Serial Ppp Cisco

Cisco CCNA concept of Configuring HDLC and PPP Encapsulation.doc for your CCNA certification exam.

How to Configure PPP on Cisco Router Before you actually configure PPP on a serial interface, we will look at the commands and the syntax of these commands as.

HDLC vs. PPP. HDLC and PPP are considered to be in layer 2 of the OSI model. The default setting of a Cisco serial interfaces is to use HDLC encapsulation.

PPP Components. PPP provides a method for transmitting datagrams over serial point-to-point links. PPP contains three main components: A method for encapsulating.

Enabling the CGR 1000 Serial Port

Configuring the Line Parameters

This chapter describes how to configure the Point-to-Point Protocol PPP on serial ports on

Cisco 1000 Series Connected Grid Routers hereafter referred to as the Cisco CG-OS router or

PPP over the serial port provides IP connectivity to downstream systems within the Supervisory Control and Data Acquisition SCADA system.

Additionally, this chapter provides details on enabling and configuring serial ports with either a RS232 DCE or RS485 interface.

This chapter includes the following sections:

PPP over the serial port provides IP connectivity to downstream systems within the SCADA system.

Figure 2-1 provides an example in which you enable the serial port on a CGR 1000 and configure PPP encapsulation on that port to provide connectivity to a low voltage concentrator LVC. Data from the LVC is then transmitted over a secure IPSec tunnel network to a Control Center for processing.

Challenge Handshake Authentication Protocol CHAP provides authentication for communications between the LVC and the CGR 1000. With CHAP, the secret must be in plaintext form. However, the router also supports encrypted passwords.

Figure 2-1 CGR 1000 Serial Port Configured with PPP Encapsulation Provides IP Connectivity within SCADA System

See the Before You Begin sections below.

Verify that the serial port is not configured with another encapsulation method before configuring the serial port for PPP encapsulation by entering the show interface serial slot/port command.

Table 2-1 lists the default settings for the serial ports, line, and PPP parameters.

Table 2-1 Default Settings for Serial Port and PPP

You can configure the two serial ports on the Cisco CG-OS routers to operate as either a RS232 or RS485 interface to provide IP connectivity to systems within the SCADA system.

For hardware details on the serial ports, see the Cisco 1120 and 1240 Hardware Installation Guides.

Determine availability of serial port on the Cisco CG-OS router.

Router enters the global configuration mode.

Enters the interface command mode for the serial slot/port.

Note The slot/port configuration for the serial port can be 1/1 or 1/2.

Provides a textual description of the interface being configured.

text-Allows 80 alphanumeric, case sensitive, characters.

ip address ip address mask secondary

Specifies a primary or secondary IPv4 address for an interface.

ip address mask-The network mask can be a four-part dotted decimal address. For example, 255.0.0.0 indicates that each bit equal to 1 means the corresponding address bit belongs to the network address.

The network mask can be indicated as a slash / and a number a prefix length. The prefix length is a decimal value that indicates how many of the high-order contiguous bits of the address comprise the prefix the network portion of the address. A slash must precede the decimal value and there is no space between the IP address and the slash.

Brings up the port, administratively.

Specifies the media type on the serial port. RS232 is the default.

Sets the maximum bytes per frame.

number-Values of 1 to 512. Default setting is 100 bytes.

number-Value of 1 to 1000. Default setting is 10 ms.

Defines the period of time before the software notifies a connecting system of an up or down state enabled/disabled of the serial port or its link.

number-Value of 1 to 3000. Default setting is 500 ms.

Configures the serial port to operate in either full-duplex or half-duplex mode. Default setting is full-duplex.

copy running-config startup-config

Optional Saves this configuration change.

This example shows how to enable serial port interface 1/1 on the router, define that interface as a RS232 media-type, enable PPP encapsulation on the interface, and add a description.

router config interface serial 1/1

router config-if encapsulation ppp

router config-if media-type RS232

router config-if description Adding PPP encapsulation to serial port

When debugging a connection issue, you can use any of all of the following commands to clear the counters.

