13 votes

How to verify that a UDP port is open?

This is a quick recipe: 1) Start a packet sniffer: sudo tcpdump -n -i eth2 icmp & [1] 1409 $ tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth2, link-...
Everton's user avatar
  • 1,636
13 votes

How to verify that a UDP port is open?

UDP is obviously a send-and-forget protocol. For example, during an NMap UDP scan, the only way to definitively prove that a UDP port is open is if you receive a response from that port. Keep in mind ...
Goodies's user avatar
  • 445
7 votes
Accepted

Fix our Cisco switch overwhelmed by console logging

I have a setup with dual redundant Cisco catalyst 4506-E using the WS-X45-SUP8L-E supervisor engine and two 48-port switches below it. My task is to access and modify the switch configuration. ...
Mike Pennington's user avatar
6 votes

How to verify that a UDP port is open?

"nc -uvz ip port" isn't somehow accurate, you probably should use "nmap -sU -p port ip", if the result shows "open" then the udp port probably is open, if it shows "open|filtered" then probably it is ...
Jailman Tough's user avatar
5 votes

Cisco 1800 series: sys-2-chunkexpanfail: error every time

If you are on release 12.4(15)T12 | 12.4(24)T2 then this could be related to a Cisco bug CSCtg26917 with more details on it here Symptom: Chunk manager consume almost all memory. We found the ...
SleepyMan's user avatar
  • 2,016
5 votes
Accepted

PAT not translating at all

Looks like the NAT access-list 1 is too tight. host 10.0.0.0 will only match the single host with address 10.0.0.0, and you have no such host. You almost certainly wanted this: access-list 1 ...
jonathanjo's user avatar
  • 16.2k
4 votes
Accepted

Is it possible to tell if a firewall is present on my network path?

Traffic can be routed between VLANs by a router, there is no requirement for a firewall (which ultimately is really a router with some filtering features, anyway). There is no way to say for sure ...
Jeremy Gibbons's user avatar
4 votes
Accepted

Steps on troubleshooting a network

Handy way to start troubleshooting is from Physical Layer following OSI Layer Model Check physical port/cable (as mentioned by @Zac67 also) Check port negotiation & Duplex, speed on both ends. ...
engineerbaz's user avatar
3 votes

Output drops/errors on Cisco 3850

You're running the interface at 94% of capacity -- 938M out of 1000M -- without flow control. The hardware buffer space is very limited (and shared between several ports in a group) so this is ...
Ricky's user avatar
  • 32.2k
3 votes

I can't ping 8.8.8.8 from 2960 L2 switch?

That's pretty normal. Looks like your switch doesn't have the default gateway set, try ip default-gateway <IP of your gateway> in the enable mode. Since clients connected to this switch can ...
drookie's user avatar
  • 243
3 votes

DLink switch loses configuration on power off!

This has just happened to me with my Dlink DGS 3120-24SI. Apparently you need to go to the web gui then select Save ... Save Configuration / Log. Change Type to All, then Apply. This saves everything ...
Patch67's user avatar
  • 31
3 votes

How to verify Voice QoS dscp and IP predecence traffic session?

To see actual traffic for each class: show policy-map interface gi0/0 In order to see actual traffic passing through the interface you'll need some kind of network sniffer with combination of port ...
ar_'s user avatar
  • 1,133
3 votes
Accepted

Adding encrypted password on Cisco 3560-CG switch

You are receiving the error because the "password" you are entering is not a proper MD5 hash. Check to verify that you are trying to enter a complete MD5 hash and that it is not another type of hash. ...
TDurden's user avatar
  • 1,241
3 votes
Accepted

What can I infer about a network connection using a cable tester (if anything)

According to the instructions for the NS-468, it is designed and engineered to use the master unit at one end of the cable and the remote unit at the other end of the cable. Based on the instructions,...
YLearn's user avatar
  • 27.3k
3 votes

What can I infer about a network connection using a cable tester (if anything)

This type of cable tester is intended for bare cables only. Connecting it to a live network can damage the network component on the other side or (more likely) the tester itself, esp. on a PoE link. ...
Zac67's user avatar
  • 84.4k
3 votes

Cisco Router IOS update

Don't delete the IOS from the flash> PLease share the output # show boot -- check the NEW IOS is in the next reboot if not do below command. conf t boot system flash:IOS image name.bin Save the ...
Kamaleshwar Kumar's user avatar
3 votes

How can you determine the result of a load-balancing hashing algorithm (such as ECMP/LAG) for troubleshooting?

