Whisper asked an interesting question …

“What I would like to know is, on my PPP negotiated ADSL connection, how the ISP assigns me a /32 ip address.”

… which prompted me to test various WAN encapsulations and address assignment rules. Here are the results:

• On all WAN encapsulations you can configure subnet masks down to /31 (/30 in old IOS releases).
• The same IP address can be used on more than one interface as long as both IP address and subnet mask match.
• Two WAN interfaces can have different IP addresses but still belong to the same IP subnet. You would use this on Frame Relay when you have multiple interfaces into the same FR cloud for bandwidth reasons.
• If you configure IP address with IPCP (with the ip address negotiated command), the subnet mask becomes /32 as IPCP does not carry subnet mask (and you get the host route toward the PPP peer unless you turn off the PPP peer route option)
• If you configure IP address with SLARP (Serial Line ARP) on HDLC, the subnet mask is inherited from the peer (HDLC SLARP carries subnet mask) and the IP address is determined by flipping the low-order bits in the neighbor's IP address.

In a previous post, I've described how you can use Tcl shell to upload text content into router's flash if the router has no connectivity to a suitable file server (or you don't have FTP or TFTP server handy).

The trick works flawlessly, but typing the same obscure Tcl commands gets tedious after a while, so the first time I had to use this solution to develop a Tcl script, I've quickly written another script that takes file name as the parameter and hides all the other murky details.

To use it, transfer the contents of storeFile.tcl (available from my web site) to the router's flash (using the previously described trick), follow the installation instructions in the source and you're ready to go.

Note: You can adapt the Tcl script to your needs; for example, you could add instructions to re-register EEM Tcl policy every time you upload the new code.

Debugging time-based configurations could be a nightmare, as you have to switch router's time back and forth trying to debug your configuration and wait for the desired event to occur. When I was debugging my EEM-based solution to time-based BGP policy routing, I simply defined two aliases that would set the clock to 30 seconds before the event I wanted to test:

alias exec 859 clock set 08:59:30
alias exec 900 clock set 09:00:30

Obviously, these tests are best done in a lab setup … and you have to turn off NTP or any other form of time synchronization.

When Kate Gerwig, my wonderful editor from SearchTelecom.com, and myself agreed on the contents of the “Building customer-resilient BGP networks” article, we had no idea that it would become extremely relevant just days before it was published. The article describes the tools a Service Provider should use to ensure that its customers cannot harm its BGP routing data (and consequently its other customers and the Internet at large).

On February 24th, someone in Pakistan decided to block local access to YouTube … and someone else decided that the best way to approach the problem was to block the whole world’s access to YouTube.

Rob van Ooijen has sent me a really interesting question:

I've configured a switch port to be unconditionally a trunk with the switchport mode trunk configuration command. However, when the interface was enabled, I've got a dynamic trap indicating the trunking was still dynamic (and the show commands also showed negotiation of trunking is enabled).

In fact, adjacent layer-2 devices can negotiate a number of things these days, among them:

• Speed and duplex settings (we'll leave this can of worms safely closed);
• Trunking (a port can be access or trunk port);
• Trunk encapsulation (ISL or 802.1q);

With the switchport mode trunk command you just disable the trunking negotiation (the port will never become an access port), but the switch still runs Dynamic Trunking Protocol (DTP) on it, unless you disable DTP with switchport nonegotiate, which (in combination with all the other configuration commands) makes the port configuration totally static. More configuration can be found in Cisco's documentation and in the Basics of DTP document.

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

Someone has sent me an interesting question a while ago: he's changed the configuration of a single VTY line and got three blocks of VTY configuration commands, similar to this:

line vty 0 2
 login
line vty 3
 password secret
 login
line vty 4
 login

He wanted to merge the three configuration blocks back into a single one but somehow didn't know how to do it.

To realize what's going on, you have to understand how the IOS generates line configurations. It takes the first line (VTY 0, for example) and generates its configuration. If the next line (VTY 1) has exactly the same configuration, the range of numbers is expanded (becoming VTY 0 1) and so forth until the pool of similar lines is exhausted or a line is found that has at least one parameter different from the starting one, in which case a new block is started. That's why the sample configuration has three blocks (0-2, 3 and 4) even though the first and the third block are identical.

However, if you change the offending parameter, the VTY lines will have identical configurations and will be automatically merged. If you want to be on the safe side, you should change the parameter for all lines, for example:

line vty 0 4
 login
 password secret

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

One of the most tedious tasks in the initial lab setup (at least for me) is the IP address configuration, which usually includes a number of typos and mixups on the WAN links. You can simplify then WAN address configuration if you configure only one end of the WAN link and let PPP do the rest. For example, you could use the following configuration to configure WAN link on your core router …

hostname Core-2
!
interface Serial1/0
 description link to POP
 ip address 10.0.2.1 255.255.255.252
 encapsulation ppp
 peer default ip address 10.0.2.2

… and use IPCP negotiation on the POP router to pick up the WAN IP address:

hostname POP
!
interface Serial1/0
 description link to Core-1
 ip address negotiated
 encapsulation ppp

You should not configure no peer neighbor-route on the router that gets dynamic IP address, as the subnet mask is not assigned with IPCP; you need the IPCP-generated host routes if you want to do hop-by-hop telnet between the routers.