Clears counters on all interfaces.

clear counters interface serialslot/port

Clears all interface counters for a specified interface.

You can set and modify the line parameters using the Linux TTY application for each of the Cisco serial ports on the CG-OS router.

Enable the serial port on the CG-OS router and define the interface as a RS232 or RS485.

See Enabling the CGR 1000 Serial Port.

Enter this command at the global configuration mode to modify line settings.

1-Configures the line for serial port 1/1

2-Configures the line for serial port 1/2

Defines the number of data bits per character.

number-Values are 5 to 8. Default values is 8.

The no form of the command disables the function.

Enables or disables the use of flow control on the line.

hardware-Enables CTS and RTS as the flow control mechanism.

none-Select this option when the configuration does not want or require flow control.

Note Setting must match that of the peer.

Sets or disables the terminal parity.

Note Setting must match that of the peer

Sets the transmit and receive speeds for the line.

Value-Any value between 300 and 115200 baud rate. Default value is 9600.

The no form of the command removes the setting.

Defines the asynchronous line stop bits. Default value is 1.

Note When you set the stop bits for a value of 2 and the data bits for a value of 5, the stop bits setting becomes 1.5

Specifies the location of the router.

This example shows how to configure line settings on serial port 1/2.

router config-line flowcontrol none

router config-line parity even

router config-line speed 56000

You must enable the PPP feature on the Cisco CG-OS router. It is not enabled by default.

Enters the global configuration mode.

This example shows how to enable PPP on the CG-OS router.

You can configure one or both of the CGR 1000 serial ports to run PPP.

Enable the serial port on the CG-OS router and define the interface as a RS232 or RS485. See Enabling the CGR 1000 Serial Port.

Enable PPP on the CG-OS router. See Enabling PPP.

Optional feature password encryption aes

Enables AES encryption on a system level.

Note Only required when configuring a type 6 password in Step 7.

Adds or modifies the master key at the system level.

After entering this command, you are prompted for the master key.

slot/port-The slot/port configuration for the serial port can be 1/1 or 1/2

Enables PPP encapsulation on the serial port.

no ppp authentication chap callin

Enables CHAP authentication on the serial port as either a server or client.

callin-Enter this option to provide authentication as a client.

By default, not entering the command option, callin, provides authentication as a server.

Note Enter the no form of this command to disable authentication.

Defines a hostname for PPP CHAP authentication.

ppp username s1 passwd 0 s2 6 type6pwd 7 type7pwd

Defines the password in plain text or as encrypted type 6 or 7.

Encrypted passwords must be copied and pasted from another session.

s1- Name of the PPP peer the downstream device to which the router connects.

s2-PPP password in plain text.

Optional ppp peer-address ip-addr

Provides an IP address to the peer.

Enter an IP address when the peer requires an address from the Cisco CG-OS router for IPCP negotiation.

ipaddr-IPv4 address for the peer format: x.x.x.x

Sets the delay interval that the router waits before attempting to restart protocol negotiation with the PPP peer after a disconnect.

delay-Values range from 5 to 86400 seconds. Default value is 30.

Note A PPP peer might disconnect after completion of a successful PPP link.

This example shows how to configure PPP as a server with encrypted authentication of type 6 on the enabled serial port 1/1.

router config-if encapuslation ppp

router config-if ppp authentication chap callin

router config-if ppp chap hostname cgr1120

router config-if ppp username lcv-va07 passwd 0 secretword

To display PPP or serial port configuration information, perform one of the following tasks.

Displays configuration details for all interfaces on the router.

show interface serial slot/port

Displays configuration for a specific serial port.

Note When you enable PPP encapsulation on the serial interface, the interface only appears as up after successful PPP negotiation.

Displays all configuration information for the line.

Displays the line settings for the specified serial port.

router config-if copy running-config startup-config

Active serial ports on CGR 1000 routers.

Initial support of the feature on the CGR 1000 Series Routers.

This chapter describes how to configure the Point-to-Point Protocol PPP on serial ports on Cisco 1000 Series Connected Grid Routers hereafter referred to as the.

