I haven't figured out why I would need this particular IOS feature (IOS documentation says you can at least get a glimpse of what was happening before the logging buffer wrapped over), but it's such a cool one I simply have to mention it ... starting with release 12.2(8)T and 12.3, IOS can tabulate the occurence of each log message.

In designs with very low number of IP hosts, no per-destination load-sharing algorithm will work adequately. Consider, for example, an extranet design where a large number of IP hosts are NAT-ed to a single IP address which then accesses a single remote server.

In this design, all the traffic flows between a single pair of IP addresses, making per-destination load-sharing unusable.

Have you ever experienced a long wait between the moment you've configured a static route and its appearance in the IP routing table? The reason might be that Cisco IOS by default adjusts static routes every 60 seconds. IOS release 12.0(29)S (integrated in 12.3(10) and 12.4) fixes this problem with the ip route static adjust-time seconds global configuration command.

The log-adjacency-changes OSPF configuration command was improved with the detail command that logs every step of OSPF adjacency establishment (sample printout below), making it a great troubleshooting tool.

%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from DOWN to INIT, Received Hello
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from INIT to 2WAY, 2-Way Received
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from 2WAY to EXSTART, AdjOK?
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from EXSTART to EXCHANGE, Negotiation Done
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from EXCHANGE to LOADING, Exchange Done
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from LOADING to FULL, Loading Done

Although you probably don't ever want to stop all router's logging activities (at the very minimum you should collect the messages in a memory buffer with the logging buffered command), the global configuration command to do it is no logging on.

Note: this article is part of You've asked for it series.

Large amount of logging output (most often produced in a debugging process) sent to a router's console can significantly increase the router's CPU load and even stop the box from forwarding packets (high-end routers with distributed forwarding architecture are obviously an exception, but even they can lose routing adjacencies). The reason is very simple - console interrupt is one of the highest-priority interrupts on the router (otherwise you wouldn't be able to get a response to the BREAK key on a hung box).

Update January 9th 2007: The router does not check if a user is logged into the console port or a device (for example, a terminal) is attached to it; if console logging is enabled, messages are always sent to the console port (causing CPU load).

To stop the console logging, use the no logging console global configuration command (highly recommended for routers that are not usually accessed through the console port) or you might want to limit the amount of messages sent to the console with the logging console level configuration command (for example, logging console notifications).

Note: this article is part of You've asked for it series.

Analyzing Google query strings that brought visitors to my blog (StatCounter is an excellent free tool to do this job), I usually find interesting (often repeating) queries that are not yet answered in my blog. Obviously there are not too many good answers on other web sites, otherwise Google users would probably not click on a hit on the second or third page (where my blog usually appears for more generic queries).

So, to help my fellow networking engineers, I've decided to start a series of "You've asked for it" articles answering the questions that brought many of you to my site in the first place (and, don't forget, you can always send me an interesting question with the Send a message link on my bio page.

The ability to replace router uptime with date and time in the logging messages with the service timestamps log datetime command was present in IOS for a long time, but I was always annoyed at timestamps when collecting syslog messages for demonstration purposes. The command to turn them off has also been available "forever", but was too obvious for me to try out ... the no service timestamps log command.

The Configuration Rollback feature (first available in IOS release 12.3(7)T) fails completely when handling configurations containing active IP SLA probes.

Cisco Express Forwarding (CEF) can perform per-packet or per-destination (actually source/destination IP address pair) load-sharing with no performance degradation (without CEF, per-packet load-sharing requires process switching). Even though there is no performance impact on the router, per-packet load sharing will almost always result in out-of-order packets. The packet reordering might degrade TCP throughput in high-speed environments (in low-speed/few-flows scenarios, per-packet load-sharing actually improves the per-flow throughput) or severely impact applications that cannot survive out-of-order packet delivery, such as Fast Sequenced Transport for SNA over IP or voice/video streams.

To configure per-packet load-sharing, use the ip load-sharing per-packet interface configuration command (default is per-destination). This command has to be configured on all outgoing interfaces over which the traffic is load-shared.

The switch between the load-sharing modes is not immediate; sometimes you have to wait a few seconds for the ip load-sharing command to take effect, worst case a manual clearing of the CEF table (clear ip cef address) is required.