locally-configured user attempts to log onto the system, the system performs the authentication. If you have configured the user profile on an AAA server, the system must determine how to contact the AAA server to perform authentication. It does this by determining the AAA context for the session.

The following table and flowchart describe the process that the system uses to select an AAA context for a context-level administrative user. Items in the table correspond to the circled numbers in the flowchart.

Figure 2. Context-level Administrative User AAA Context

The context selection process for a subscriber session is more involved than that for the administrative users. Subscriber session context selection information for specific products is located in the Administration Guide for the individual product.

An Internet Protocol Version 4 (IPv4) address consists of 32 bits divided into four octets. These four octets are written in decimal numbers, ranging from 0 to 255, and are concatenated as a character string with full stop delimiters (dots) between each number.

For example, the address of the loopback interface, usually assigned the host name localhost, is 127.0.0.1. It consists of the four binary octets 01111111, 00000000, 00000000, and 00000001, forming the full 32-bit address.

IPv4 allows 32 bits for an Internet Protocol address and can, therefore, support 2³² (4,294,967,296) addresses.

An Internet Protocol Version 6 (IPv6) address has two logical parts: a 64-bit network prefix, and a 64-bit host address part. An IPv6 address is represented by eight groups of 16-bit hexadecimal values separated by colons (:).

A typical example of a full IPv6 address is 2001:0db8:85a3:0000:0000:8a2e:0370:7334

The hexadecimal digits are case-insensitive.

IPv6 allows 128 bits for an Internet Protocol address and can support 2¹²⁸ (340,282,366,920,938,000,000,000,000,000,000,000,000) internet addresses.

Classless Inter-Domain Routing (CIDR) notation is a compact specification of an Internet Protocol address and its associated routing prefix. It is used for both IPv4 and IPv6 addressing in networking architectures.

CIDR is a bitwise, prefix-based standard for the interpretation of IP addresses. It facilitates routing by allowing blocks of addresses to be grouped into single routing table entries. These groups (CIDR blocks) share an initial sequence of bits in the binary representation of their IP addresses.

CIDR notation is constructed from the IP address and the prefix size, the latter being the number of leading 1 bits of the routing prefix. The IP address is expressed according to the standards of IPv4 or IPv6. It is followed by a separator character, the slash (/) character, and the prefix size expressed as a decimal number.

The address may denote a single, distinct, interface address or the beginning address of an entire network. In the latter case the CIDR notation specifies the address block allocation of the network. The maximum size of the network is given by the number of addresses that are possible with the remaining, least-significant bits below the prefix. This is often called the host identifier.

The number of addresses of a subnet defined by the mask or prefix can be calculated as 2^{address size - mask}, in which the address size for IPv4 is 32 and for IPv6 is 128. For example, in IPv4, a mask of /29 gives 2^32-29 = 2³ = 8 addresses.

The alphanumeric character set is a combination of alphabetic (Latin letters) and/or numeric (Arabic digits) characters. The set consists of the numbers 0 to 9, letters A to Z (uppercase) and a to z (lowercase). The underscore character ( _ ) and dash/hyphen (-) are also considered to be members of the alphanumeric set of characters.

Blank spaces (whitespaces or SPACE characters) should mostly be avoided in alphanumeric strings, except in certain ruledef formats, such as time/date stamps.

If descriptive text requires the use of spaces between words, the string must be entered within double quotation marks (" "). For example:

interface "Rack 3 Chassis 1 port 5/2"