Configuring the Serial Interface

This chapter describes configuring serial interface management in the following sections:

The Cisco 819 Integrated Services Router ISR supports synchronous by default and asynchronous serial interface protocols.

Configuring the serial interface in the Cisco 819 ISR allows you to enable applications such as WAN access, legacy protocol transport, console server, and dial access server. It also allows remote network management, external dial-modem access, low-density WAN aggregation, legacy protocol transport, and high port-density support.

Serial interfaces enables the following features:

Serial interfaces can be used to provide WAN access for remote sites. With support for serial speeds up to 8 Mbps, it is ideal for low- and medium-density WAN aggregation.

To configure serial interfaces, you must understand the following concept:

Cisco High-Level Data Link Controller HDLC is the Cisco proprietary protocol for sending data over synchronous serial links using HDLC. Cisco HDLC also provides a simple control protocol called Serial Line Address Resolution Protocol SLARP to maintain serial link keepalives. Cisco HDLC is the default for data encapsulation at Layer 2 data link of the Open System Interconnection OSI stack for efficient packet delineation and error control.

Note Cisco HDLC is the default encapsulation type for the serial interfaces.

When the encapsulation on a serial interface is changed from HDLC to any other encapsulation type, the configured serial subinterfaces on the main interface inherit the newly changed encapsulation and they do not get deleted.

Cisco HDLC uses keepalives to monitor the link state, as described in the Keepalive Timer section.

PPP is a standard protocol used to send data over synchronous serial links. PPP also provides a Link Control Protocol LCP for negotiating properties of the link. LCP uses echo requests and responses to monitor the continuing availability of the link.

Note When an interface is configured with PPP encapsulation, a link is declared down and full LCP negotiation is re-initiated after five echo request ECHOREQ packets are sent without receiving an echo response ECHOREP.

PPP provides the following Network Control Protocols NCPs for negotiating properties of data protocols that will run on the link:

IP Control Protocol IPCP to negotiate IP properties

Multiprotocol Label Switching control processor MPLSCP to negotiate MPLS properties

Cisco Discovery Protocol control processor CDPCP to negotiate CDP properties

IPv6CP to negotiate IP Version 6 IPv6 properties

Open Systems Interconnection control processor OSICP to negotiate OSI properties

PPP uses keepalives to monitor the link state, as described in the Keepalive Timer section.

PPP supports the following authentication protocols, which require a remote device to prove its identity before allowing data traffic to flow over a connection:

Challenge Handshake Authentication Protocol CHAP CHAP authentication sends a challenge message to the remote device. The remote device encrypts the challenge value with a shared secret and returns the encrypted value and its name to the local router in a response message. The local router attempts to match the remote device s name with an associated secret stored in the local username or remote security server database; it uses the stored secret to encrypt the original challenge and verify that the encrypted values match.

Microsoft Challenge Handshake Authentication Protocol MS-CHAP MS-CHAP is the Microsoft version of CHAP. Like the standard version of CHAP, MS-CHAP is used for PPP authentication; in this case, authentication occurs between a personal computer using Microsoft Windows NT or Microsoft Windows 95 and a Cisco router or access server acting as a network access server.

Password Authentication Protocol PAP PAP authentication requires the remote device to send a name and a password, which are checked against a matching entry in the local username database or in the remote security server database.

command in interface configuration mode to enable CHAP, MS-CHAP, and PAP on a serial interface.

Note Enabling or disabling PPP authentication does not effect the local router s willingness to authenticate itself to the remote device.

Multilink Point-to-Point Protocol MLPPP is supported on the Cisco 819 ISR serial interface. MLPPP provides a method for combining multiple physical links into one logical link. The implementation of MLPPP combines multiple PPP serial interfaces into one multilink interface. MLPPP performs the fragmenting, reassembling, and sequencing of datagrams across multiple PPP links.

