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Posted on in Networking
Sending log messages from your Cisco network equipment to a centralized syslog server is a fairly normal activity. Most of the time, this setup works fine and there's no reason to do anything out of the ordinary. Occasionally though, you might need to run a debugging session over an extended …
Posted on in Networking
Anyone who has ever done anything remotely "interesting" with a run-of-the-mill broadband router is undoubtedly familiar with the concept of port forwarding. In the case of some applications (<acronym title="Peer-to-Peer">P2P</acronym> comes immediately to mind, but the <acronym title="Real-time Transport Protocol">RTP</acronym> part of <acronym title …
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If you've spent any amount of time in Cisco networking with Catalyst switches, you've probably run across a port that is in err-disabled state. The most likely place to find this is in the output of show interfaces status. A switch port can end be error disabled if the software …
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A remote network change becomes an outage when the change breaks the only path back to the device. It does not matter whether the mistake is a bad ACL, an incorrect default route, an interface change, or an authentication policy that locks out your SSH session. If you have no …
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Ars Technica recently discussed the effect YouTube has had on the popularity of P2P networking. According to a study done by Ellacoya Networks, HTTP traffic has surpassed P2P protocols as the dominant form of Internet downloads for the first time in four years. Considering my recent discussion of the top …
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Robin Gareiss of Nemertes Research has a put together a nice list of questions for potential WAN providers. Most of the questions are focused on non-technical issues like billing and customer service. In fact, the most technical question in the list is
- How many classes of service do you provide …
Posted on in Networking
Diagnose and allow trusted DHCP Option 82 requests through Cisco relay agents without using a global trust setting that weakens the relay boundary.
Posted on in Networking
If you've ever needed to reset a Cisco Catalyst switch to it's factory default (locked out, perhaps?), you've probably spent some time digging around on Cisco's web site. The Catalyst password recovery procedure starts off by reseting the Catalyst to it's original factory settings. Below, I'm going to give you …
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Configure Cisco Catalyst DHCP snooping and Option 82 safely with a small pilot VLAN, explicit trusted uplinks, verification commands, and a rollback plan.
Posted on in Networking
How many times has this happened to you?
SLAP#write memory
startup-config file open failed (Device or resource busy)
If you work in an environment where multiple users have enable access on Cisco devices, you've probably seen it more than once. This particular error is caused when another user has …
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DS3 (also known as T3) scrambling is not an issue that comes up very often. It's something every network administrator should know about, though. After all, it just might save your bacon.
Some physical layer protocols depend on the transition from 1 to 0 in a frame to maintain clocking …
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Most network administrators should be familiar with Spanning Tree Protocol (STP; IEEE 802.1D). <acronym title="Spanning Tree Protocol">STP</acronym> is tasked with ensuring there are no loops in a bridged network. Unfortunately, STP is an older protocol and doesn't quite mesh with today's high speed networks. Here are …
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Is your network bandwidth being consumed by Peer-to-Peer (P2P) traffic? (Hint: If you don't know, it's time to fire up NBAR and do a little investigating.) One way to stop P2P traffic is to use an access-list to block traffic on the well-know P2P ports. Unfortunately, many P2P technologies no …
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I recently wrote about using Cisco's NBAR to investigate what protocols were in use on your network. I thought it might be interesting to take a look at the top 10 protocols on a residential broadband network. If you're running a large network, or planning a large network, perhaps this …
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The following is a list of alarms you may receive on your T1 or T3 (or E1/E3) interface. Where applicable, troubleshooting hints are included.
A Loss of Signal (LOS) alarm indicates that the interface is not receiving a valid T1 or T3 signal. This is …
