If you want to build a truly redundant LAN infrastructure, you should also have redundant DHCP servers. If you decide to do the DHCP address allocation locally (on the router), you should take care that the two routers acting as DHCP servers don't assign overlapping addresses.

If the address space assigned to a LAN is at least twice as large as the number of LAN-attached devices, you can use the ip dhcp excluded-addresses command to exclude half of the address pool on each router, for example:

If you’ve ever had to configure OSPF on a Cisco router, you’re well familiar with the venerable network statement, which effectively assigns interfaces into OSPF areas based on their IP addresses. Although our life became simpler when the network statements stopped being order-dependent (the order dependency allowed for a few nasty surprises in the troubleshooting part of the CCIE lab when the CCIE title still implied you had to be able to fix other people’s mistakes :), it was still an awkward way of configuring what belongs where.

Every now and then I'm getting questions from my readers regarding the suitability of my MPLS books for the CCIP exam, for example:

I'm pursuing my CCIP and have a hard time finding the right MPLS study guide. I know you have the CCIP edition that was written in 2002, but I think the exam topics have changed. Can you recommend what book or books are best for the CCIP MPLS exam?

Are MPLS VPN Architectures Volume 1 & 2 two completely separate books or is Volume 2 a newer release. I was thinking of going for the CCIP and wanted to know if I should get both books or just the more recent one.

Here’s an interesting OSPF-related question I got::

“Which one is better: default-information originate or default-information originate always?”

As always, the answer is it depends. If your OSPF edge routers have external default routes (for example, static default routes toward the Internet, see the next diagram), you'd want them to announce the default route only when they have a default themselves (otherwise, they would attract the traffic and then blackhole it). In this case, you’d use default-information originate.

Another Cisco IOS OSPF implementation trivia: if you’re redistributing a default route into OSPF (for example, you have a static default route configured with ip route 0.0.0.0 0.0.0.0 … and you use redistribute static subnets within the OSPF process), the default route will not be entered into the OSPF database unless you configure default-information originate within the router ospf configuration.

Similarly, if you configure default-information originate always, the router will inject the type 5 LSA for the default route into the OSPF topology database even if the router itself does not have a default route (or gateway of last resort).

Similar to the show processes cpu sorted command, the show processes memory sorted printout displays the top memory consumers (see example below).

Recently, I was investigating MTU-related problems and got mightily upset when I had to search for the interface IP MTU size in the long printout produced by the show ip interface command. Obviously, I could display the IP MTU for a single interface with the show ip interface name | include MTU filter, but I wanted a nice tabular printout. Obviously, it was time for another Tcl script.

To use it, download it and store it in the flash memory of your router. Configure alias exec ipconfig tclsh flash:ipInterfaces.tcl and you can use ipconfig or ipconfig active to display interface IP addresses.

One of the hardest troubleshooting problems within an MPLS VPN network has always been finding a broken LSP. While you could (in theory) use the IP ping or traceroute (assuming all hops support ICMP extensions for MPLS), the results are not always reliable… and interpreting them is not so easy. For example, after I've disabled LDP on an interface with the no mpls ip configuration command, the routers in the LSP path still reported outgoing MPLS labels in ICMP replies for a few seconds (until the LDP holddown timer expired on both ends of the link).

As a side note, would you deduce from the printout that the break in the LSP path happened on the router with the IP address 192.168.201.1?

Due to the way Cisco routers behave when dropping packets with an inbound access list, whenever you use access lists to protect the router from the outside attacks (or port scans), the protected ports (even though they're not active on the router) will appear filtered (some scanners might use the term stealth), which is almost an invitation to a determined hacker.

Sometimes (it depends on the application you're protecting) you can configure application-layer protection in Cisco IOS. For example, you can protect HTTP server with ip http access-class global configuration command or the Telnet server with the access-class in line configuration command (and BGP will not accept incoming TCP SYN packets unless you've configured a BGP neighbor). The access-class configuration causes the incoming request to be rejected within application (in control plane after the TCP stack), resulting in TCP RST packet being sent back. The port scanner thus reports the protected TCP port as closed.

Under some circumstances, you might want to tune the ARP timers on the router (for example, when using ARP as a keepalive mechanism to detect whether the host is up). Unfortunately, although you can set the per-interface arp timeout in seconds, the actual timer resolution is in minutes. For example, if you set the ARP timeout to 10 seconds, the router will age the ARP entries once per minute.