MLPPP provides the same features that are supported on PPP Serial interfaces with the exception of QoS. It also provides the following additional features:

Fragment sizes of 128, 256, and 512 bytes

Lost fragment detection timeout period of 80 ms

Minimum-active-links configuration option

LCP echo request/reply support over multilink interface

Full T1 and E1 framed and unframed links

Cisco keepalives are useful for monitoring the link state. Periodic keepalives are sent to and received from the peer at a frequency determined by the value of the keepalive timer. If an acceptable keepalive response is not received from the peer, the link makes the transition to the down state. As soon as an acceptable keepalive response is obtained from the peer or if keepalives are disabled, the link makes the transition to the up state.

Note The keepalive command applies to serial interfaces using HDLC or PPP encapsulation. It does not apply to serial interfaces using Frame Relay encapsulation.

For each encapsulation type, a certain number of keepalives ignored by a peer triggers the serial interface to transition to the down state. For HDLC encapsulation, three ignored keepalives causes the interface to be brought down. For PPP encapsulation, five ignored keepalives causes the interface to be brought down. ECHOREQ packets are sent out only when LCP negotiation is complete for example, when LCP is open.

command in interface configuration mode to set the frequency at which LCP sends ECHOREQ packets to its peer. To restore the system to the default keepalive interval of 10 seconds, use the

keyword. To disable keepalives, use the

disable command. For both PPP and Cisco HDLC, a keepalive of 0 disables keepalives and is reported in the

When LCP is running on the peer and receives an ECHOREQ packet, it responds with an ECHOREP packet, regardless of whether keepalives are enabled on the peer.

Keepalives are independent between the two peers. One peer end can have keepalives enabled; the other end can have them disabled. Even if keepalives are disabled locally, LCP still responds with ECHOREP packets to the ECHOREQ packets it receives. Similarly, LCP also works if the period of keepalives at each end is different.

When Frame Relay encapsulation is enabled on a serial interface, the interface configuration is hierarchical and comprises the following elements:

The serial main interface comprises the physical interface and port. If you are not using the serial interface to support Cisco HDLC and PPP encapsulated connections, then you must configure subinterfaces with permanent virtual circuits PVCs under the serial main interface. Frame Relay connections are supported on PVCs only.

Serial subinterfaces are configured under the serial main interface. A serial subinterface does not actively carry traffic until you configure a PVC under the serial subinterface. Layer 3 configuration typically takes place on the subinterface.

Point-to-point PVCs are configured under a serial subinterface. You cannot configure a PVC directly under a main interface. A single point-to-point PVC is allowed per subinterface. PVCs use a predefined circuit path and fail if the path is interrupted. PVCs remain active until the circuit is removed from either configuration. Connections on the serial PVC support Frame Relay encapsulation only.

Note The administrative state of a parent interface drives the state of the subinterface and its PVC. When the administrative state of a parent interface or subinterface changes, so does the administrative state of any child PVC configured under that parent interface or subinterface.

To configure Frame Relay encapsulation on serial interfaces, use the

encapsulation Frame Relay VC-bundle

Frame Relay interfaces support two types of encapsulated frames:

PVC configuration mode to configure Cisco or IETF encapsulation on a PVC. If the encapsulation type is not configured explicitly for a PVC, then that PVC inherits the encapsulation type from the main serial interface.

Note Cisco encapsulation is required on serial main interfaces that are configured for MPLS. IETF encapsulation is not supported for MPLS.

Before you configure Frame Relay encapsulation on an interface, you must verify that all prior

Layer 3 configuration is removed from that interface. For example, you must ensure that there is no IP address configured directly under the main interface; otherwise, any Frame Relay configuration done under the main interface will not be viable.

The Local Management Interface LMI protocol monitors the addition, deletion, and status of PVCs. LMI also verifies the integrity of the link that forms a Frame Relay UNI interface. By default,

the default LMI type, the maximum number of PVCs that can be supported under a single interface is related to the MTU size of the main interface. Use the following formula to calculate the maximum number of PVCs supported on a card or SPA:

MTU - 13 /8 maximum number of PVCs

Note The default setting of the mtu command for a serial interface is 1504 bytes. Therefore, the default numbers of PVCs supported on a serial interface configured with cisco LMI is 186.

This section contains the following tasks:

To specify a synchronous serial interface and enter interface configuration mode, use one of the following commands in global configuration mode.

