Provides the missing software ingredient to create a fully functional back up sysetm; supported in Cisco IOS ver 9 or later; supports IP, IPX, Aplletalk, DECnet, OSI/CLNS; DDR can be used over sync/Async serial links & ISDN; supports only Static routing(not always active to support dynamic).
X.25: std defines conn between a term & a pkt switching net. works on switched/perm virtual ckts, uses addsng defined by X.121(1-15 decimal digits long), first 4 bits used for DNIC (Data Net Identifier Code), rest is NTN (Net Term No.) & can be assigned by admnr/PSN provider. X.25 defines p-p comms between DTE (router) & DCE (modem or csu/dsu). Can be placed in b types of eqpmts. Works at layer3 using LAPB at layer2. reliable & sliding windows.

DTE , Data Ckt Term Eqpt (DCE), Pkt switching Exge (PSE)

X.25 Sessions: used for DTE to DTE commn. process estblmt > One dte contacts other, sec one accepts/refuses, if accepted full duplex comm begins, either one can terminate conn. Once terminated must repeat same process.

Virtual Ckts: is a logical bi-directional path from one DTE to other, can pass thro X no.of nodes, can be mux/demuxed,

SVC: Switched Virt Ckt is a temp conn made & terminated for intermittent use. max 8 svc/proto/destn. max 9 proto/svc.

PVC: Perm Virt Ckt is always active once setup, DTE’s can use it any time w/o make/termn.

X.25 Proto suite : maps to lowest 3 layers. Pkt layer proto(PLP), LAPB, X.21 bis are typically used in X.25 imples.

PLP (Pkt layer proto): X.25’s net layer proto, manages pkt exges between DTE devices, can also run over LLC2 imples on LAN, and over ISDN int running LAP on D chnl (LAPD). Five modes of opns : 1.Call set up mode: creates SVC between DTE’s, not used with PVC’s; 2.Data Xfer Mode: used for data xfer between DTE’s via virt ckt, segmentation, reassembly, bit padding, and error & flow control, used with svc & pvc 3.Idle Mode: used when a svc estbed, not with pvc 4.Call clearing mode: used to terminate commn sessions & svc’s, not with pvc’s 5. Restarting mode: used to synch commn between DTE & local DCE,
Link Access Procedure Balanced (LAPB): Bit oriented proto, makes sure that frames are error free & properly sequenced, 3 diff frame types 1. Info frame (I-frame): transports upper layer & some control info, helps sequencing, flow control, error detection & recovery jobs. 2. Supervisory frames (S-frame): Bearing control info, handle req for and the suspension of xmission, 3.Unnumbered frames (U-frames): Bearing control info, handle link setup & termn.

X.21bis: used in X.25 physical layer, at 19.2KB x.21bis supports p to p conn and synch, full duplex Xmission over 4 Wires.

X.25 on Cisco: # config t, # int s0, # encap x25 - x.25 enabled; If# x25 address 12345-config local x.121 add on x25 ser int;

Mapping proto: # x25 map[proto] [add] [ x.121add] [options], # x25 map ip 10.6.8.1 123456 broadcast

config i/p, o/p pkt size: If# x25 ips 256, # x25 ops 256 (def-128,max-4096B), config VC range: # x25 pvc xx (1-4095)

config i/p, o/p window sizes If# x25 win 7, # x25 wout 7 (def-2, max-127).

Monitoring X.25: R# sh int s0- shows x.25 config & encap

FRAME RELAY: A Layer2 proto, efficient(sharing), faster(no error check), shares b/w with other users, less cost.

CIR-Committed info rate (min guaranteed b/w)-Gives commn int bet Dte & Dce, uses more pvc than svc. Gives conn oriented Dllayer comm by using virtual ckts like x.25. uses Inv ARP(find remote IP from dlci values)

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