Cabrillo College and Los Medanos College

Building Cisco Remote Access Networks - Building Cisco Remote Access Networks - X.25X.25Rick Graziani, Mark McGregor

February 27, 2001February 27, 2001

X.25X.25 is a standard that defines the connection

between a terminal and a packet-switching network. – Virtually every nation uses some X.25– originated in the early 1970s

Developed in the days of analog circuits when error rates were much higher.

With today’s digital and fiber optic technologies, error rates have dropped dramatically

.

Many different network-layer protocols can be transmitted across X.25 virtual circuits (VCs) which results in tunneling or Layer 3 packets with the X.25 Layer 3 packet.• Each Layer 3 packet keeps addressing legal for its respective protocol, whereas the X.25 VC transports the packet across the WAN

X.25 Protocol Stack

X.25 reliability

X.25 is used as an over-engineered data link in the internetworking world. – Both X.25 at Layer 3 and Link Access

Procedure, Balanced (LAPB) at Layer 2 provide reliability and sliding windows

– Layers 2 and 3 were designed with strong flow control and error checking to reduce the requirement for these functions external to X.25.

X.25 Components

X.25 Components

The X.25 protocol implements virtual circuits between the X.25 DTE and X.25 DCE.

X.25 Components

The X.25 DTE is typically a router or a packet assembler/disassembler (PAD).

The X.25 packet-level DCE typically acts as a boundary function to the public data network (PDN) within a switch or concentrator.– The X.25 switch at the carrier site may also

be called data switching equipment (DSE).

X.25 Components

Identifying the PAD

X.25 Components

The PAD is a device that collects data from a group of asynchronous terminals and periodically outputs the data in X.25 packets.

A PAD also takes data packets from a host and turns them into a character stream that can be transmitted to the terminals.

X.121 Addressing Format

X.121 Addressing

The first four digits specify the Data Network Identification Code (DNIC). – The first three digits specify the country code. – The fourth digit is the provider number assigned by

the ITU-T. – Countries that require more than ten provider

numbers are assigned multiple country codes. The remaining 8 to 10 or 11 digits specify the

network terminal number (NTN) that is assigned by the packet-switched network (PSN) provider.

X.25 Addressing

For different network protocols to connect across X.25, statements are entered on the router to map the next-hop network-layer address to an X.121 address. – An IP network-layer address is mapped to

an X.121 address to identify the next-hop host on the other side of the X.25 network.

X.25 Maps (like ARP)

X.25 Encapsulation

Virtual Circuits (VCs)

X.25 typically uses SVCs. An SVC exists only for the duration of the session.

Three phases are associated with SVCs:

•Call setup

•Information transfer

•Call clear

X.25 Multiprotocol Support

In Cisco IOS® Release 10.2 and later releases, a single VC to a host can carry traffic from multiple protocols.

One X.25 map statement contains several protocol addresses, mapped to a single X.121 address associated with the destination host.

X.25 Multiprotocol SupportTwo X.25 Encapsulations - Encapsulation of IP

and other layer 3 network protocols over X.25 networks.

Cisco Internet Engineering Task Force’s (IETF's) -

RFC 1356allows hosts to exchange several protocols over a

single VC.

The Cisco encapsulation method is the default (for backward compatibility), unless the interface configuration command specifies IETF.

Configuring X.251. Setting the Encapsulation

Router(config-if)# encapsulation x25 [dte|dce] dte is the default, which means the X.25 switch is

the DCE

2. Assign the X.121 address (supplied by provider)

Router(config-if)# x25 address x.121-address

3. Define map statements to associate x.121 address with layer 3 (ip) address

Router(config-if)# x25 map protocol protocol-address x.121-address [options]

ip-address and x.121-address are remote router addresses

SVC Configuration Example

2

1

PVC Configuration Example

Instead of defining a SVC with the X25 map statement, you establish a PVC.

X.25 PVCs are the equivalent of a leased lines, they never disconnect.

Router(config-if)# x25 pvc circuit protocol-address x.121-address [options]

ip-address and x.121-address are remote router addresses

X.25 VC RangesVC TypesIncoming-only trafficTwo-way trafficOutgoing-only traffic

X.25 VC RangesThe incoming-only, two-way, and outgoing-only ranges define the

virtual circuit numbers over which a switched virtual circuit (SVC) can be established by placing an X.25 call, much like a telephone network establishes a switched voice circuit when a call is placed.

The rules about DCE and DTE devices initiating calls are as follows: Only the DCE device can initiate a call in the incoming-only range. Only the DTE device can initiate a call in the outgoing-only range. Both the DCE device and the DTE device can initiate a call in the

two-way range.

X.25 VC Ranges - OrderCircuit number must be assigned in a specific order, I.e.

that an incoming range of SVCs come before a two-way range.

PVC 1-4095 None x25 pvc circuit

SVCs Incoming 1-4095 0 x25 lic circuit

1-4095 0 x25 hic circuit Two-way 1-4095 1 x25 ltc circuit

1-4095 1 x25 htc circuit Outgoing 1-4095 0 x25 loc circuit

1-4095 0 x25 hoc circuit

i = incoming, t = two-way, o = outgoing

l = low, h = high, c = circuit

X.25 VC Ranges - OrderP-I-T-O = PVC, Incoming, Two-way, Outgoing

If you acquire 7 circuits form your SVC provider, you must partition them according to their different calling situations - incoming, outgoing, two-way:

1 Incoming

2 Incoming

3 Incoming

4 Two-Way

5 Two-way

6 Outgoing

7 Outgoing

X.25 VC Ranges

The following example illustrates how to set a valid two-way virtual circuit range of 5 to 25:

interface serial 0 x25 ltc 5 x25 htc 25

X.25 Packet SizesTo set the default maximum input packet size:

Router(config-if)# x25 ips bytes

To set the default maximum output packet size:

Router(config-if)# x25 ops bytes

bytes = 16, 32, 64, 128, 256, 512, 1024, 2048, 4096. Default is 128 bytes.

Input and output values should match unless network supports asymmetric transmissions. Ask you PSN.

X.25 Window Parameters Window size specifies the number of packets that can be

received or sent without receiving or sending an X.25 acknowledgement. Both ends of the link must use the same default window size.

To set the default unacknowledged packet limits:

Router(config-if)# x25 win packets

Router(config-if)# x25 wout packets

Router(config-if)# x25 modulo modulus

packets = Packet window size, assumed for VCs that do not negotiate a size. Range is one to less than the modulus. The default is 2 packets.

Modulus = Specifies the packet-number modulus that affect the maximum window size. Either 8 or 128. 8 is widely used and allows VC sizes up to 7 packets. 128 (127) is rare.

Sample Configuration

interface serial 0

ip address 131.108.9.1 255.255.255.0

encapsulation X25

x25 win 7

x25 wout 7

x25 ips 512

x25 ops 512

x25 address 31370054065

x25 ltc 5

x25 htc 25

x25 map IP 131.108.9.3 31370019134 broadcast