Blog Posts in October 2008
OSPF Challenge: Mixing Numbered and Unnumbered Interfaces
Assuming you have the following configurations on R1 and R2:
hostname R1
!
interface Loopback 0
ip address 10.0.0.1 255.255.255.255
!
interface Serial 0
encapsulation ppp
ip unnumbered Loopback0
ip ospf 1 area 1
!
router ospf 1
OSPF LAN Adjacency Challenge: Final Results
I’ve received several e-mails responding to the mismatched OSPF subnet challenge. Some of the readers claimed that the configuration would work as-is; if you were one of them, I would advise you to do some lab tests the next time.
A few of the respondents also noted that it was more a review question than a challenge (since I’ve been writing about this topic a few days back), and everyone who decided the configuration has to be fixed has provided the correct solution: you have to configure the Fast Ethernet as a point-to-point OSPF interface and the routers stop complaining about the OSPF subnet mask mismatch.
Who is an associate?
One of my readers made an interesting observation in response to my “Knowledge or recipes” post: maybe network associates need recipes more than knowledge. My first reaction was to disagree; in my understanding, technicians work with recipes, engineers need to know what they’re doing and why. But then I tried to figure out what the term “networking associate” really means.
Telnet access restrictions
A while ago I've got an interesting question from one of the readers:
I'd like to be able to configure a set of routers to only be manageable from each other. Something like an access-class matching minimum packet TTL would probably be good enough, better if some connected routes could be tagged and access granted based on that. The idea is to keep router-by-router logins in case of routing problems, without opening up access too widely.
I did a few tests with IOS release 12.4(15)T and neither access-class nor control-plane policing recognizes the TTL field in ACL (various bits and pieces of IOS use the same data structures in different procedures, thus resulting in inconsistent behavior). Alternatively, you could deploy inbound access lists on all interfaces, but this is probably way too cumbersome to manage.
IP address lookup
Someone recently asked me how to get the physical location of an IP address. One of the better (free) services available on the Internet is the IP2Location (demo) service.
This feature might come handy if you're trying to figure out who's attacking your application servers (when the TCP session has already been established). Denial-of-service attacks commonly use fake source IP addresses.
Gaining Knowledge - what’s the best way to do it?
A few days after my “Knowledge or Recipes” post, Greg Ferro started his “Experience or Certifications” series with a radical “I would always choose certification over experience” approach that quickly moderated into “Knowledge is more fundamental than experience … but you need both”. It’s nice to see someone else thinking along the same lines as yourself.
Challenge: Establish OSPF Adjacency on a LAN Interface
You could get something like this only in a CCIE lab (I would hope): R1 and R2 should establish OSPF adjacency, but you cannot change or remove any of the existing configuration commands (you can add new commands).
OSPF Breaks When Faced With Overlapping IP Addresses
A while ago cciepursuit described his problems with PPP-over-Frame Relay. Most probably his problems were caused by a static IP address assigned to the virtual template interface (this address gets cloned to all virtual access interfaces and IOS allows you to have the same IP address on multiple WAN point-to-point links). I recreated a very similar (obviously seriously broken) scenario in my lab using point-to-point subinterfaces over Frame Relay to simplify the setup.
OSPF Ignores Subnet Mask Mismatch on Point-to-Point Links
The common wisdom says that the subnet mask mismatch will stop the OSPF adjacency from forming. In reality, the subnet mask is checked only on the multi-access interfaces and is ignored on point-to-point links. The source of this seemingly weird behavior is the Section 10.5 of RFC 2328, which says:
The generic input processing of OSPF packets will have checked the validity of the IP header and the OSPF packet header. Next, the values of the Network Mask, HelloInterval, and RouterDeadInterval fields in the received Hello packet must be checked against the values configured for the receiving interface. Any mismatch causes processing to stop and the packet to be dropped. In other words, the above fields are really describing the attached network's configuration. However, there is one exception to the above rule: on point-to-point networks and on virtual links, the Network Mask in the received Hello Packet should be ignored.
Troubleshooting OSPF Adjacencies
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.
Quality in training: you can make a difference
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!
Display Interfaces Belonging to a Single OSPF Process
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).
IP QoS: Two generations of class-of-service tools
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:
AAA command authorization gotchas
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.
Specify MPLS TE bandwidth as percentage of interface bandwidth
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.
Leak Map Confusion
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.
Internet Access Russian Dolls
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.
RIP Rocks in Low-End Hub-and-Spoke Networks
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).