Troubleshooting OSPF adjacencies can be a nightmare: if you’ve misconfigured the OSPF interface parameters (the timers or the subnet mask), the adjacency will not form, but the router will not tell you why. The only mechanism you can use to detect the mismatch is the debug ip ospf hello command … just don’t try to use it on a console session of a router running OSPF across hundreds of interfaces.

The OSPF hello event debugging does not display OSPF packets received from a different subnet. If you configure mismatched IP subnets (not the subnet mask) on adjacent routers, you will not see any received hello packets.

Several comments I’ve received in response to my “Knowledge or recipes” post were slightly resigned, leading me to the unfortunate conclusion that you all gave up and decided to live with the current state of the IT training business. But you can do something about it – go out and vote!

I’m constantly receiving interesting OSPF-related queries – the many hidden details of the OSPF specs result in slightly unexpected behavior and constant amazement of engineers studying OSPF. During this week, I’ll focus on a few intriguing OSPF details.

Let’s start with an easy one: you can use the show ip ospf interface brief command to display the OSPF interface status (including the interface area, OSPF cost, link type, and router status on broadcast links). Unfortunately, this command does not allow you to specify the OSPF process ID and displays interfaces belonging to all OSPF processes (if you run multiple OSPF processes on the router).

My IP QoS: Two generations of class-of-service tools article published by SearchTelecom gives you a very high-level overview of IntServ and DiffServ approaches to IP QoS as well as brief description of various DiffServ tools.

More to explore:

• More QoS articles
• QoS Fundamentals webinar

Once upon a time, AAA command authorization in Cisco IOS queried the TACACS+ server for every single command a user entered. Rules have changed drastically in the meantime (at least for IOS release 12.4):

• Non-privileged show commands are executed without TACACS+ authorization. Privileged show commands (show running or show archive log config) are still authorized.
• Some commands that can be executed in non-privileged (aka disable) mode (enable, disable, help, logout) are authorized only if you configure aaa authorization commands 0 methods regardless of the current privilege level.
• Other commands (for example, ping) are authorized based on the current privilege level.

For example, if you’ve configured AAA command authorization only for privilege level 15, the ping command will be authorized if you’re working in enable mode, but not otherwise.

• Command authorization is not performed on console unless you’ve configured aaa authorization console.

When configuring MPLS Traffic Engineering in your network, you have to specify the amount of bandwidth that the MPLS TE tunnels can request on each MPLS TE-enabled interface with the ip rsvp bandwidth command.

Until recently, this command accepted only fixed bandwidth (in kilobits), which could be pretty inconvenient if you wanted to use common interface templates or deployed MPLS TE on links with varying bandwidth (for example, Multilink PPP bundles). IOS release 12.2SRC introduced a variant of the same command (ip rsvp bandwidth percentage) that allows you to specify reservable bandwidth as percentage of the current interface bandwidth. Unfortunately this feature didn’t make it into 12.4(20)T.

A short question I've got from Shahid Rox:

Today I read your article about scaling EIGRP using stub routers. I was wondering whether you can use the leak map only for routes learned from other EIGRP neighbors? Is it also usable to filter connected routes?

Leak-map controls what its name implies: the leakage of routes received from EIGRP neighbors to other EIGRP neighbors. To filter connected prefixes redistributed into EIGRP, use the route-map on redistribute connected command. The only way I've figured out to filter announcements of directly connected networks that are part of the EIGRP process is the distribute-list out command.

When the local Telco installed my blindingly fast 20 Mbps Internet-over-fiber-cable service, I was expecting to use DHCP on the router’s outside interface to connect to the Internet. After all, they’re running switched Ethernet VLANs over the fiber cable, and using DHCP seemed a logical choice. Imagine my surprise when I had to configure PPP-over-Ethernet (PPPoE) – it was as if I would be using a DSL connection, not a fiber-optic cable.

Yesterday, I introduced a scenario where RIP would (in my opinion) work much better than OSPF. If you were not persuaded by the “management-level” arguments, let’s focus on the technical details (but make sure you read the scenario first).

One of my readers called RIP “Rest in Piece” routing. Although it’s probably the routing protocol that dinosaurs used to find their way around, it’s still useful in modern networks. Imagine that you have to deploy hundreds (or thousands) of low-cost remote sites with dual uplink capability (for backup purposes). They could be automated kiosks, point-of-sale terminals or even ATM machines.

If you’re infinitely lucky (and have huge budget), you could afford an ISR router at each location and use different design options that Cisco IOS gives you. In most cases, you have to work with devices that barely know what routing is … but you still need dynamic routing protocol to give them the ability to detect primary route failure and switch over to the backup route.

Assuming your purchasing department didn’t buy boxes that don’t have enough memory to run OSPF, you could usually choose between RIP and OSPF as the routing protocol … and I would always select RIP in this scenario. Let’s start with the “management-level” arguments: RIP is simpler to design (there is almost nothing to design) and troubleshoot than OSPF. It uses less memory and CPU cycles and I would also expect low-end boxes to have fewer bugs in RIP than in OSPF. More in-depth arguments are coming in the follow-up post.