Serial interface "Line Protocol" up/ down question

I have multiple remote routers with the same type of ISDN backup

setup. I have one that will not go UP, UP when doing a Show Int. What

dictates if Line Protocol is UP? (See Sh Int below.

#sh int s4/0

Serial4/0 is up, line protocol is down

Hardware is cxBus Serial

Internet address is

MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation PPP, crc 16, loopback not set

Keepalive set (10 sec)

Restart-Delay is 0 secs

LCP Closed


Last input 00:00:08, output 18:55:50, output hang never

Last clearing of "show interface" counters 22w3d

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops:


Queueing strategy: weighted fair

Output queue: 0/1000/64/0 (size/max total/threshold/drops)

Conversations 0/1/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

Available Bandwidth 48 kilobits/sec

5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 2322 packets input, 53834 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 1856 packets output, 38512 bytes, 0 underruns 0 output errors, 0 collisions, 500 interface resets 0 output buffer failures, 0 output buffers swapped out 4 carrier transitions

RTS up, CTS up, DTR up, DCD up, DSR up

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you have configured "PPP" as the upper protocol (encapsulation) for the serial link and this wents from DOWN to UP only when the PPP-negotiation between your router and the remote one was successful. maybe the remote router was not configured for PPP? the default is HDLC, e.g. when "encapsulation" is omitted completely.

shut/no shut together with "debug ppp ..." may help also.


Reply to
Gerald Krause

maybe an IP subnet mismatch?


Reply to
Andre Janssen

cannot be an ip subnet mismatch because the problem is layer 2.

as you can see in the show interface... "LCP Closed" if ever the ppp negotiation went fine, this should be LCP open.

Reply to

I think that it is a simple line problem , Or on the sending link , so you don't see any error o crc , but the troble exist . Or in a layer 2 not directly connect at your interface .

Try to open a ticket at you carrier .

See :

500 interface resets

Best Regards


Reply to

Gerald Krause wrote in news:3lsdltF1441n2U1

generally, interface up/down refers to carrier detect, and line proto refers to keepalive state. Since interface is up, but line proto down, checking the encaps would be a good start.

Reply to
Chris Marva

I would recommend to check if the encapsulation is the same on both sides. See comments below. "PPrash" schrieb im Newsbeitrag news:E9-dnSRzEeyURWXfRVn

Is PPP used on both sides? For serial interfaces, the default is HDLC! .... Both sides sent a lot of packets for initialization, but do not understand each other at all. There were no errors counted since weeks. So, L1 seems o.k., but L2 is not. You should check the encapsulation settings on both sides.

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