Information Technology Reference
In-Depth Information
case, the node with a Layer-3 problem can automatically or administratively enter SRP pass-
through mode. In this state, any packet entering the node is forwarded at the MAC layer to the
next node. The problem node is invisible to the ring. It does not perform any address lookup on
any received packets, it does not pass packets up to Layer 3, and it does not transmit packets
that are received from Layer 3. It does not participate in the SRP fairness algorithm.
X.25
This section covers the X.25 topics that are listed in the blueprint for the CCIE written test.
Although newer WAN technologies are popular, networks still use X.25. The CCIE candidate
should know that this protocol is used primarily on unreliable links because it implements error
correction, unlike Frame Relay, on each link. X.25 is connection-oriented, which provides for
error checking and recovery.
X.25 is defined by the ITU. Routers and other devices perform the roles of data terminal
equipment (DTE) and data circuit-terminating equipment (DCE). Routers are typically DTEs
that are connecting to modems or packet switches, which perform the DCE function.
X.25 is a three-layer protocol, corresponding to the first three layers of the OSI model. At Layer
1, the physical layer, X.25 uses protocols such as RS-232, V.24, V.35 and X.21. The data-link
level uses HDLC LAPB. The network layer uses the X.25 Packet Layer Protocol (PLP). The
X.25 PLP is concerned with the functions required to move data, such as virtual circuit (VC)
initialization and termination, data transfer between two DTEs, flow control on a VC, network
level error detection and recovery, and data integrity.
X.25 VCs
X.25 implements VCs as a logical connection between two end devices (DTEs), as shown in
Figure 5-1. Physically, the connection can pass through many interconnecting nodes (switches)
on the network. X.25 circuits can be switched or permanent. PVCs are always available to
transfer data. SVCs are temporary connections for sporadic data transfers. SVCs need to be
established before data is transferred. After the data is transferred, the SVC is terminated.
X.121 Addressing
X.25 uses X.121 addresses. The address field includes the International Data Number (IDN).
The IDN consists of two fields: the Data Network Identification Code (DNIC) and the National
Terminal Number (NTN). Figure 5-2 shows the format for the addresses. The DNIC is an
optional field that identifies the exact packet switching node in which the destination DTE is
located. The DNIC has Country and PSN subfields.
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