Petr Lapukhov describes an interesting scenarion in his post BGP Time-Based Policy Routing: a multi-homed customer that uses one upstream link (for example, more reliable but slower one) during the work hours, switching to the other upstream link (faster, less reliable) after that.

He uses BGP communities to achieve the switch (perfect solution if your ISP supports them) and time-based ACL in a route-map to set the community based on time-of-day. As Cisco changed the way BGP imports local routes in IOS release 12.3T, he then devises an ingenious solution based on reliable static routing to trigger a change in the IP routing table.

The optimum solution is way simpler: you just configure two EEM applets to perform clear ip route network command at appropriate times.

I've finally found the EEM reference documentation that specifies the side effects (changes in environment variables) of all action commands. You can use the changed environment variables in subsequent action commands by prefixing the variable name with the $ sign (similar to the EEM applet where I've included router's name in an outgoing e-mail).

Network World has published an article describing 20 useful sites for Cisco networking professionals, featuring among other sites the Cisco IOS Hints blog. I felt even better when I've discovered my blog was one of the three non-US sites mentioned in the article :)

It's well worth browsing through the list. I already had most of these sites in my feed reader and I've discovered a few new nuggets.

BGP implementation on Cisco IOS gives you a number of filtering options, including prefix filters, AS path filters and route maps. While it might be tempting to learn just the most versatile tool available (route maps) and discard all the others, judicious use of all available tools can simplify your router configurations.

For example, an Internet Service Provider might want to filter incoming updates received from the customers to ensure they’re not advertising transit routes and advertise only IP prefixes they actually own. Inbound route maps might also be needed to attach BGP communities to inbound routes or set BGP attributes (for example, local preference) based on communities attached to incoming routing updates.

You can perform all these tasks with route-maps, but then you’d probably have to create a dedicated route-map for each customer (as the inbound prefix filter has to be customer-specific). Changing your routing policies or community definitions would require changing a lot of route maps.

Update 21-feb-08@15:39: distribute lists and prefix lists can't coexist (they cannot be configured in the same direction on the same neighbor)

On the other hand, if you use all the filters available in the BGP routing process, you could:

• Use neighbor filter-list in to check the customer-specific AS path requirements;
• Use neighbor prefix-list in to filter customer prefixes, reject too long prefixes or prefixes not belonging to the customer;
• Use neighbor route-map in to drop prefixes belonging to your own address space, implement various routing policies and set BGP communities.

The solution scales even better if you configure common filters (route-map in and filter-list in in our scenario) in a BGP peer session template.

One of my readers asked a very interesting question:

“Is there a way to have a port on a router open for legitimate use while closed to port scanning software and the such. For example. I have SSL VPN configured on my IOS router. Is it possible to have the port seem stealthed to port scanners while still allowing legitimate access to the service? An example being, allowing a web browser to connect using the port but making sure that a port scanner doesn't detect it as open.”

The short answer is no, unless you can differentiate legitimate users by their IP addresses. The port scanners (when using SYN scan) simply open and close a TCP session, and there is no way for a router to differentiate between the legitimate users (who would send valid HTTP GET requests) and port scanners (that would close the session as soon as it's established).

If you can distinguish between legitimate users and everyone else based on their IP address, the task becomes simpler: either you apply inbound access list on Internet-facing interface of the router or configure per-service access-list (for example, ip http access-class acl). When using the inbound access-list, the port appears filtered or stealth, when configuring per-service access-class, it's reported as closed.

When IOS started supporting dynamic allocation of IPCP (IP over PPP) addresses, it also got the mechanism to insert a dynamic host route toward the neighbor's IP address once the IP addresses were negotiated. This mechanism makes perfect sense in dynamic address allocation environments, as the subnet mask is not negotiated with IPCP. Without a host route toward the other end of the PPP link, there would be no easy way to reach the IP prefixes reachable via the PPP link.

Sometimes it would be nice to have the full complement of Unix utilities available on Cisco IOS. That's not going to happen for a while, but we can use Tcl to make our life simpler in the meantime. Xavier Brouckaert, a regular contributor to my blog, has sent me the Tcl implementation of line counting utility (equivalent to wc -l on Unix).

First you have to define the wc Tcl procedure:

proc wc { cmd } { llength [split [exec $cmd] "\n" ] }

You can define the procedure interactively in Tclsh (but then you have to do it every time you start Tclsh) or you could store the code in a flash file and execute the file every time the Tclsh is started with the scripting tcl init filename global configuration command.

Once the wc procedure is defined, execute wc { IOS command } in Tclsh and you'll get the line count. For example, to get the number of directly connected routes use

wc { show ip route ¦ include ^C }

The include ^C filter includes all lines that start with letter C; in our case all directly connected routes

Obviously you could turn this idea into a full-blown Tclsh script that would accept CLI arguments … but I'll leave this as an exercise for the readers (you can probably tell I've been reading some academic literature lately :). However, if you find the time to write a more complete wc implementation on IOS, please do post the URL here.

When trying to extract the OSPF route selection rules from RFC 2328, I've stumbled across a very weird rule (section 16.4.1): if an ASBR within a non-backbone area advertises an external route (or if the forwarding address is within the non-backbone area), it's preferred over external routes advertised by ASBRs in other areas regardless of its metric. I simply had to test this on Cisco IOS … and found out that Cisco engineers prefer common sense to OSPF RFC.