Enters interface configuration mode.

By default, synchronous serial lines use the High-Level Data Link Control HDLC serial encapsulation method, which provides the synchronous framing and error detection functions of HDLC without windowing or retransmission. The synchronous serial interfaces support the following serial encapsulation methods:

Synchronous Data Link Control SDLC

Cisco Bisync Serial Tunnel BSTUN

To define the encapsulation method, use the following command in interface configuration mode.

Configures synchronous serial encapsulation.

Note You cannot use the physical-layer async command for frame-relay encapsulation.

Encapsulation methods are set according to the type of protocol or application you configure in the Cisco IOS software.

By default, synchronous interfaces operate in full-duplex mode. To configure an SDLC interface for half-duplex mode, use the following command in interface configuration mode.

Configures an SDLC interface for half-duplex mode.

Binary synchronous communication Bisync is a half-duplex protocol. Each block of transmission is acknowledged explicitly. To avoid the problem associated with simultaneous transmission, there is an implicit role of primary and secondary stations. The primary sends the last block again if there is no response from the secondary within the period of block receive timeout.

To configure the serial interface for full-duplex mode, use the following command in interface configuration mode.

Specifies that the interface can run Bisync using switched RTS signals.

The synchronous serial port adapters on Cisco 819 ISRs support half-duplex and Bisync. Bisync is a character-oriented data-link layer protocol for half-duplex applications. In half-duplex mode, data is sent one direction at a time. Direction is controlled by handshaking the Request to Send RST and Clear to Send CTS control lines. These are described in the Configuring Bisync section.

You can configure point-to-point software compression on serial interfaces that use HDLC encapsulation. Compression reduces the size of a HDLC frame via lossless data compression. The compression algorithm used is a Stacker LZS algorithm.

Compression is performed in software and might significantly affect system performance. We recommend that you disable compression if CPU load exceeds 65 percent. To display the CPU load, use the

If the majority of your traffic is already compressed files, you should not use compression.

To configure compression over HDLC, use the following commands in interface configuration mode.

The nonreturn-to-zero NRZ and nonreturn-to-zero inverted NRZI formats are supported on the Cisco 819 serial ports.

NRZ and NRZI are line-coding formats that are required for serial connections in some environments. NRZ encoding is most common. NRZI encoding is used primarily with EIA/TIA-232 connections in IBM environments.

The default configuration for all serial interfaces is NRZ format. The default is

To enable NRZI format, use one of the following commands in interface configuration mode.

When a DTE does not return a transmit clock, use the following interface configuration command on the router to enable the internally generated clock on a serial interface:

Systems that use long cables or cables that are not transmitting the TxC signal transmit echoed clock line, also known as TXCE or SCTE clock can experience high error rates when operating at the higher transmission speeds. For example, if the interface on the PA-8T and PA-4T synchronous serial port adapters is reporting a high number of error packets, a phase shift might be the problem. Inverting the clock signal can correct this shift. To invert the clock signal, use the following commands in interface configuration mode.

It is possible to send back-to-back data packets over serial interfaces faster than some hosts can receive them. You can specify a minimum dead time after transmitting a packet to remove this condition. This setting is available for serial interfaces on the MCI and SCI interface cards and for the HSSI or MIP. Use one of the following commands, as appropriate for your system, in interface configuration mode.

Sets the transmit delay on the MCI and SCI synchronous serial interfaces.

Sets the transmit delay on the HSSI or MIP.

You can configure pulsing Data Terminal Ready DTR signals on all serial interfaces. When the serial line protocol goes down for example, because of loss of synchronization, the interface hardware is reset and the DTR signal is held inactive for at least the specified interval. This function is useful for handling encrypting or other similar devices that use the toggling of the DTR signal to reset synchronization. To configure DTR signal pulsing, use the following command in interface configuration mode.

Configures DTR signal pulsing.

By default, when the serial interface is operating in DTE mode, it monitors the Data Carrier Detect DCD signal as the line up/down indicator. By default, the attached DCE device sends the DCD signal. When the DTE interface detects the DCD signal, it changes the state of the interface to up.