There are some things to test in certain circumstances, but the basic idea is that PAgP or LACP aggregations are between two directly-connected devices, and anything sent from one device to the other ...
Ron Maupin's user avatar
  • 99.6k
3 votes

Do wiremap testers detect split pairs

Most do, but not necessarily all of them - some just test for DC continuity and shorts. While the term is widely understood among professionals, there may be vendors trying to fool you. After all, ...
Zac67's user avatar
  • 84.4k
3 votes
Accepted

Wireshark filter for specific SYN packet which never received a SYN/ACK

According to https://osqa-ask.wireshark.org/questions/60995/how-do-i-filter-tcp-connection-with-syn-and-syn-ack-without-ack-response/ you can do this by applying a filter (tcp.flags==0x12) and not tcp....
Paul's user avatar
  • 154
2 votes

Interpreting TTL in ping results

TTL values are determined by the remote machine. For example, if you ping a Linux machine, its originating TTL value is 64. Depending on how many routers it crosses to get back to you, the TTL is ...
dubu2's user avatar
  • 21
2 votes

DLink switch loses configuration on power off!

Previous answer given by patch67: go to web gui, Save Configuration / Log. Change Type to All, then Apply. works perfectly for my device: dgs-3120-48tc
matteo's user avatar
  • 21
2 votes

Mixed Use Four-pair Cable

The first thing I would do is disconnect the phone and CCTV temporerally. If the Ethernet suddenly starts working you have an interference problem. If not then there is some problem with the ethernet ...
Peter Green's user avatar
  • 13.3k
2 votes

Trouble connecting a small network.. pinging

The issue is most likely that the BR has not route to your 172.16.0.0/16 network so cannot send any response. You could NAT all traffic coming from your network in 172.16.0.0/16 to your 'external' IP ...
JFL's user avatar
  • 19.6k
2 votes

Sniffing a Device's Traffic on LAN

If you are using a switch, i guess the only passive option to sniff the traffic for/from that machine is configuring port mirroring(on a capable switch) on the interface connected to that device. ...
Orlando Gaetano's user avatar
2 votes

MLPPP_Bundle_load sharing

Different devices and software/firmware use different load-balancing hashing algorithms, what equipment are you using and what load-balancing method are you using? Based on the minimal information ...
Baldrick's user avatar
  • 5,300
2 votes

virtual box to other virtual box connectivity issue

Did you change the mac addresses for the networks cards for those two virtual box machines? I mean... are those addresses different? That may cause problems. Bridge connection is the same as direct ...
reybanger's user avatar
  • 305
2 votes

Cisco ASA 5505 ver 8.2 Firewall

Your access list allow only echo-reply, but not requests. Also Cisco ASA is statefull firewall, so you don't need your access list applied on every interface, only on outside interface. When icmp ...
Evgenii Berezhnoy's user avatar
2 votes
Accepted

Juniper EX4200 firewall filter applied to trunk port not matching specific VLAN

This appears to be a defect - I just recreated a similar environment on an EX42000-VC running 15.1R3.6. family ethernet-switching { filter LIMIT-VLANS { term VID10 { from { ...
Benjamin Dale's user avatar
2 votes
Accepted

Time exceeded ICMP in Serial Interface [Cisco Router 2811]

Serial0/0/0 is up, line protocol is up (looped) You interface is looped and hence when you ping the IP 210.220.61.1 on the other end you are stuck in the loop and hence TTL(hop count) gets expired, ...
Anirudh Malhotra's user avatar

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