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Posted on in Networking
Use Cisco ip helper-address deliberately: verify the default UDP broadcasts it forwards, keep DHCP working, and restrict only the protocols your topology does not need.
Posted on in Networking
This isn't late breaking information, but there's an interesting post in the Cisco WAN, Routing and Switching Forum regarding a particular regular expression that, when used in conjunction with the show ip bgp regexp, results in a router crash. Here is the full command.
show ip bgp regexp (.*)(_\1 …Posted on in Networking
Let me start by saying this article is not about server load-balancing. That's a different topic, and I apologize if that's what you were looking for. That said, interface load-balancing is an interesting topic and one that comes up quite frequently.
A time will come when it becomes necessary to …
Posted on in Networking
On most networks, there is a subset of IP addresses assigned to "management" hosts. These hosts might be the workstations of network administrators or monitoring servers. One of the keys to network security is restricting who has access to the device. Generally, we think of access restriction in terms of …
Posted on in Networking
The last thing I want to do when some network administrator leaves my organization is change all the login passwords on the Cisco network devices. (For now, let's just assume the administrator in question didn't know the enable secret.) If you have relatively few machines, it's easy to setup username …
Posted on in Networking
If you followed my recent Cisco Catalyst rate-limiting post, you already know that policing traffic on a Cisco Catalyst switch requires a bit of thought. Mainly, you have to do a bit of calculating to determine the best bucket size for your application. The good news is that achieving the …
Posted on in Networking
When a Cisco switch or router reports a power problem, start by finding out whether the issue is input power, a failed supply, redundancy, PoE budget, or a sensor alarm. Those are related, but they are not the same failure.
For most Cisco IOS and IOS XE platforms, this is …
Posted on in Networking
If you work on a fairly spread out network (or have remote clients), you might be managing Cisco Catalyst switches that are far, far away. For a long time, I thought the only option was to have someone on site to pre-configure the switch just enough to get it on …
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
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 …
Posted on in Networking
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 …
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
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 …
Posted on in Networking
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 …
