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-...
- 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 ...
- 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.
...
- 29.9k
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 ...
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 ...
- 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 ...
- 16.2k
4
votes
How to find the end-device causing high cpu usage in a Cisco switch?
What is driving CPU usage in a Cisco switch is probably not a single end-device. Cisco switches can normally handle a full switching load without driving the CPU utilization unusually high. This is ...
- 98.8k
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 ...
- 2,449
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.
...
- 165
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 ...
- 243
3
votes
Latency in P2P Circuit
This is to be expected if your routers are filling the buffers. If you are using ping to test this, understand that ping has the lowest priority.
Depending on your traffic mix, you may want to use ...
- 98.8k
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 ...
- 31
3
votes
Effects of local switch time on network performance
Regarding your first question local time on certain switch or switching equipment won't affect any switching Performance. You may face this problem and in the devices which are multi homed to multiple ...
- 3,444
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 ...
- 31.8k
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 ...
- 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. ...
- 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,...
- 27.2k
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.
...
- 82.8k
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 ...
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 ...
- 98.8k
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, ...
- 82.8k
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....
- 146
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 ...
- 21
2
votes
Is NAT loopback the only reason I can't view my own website from within my LAN?
You have diagnosed the problem correctly. The problem is that you try to access your webserver which is located within your local network by its public ip address.
Your router will not NAT your DST ...
- 767
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
- 21
2
votes
Input packet drops on port channel
You get a high number of input drops if your device cannot service the input queue fast enough. There can be many reasons for this. For instance, routing policies or other services which cause process ...
- 98.8k
2
votes
Accepted
Static NAT using IP address from different address space
The non-NAT router is missing a route for the address 130.130.130.130.
Try injecting the required route as static route and then redistribute it into OSPF of NAT router:
router ospf 1
redistribute ...
- 1,636
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 ...
- 5,290
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 ...
- 13.2k
Only top scored, non community-wiki answers of a minimum length are eligible