3troubleshootingjunos

[edit interfaces so-0/1/1]

1

user@London# run ping 10.0.22.1 count

PING 10.0.22.1 (10.0.22.1): 56 data bytes

36

bytes from 10.0.22.2: Time to

live

exceeded

Dst

Vr HL TOS

Len

ID Flg

off TTL

Pro

cks

Src

4

5

00

0054

601b

0

0000

01

01

198c

10.0.22.2

10.0.22.1

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--- 10.0.22.1 ping statistics ---

received, 100% packet loss

1 packets transmitted, 0 packets

Not

for

Reproduction

Layer 2 Protocols and Loo backs

Many Layer 2 protocols make use of a keepalive mechanism that, among other things,

can detect the s nce of a loopback. Whether local or remote, the detection of a

loop condition results in a link down declaration for that interface. When the interface

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is ma ked as down at the Data Link Layer, the related interface route is removed from

the outing table, which prevents ping testing for the duration of the loopback. (We

desc ibe ping testing over a loopback on subsequent pages.)

In m st cases you can work around this issue by configuring the interface with a

no-keepalives statement, but, as shown on the slide, this workaround only works

r the frame-relay, atm, and cisco-hdlc encapsulation types. Even with

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keepalives (LCP) disabled, PPP still detects the presence of a loopback when the

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Looping Line and K

ing Interface Up

In the example the slid

, we looped the line externally. This loop might be a hard

loop, a telco loop, or a r mote interface loop for the purposes of this example.

Because some Data Link Layer protocols detect the looped condition, and disable the

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inte face as a result, you must use either ATM Adaptation Layer 5 (AAL5), Frame Relay,

Cisco HDLC encapsulation with keepalives turned off. PPP encapsulation generally

d es not work, because the looped condition prevents the NCP from completing its

initialization, thereby preventing a declaration of up for the interface.

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Pinging Remote IP Address

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Because the default TTL for locally generated pings is 255 on a JUNOS device, each

TTL expiration message indicates 255 successful transmissions and receptio

s of the

initial ping request, all at wire speed.

Setting the TTL to a lower value is useful when trying to determine marg nal ty

f a

line. That is, a TTL of 1 requires only a single transmission and recep

f he

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BERT Tests

quire a Loop

The pattern

iv through a loop is verified against the pattern sent. End-to-end

testing is difficult to coordinate. By changing the position of the loop downstream from

the device performing the test, you can locate the problem area easily. Common

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points for looping the line are the telco demarcation point (also named demark), the

emote end, and the midpoint (with help from the carrier).

Y u can configure any of the following interfaces to execute a BERT test when the

inte face eceives a request to run this test: E1, E3, T1, T3, the channelized DS-3,

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OC-3, OC-12, STM-1, the channelized DS-3 IQ, E1, and OC-12 IQ.

BERT Parameters

Start and stop the BERT test with the test interface interface-name

bert-start and bert-stop commands. Note that you cannot run a BERT test on

an interface that is administratively enabled. To start a BERT test you must first

disable the interface with a set interfaces interface-name disable

statement. After the test completes, you can use a rollback 1, commit

command sequence to re-enable the interface, or you can remove the disable

statement with a delete interface interface-name disable stateme .

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Media-Specific Int rface Troubleshooting

The slide highlights the topic we discuss next.

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