In some configurations, such as an SDLC multidrop environment, the DCE device sends the Data Set Ready DSR signal instead of the DCD signal, which prevents the interface from coming up. To tell the interface to monitor the DSR signal instead of the DCD signal as the line up/down indicator, use the following command in interface configuration mode.

On Cisco 819 ISRs, you can specify the serial Network Interface Module timing signal configuration. When the board is operating as a DCE and the DTE provides terminal timing SCTE or TT, you can configure the DCE to use SCTE from the DTE. When running the line at high speeds and long distances, this strategy prevents phase shifting of the data with respect to the clock.

To configure the DCE to use SCTE from the DTE, use the following command in interface configuration mode.

Router config-if dce-terminal-timing enable

Configures the DCE to use SCTE from the DTE.

When the board is operating as a DTE, you can invert the TXC clock signal it gets from the DCE that the DTE uses to transmit data. Invert the clock signal if the DCE cannot receive SCTE from the DTE, the data is running at high speeds, and the transmission line is long. Again, this prevents phase shifting of the data with respect to the clock.

To configure the interface so that the router inverts the TXC clock signal, use the following command in interface configuration mode.

The following sections describe the communication between half-duplex DTE transmit and receive state machines and half-duplex DCE transmit and receive state machines.

As shown in Figure 3, the half-duplex DTE transmit state machine for low-speed interfaces remains in the ready state when it is quiescent. When a frame is available for transmission, the state machine enters the transmit delay state and waits for a time period, which is defined by the

half-duplex timer transmit-delay

command. The default is 0 milliseconds. Transmission delays are used for debugging half-duplex links and assisting lower-speed receivers that cannot process back-to-back frames.

Figure 3 Half-Duplex DTE Transmit State Machine

After idling for a defined number of milliseconds ms, the state machine asserts a request to send RTS signal and changes to the wait-clear-to-send CTS state for the DCE to assert CTS. A timeout timer with a value set by the

command starts. The default is 3 ms. If the timeout timer expires before CTS is asserted, the state machine returns to the ready state and deasserts RTS. If CTS is asserted before the timer expires, the state machine enters the transmit state and sends the frames.

Once there are no more frames to transmit, the state machine transitions to the wait transmit finish state. The machine waits for the transmit FIFO in the serial controller to empty, starts a delay timer with a value defined by the

half-duplex timer rts-drop-delay

interface command, and transitions to the wait RTS drop delay state.

When the timer in the wait RTS drop delay state expires, the state machine deasserts RTS and transitions to the wait CTS drop state. A timeout timer with a value set by the

half-duplex timer cts-drop-timeout

interface command starts, and the state machine waits for the CTS to deassert. The default is 250 ms. Once the CTS signal is deasserted or the timeout timer expires, the state machine transitions back to the ready state. If the timer expires before CTS is deasserted, an error counter is incremented, which can be displayed by issuing the

command for the serial interface in question.

As shown in Figure 4, a half-duplex DTE receive state machine for low-speed interfaces idles and receives frames in the ready state. A giant frame is any frame whose size exceeds the maximum transmission unit MTU. If the beginning of a giant frame is received, the state machine transitions to the in giant state and discards frame fragments until it receives the end of the giant frame. At this point, the state machine transitions back to the ready state and waits for the next frame to arrive.

Figure 4 Half-Duplex DTE Receive State Machine

An error counter is incremented upon receipt of the giant frames. To view the error counter, use the

As shown in Figure 5, for a low-speed serial interface in DCE mode, the half-duplex DCE transmit state machine idles in the ready state when it is quiescent. When a frame is available for transmission on the serial interface, such as when the output queues are no longer empty, the state machine starts a timer based on the value of the

half-duplex timer transmit-delay

command, in milliseconds and transitions to the transmit delay state. Similar to the DTE transmit state machine, the transmit delay state gives you the option of setting a delay between the transmission of frames; for example, this feature lets you compensate for a slow receiver that loses data when multiple frames are received in quick succession. The default

interface configuration command to specify a delay value not equal to 0.

