Blog Posts in October 2008

OSPF Challenge: Mixing Numbered and Unnumbered Interfaces

Assuming you have the following configurations on R1 and R2:

R1 configuration
hostname R1
interface Loopback 0
 ip address
interface Serial 0
 encapsulation ppp
 ip unnumbered Loopback0
 ip ospf 1 area 1
router ospf 1
R2 configuration
hostname R2
interface Serial 0
 encapsulation ppp
 ip address
 ip ospf 1 area 1
router ospf 1

What IP address can you use on the loopback interface of R1 to establish adjacency between R1 and R2? Can you use more than one IP address?

You can find the solution here.

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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.

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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.

The best you can do without going into weird solutions is to allocate router loopback interfaces and inter-router links from a tightly controlled address space and only allow telnet from that address space (while at the same time filtering IP packets pretending to come from that same address space on the perimeter of your network). As the IOS supports extended access lists in the access-class line configuration command, you could allow SSH from a wider set of IP addresses and limit Telnet to the address range allocated to inter-router links.

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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.

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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.

This is not something you’d want to do in your production network.
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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.

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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.

To limit the debugging outputs to a single interface, use the debug interface command.
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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). ::: Here is a sample printout taken from a router running OSPF processes #2 and #13:

The printout contains interfaces from multiple OSPF processes
C1#show ip ospf interface brief
Interface    PID   Area    IP Address/Mask    Cost  State Nbrs F/C
Lo102        2     22        1     LOOP  0/0
Fa0/0        13    0        10    BDR   1/1
Lo0          13    0       1     LOOP  0/0
Se1/0.101    13    1          64    P2P   1/1
Se1/0.100    13    1          64    P2P   1/1

You can use an output filter to display the interfaces of a single OSPF process. The filter is quite convoluted:

Use a filter to display interfaces of a single OSPF process
C1#show ip ospf interface brief | include ^[^ ]+ +13
Fa0/0        13    0        10    BDR   1/1
Lo0          13    0       1     LOOP  0/0
Se1/0.101    13    1          64    P2P   1/1
Se1/0.100    13    1          64    P2P   1/1 

It works like this:

  • The initial caret (\^) matches the beginning of the line, ensuring that our filter matches precisely what we expect it to match. Without the initial caret, the filter could generate a match anywhere in the line, potentially resulting in false positives.
  • The [^ ]+ pattern matches any non-empty (the + sign) string of non-space characters (the [^ ] expression matches anything but the whitespace). This part of the pattern matches the interface name.
  • The  + pattern matches the string of spaces between the interface name and the process ID.
  • The final part of the pattern (13) matches the OSPF process ID.
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MPLS on 7600: the devil is in the details

I've got a simple question recently: “Can I run MPLS on a VLAN interface on 7600?” My initial response was “Sure, why not.”, as I knew we've deployed MPLS in 7600-based networks and there should be no significant difference between a routed port and a VLAN interface on a 7600 (this box treats everything as a VLAN internally).

It turned out the problem was "a small detail" that's not advertised in any 7600-related MPLS marketing material on Cisco web site: you need Advanced IP Services software to run MPLS. To make matters worse, the only mention of 7600-series devices in the Cisco IOS Packaging Product Bulletin I've finally found within the 7600 routers product literature is in the first marketing slide.

This article is part of You've asked for it series.

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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.
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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.

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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.

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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.

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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).

All you ever want to advertise to the remote sites in this design is the default route (or a network-wide summary). Alternatively, you might want to advertise only a route to a central LAN or server. Both requirements are easily met with RIP per-interface output filters. Doing something similar with OSPF is close to impossible. Either you place every remote site into a separate OSPF area (don’t even think about doing it; there could be hundreds of sites), or the routes within an area will leak between the remote sites.

RIP is also more stable than OSPF in this setup. Whenever a remote site disappears, the change in the OSPF area is unnecessarily propagated to all other remote sites in the same area. RIP doesn’t propagate the topology change; the central site’s output route filter stops all unnecessary updates.

As you know, OSPF requires hello packets and adjacencies to work correctly. Therefore, the central hub router must track the adjacency states of hundreds of neighbors. When using RIP, the central router couldn’t care less … it sends out its routes every so often, collects whatever comes back, and reports when a new remote route is received, or an old one disappears.

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