Figure 5 Half-Duplex DCE Transmit State Machine

After the transmit delay state, the next state depends on whether the interface is in constant-carrier mode the default or controlled-carrier mode.

If the interface is in constant-carrier mode, it passes through the following states:

1. The state machine passes to the transmit state when the

timer expires. The state machine stays in the transmit state until there are no more frames to transmit.

2. When there are no more frames to transmit, the state machine passes to the wait transmit finish state, where it waits for the transmit FIFO to empty.

3. Once the FIFO empties, the DCE passes back to the ready state and waits for the next frame to appear in the output queue.

If the interface is in controlled-carrier mode, the interface performs a handshake using the data carrier detect DCD signal. In this mode, DCD is deasserted when the interface is idle and has nothing to transmit. The transmit state machine transitions through the states as follows:

timer expires, the DCE asserts DCD and transitions to the DCD-txstart delay state to ensure a time delay between the assertion of DCD and the start of transmission. A timer is started based on the value specified using the

command. This timer has a default value of 100 ms; use the

half-duplex timer dcd-txstart-delay

interface configuration command to specify a delay value.

2. When this delay timer expires, the state machine transitions to the transmit state and transmits frames until there are no more frames to transmit.

3. After the DCE transmits the last frame, it transitions to the wait transmit finish state, where it waits for transmit FIFO to empty and the last frame to transmit to the wire. Then DCE starts a delay timer by specifying the value using the

command. This timer has the default value of 100 ms; use the

half-duplex timer dcd-drop-delay

4. The DCE transitions to the wait DCD drop delay state. This state causes a time delay between the transmission of the last frame and the deassertion of DCD in the controlled-carrier mode for DCE transmits.

5. When the timer expires, the DCE deasserts DCD and transitions back to the ready state and stays there until there is a frame to transmit on that interface.

As shown in Figure 6, the half-duplex DCE receive state machine idles in the ready state when it is quiescent. It transitions out of this state when the DTE asserts RTS. In response, the DCE starts a timer based on the value specified using the

command. This timer delays the assertion of CTS because some DTE interfaces expect this delay. The default value of this timer is 0 ms; use the

Figure 6 Half-Duplex DCE Receive State Machine

When the timer expires, the DCE state machine asserts CTS and transitions to the receive state. It stays in the receive state until there is a frame to receive. If the beginning of a giant frame is received, it transitions to the in giant state and keeps discarding all the fragments of the giant frame and transitions back to the receive state.

Transitions back to the ready state occur when RTS is deasserted by the DTE. The response of the DCE to the deassertion of RTS is to deassert CTS and go back to the ready state.

To return a low-speed serial interface to constant-carrier mode from controlled-carrier mode, use the following command in interface configuration mode.

To optimize the performance of half-duplex timers, use the following command in interface configuration mode.

The timer tuning commands permit you to adjust the timing of the half-duplex state machines to suit the particular needs of their half-duplex installation.

command and its options replaces the following two timer tuning commands that are available only on high-speed serial interfaces:

To specify the mode of a low-speed serial interface as either synchronous or asynchronous, use the following command in interface configuration mode.

Router config-if physical-layer sync

Specifies the mode of a low-speed interface as either synchronous or asynchronous.

This command applies only to low-speed serial interfaces available on Cisco 2520 through Cisco 2523 routers.

Note When you make a transition from asynchronous mode to synchronous mode in serial interfaces, the interface state becomes down by default. You should then use the no shutdown option to bring the interface up.

In synchronous mode, low-speed serial interfaces support all interface configuration commands available for high-speed serial interfaces, except the following two commands:

When placed in asynchronous mode, low-speed serial interfaces support all commands available for standard asynchronous interfaces. The default is synchronous mode.

Note When you use this command, it does not appear in the output of the show running-config and show startup-config commands because the command is a physical-layer command.

To return to the default mode synchronous of a low-speed serial interface on a Cisco 2520 through Cisco 2523 router, use the following command in interface configuration mode.

Multilink PPP. Multilink Point-to-Point Protocol MLPPP is supported on the Cisco 819 ISR serial interface. MLPPP provides a method for combining multiple physical.