Blog Posts in June 2007

Using MPLS VPN Books to Study for the CCIP Exam

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.

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OSPF Default Route: Design Scenarios

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.

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Inserting Default Route Into OSPF

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).

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Display per-process memory usage

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

Total: 13734272, Used: 6372068, Free: 7362204

 PID TTY  Allocated      Freed    Holding    Getbufs    Retbufs Process

   0   0     135340       1864    4734916          0          0 *Init*

  55   0     242388        188     249076          0          0 URL filter proc

  69   0     317996     143308     182184          0          0 IPSEC key engine

  62   2     277048     124752     165172          0          0 Virtual Exec

  68   0     762828     657056     109896          0          0 Crypto IKMP

  80   0      74556       1100      73772          0          0 CEF process

  91   0      25704        188      28776          0          0 NTP

  67   0       3116      51368      27904          0          0 Crypto ACL

  83   0        184          0      25060          0          0 traffic_shape

  30   0      89900          0      24700          0          0 IP Input

  46   0      32248       1776      23596          0          0 DHCPD Receive

  35   0      10236        540      16572          0          0 PPPOE discovery

  48   0      95344      51488      14724          0          0 HTTP CORE
Usually the top entry is the *Init* process, which allocates all shared buffers, but routing processes could also exhibit significant memory utilization in large networks.
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The “show ip interface” command I've always wanted to have

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 size of a single interface with the show ip interface name | include MTU filter, but I wanted to have a nice tabular printout. Obviously it was time for another Tcl script.

To use it, download it and store it into 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.
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MPLS Ping and Traceroute

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?

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Closed versus Filtered ports

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.
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ARP timeout resolution is implemented in minutes

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.
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Port number not shown in access-list log output

When I was testing the inspection of router-generated traffic, I wanted to block and log all incoming traffic (apart from inspect-generated conduits, obviously) with a simple access-list:
access-list 102 deny ip any any log
Unfortunately, the port numbers in the logging printout were always zero:
%SEC-6-IPACCESSLOGP: list 102 denied udp 10.0.0.1(0) -> 192.168.1.3(0), 1 packet
The reason for this behavior is very simple: unless a line in the IP ACL matches on the layer-4 port numbers, the router does not inspect them; the log action thus has no port number to show in the syslog printout.

To fix the printout, you have to force the router to inspect the layer-4 port numbers. If you still want to block-and-log all traffic, the minimum access-list achieving this goal is the following:
access-list 102 deny   udp any gt 0 any gt 0 log
access-list 102 deny   tcp any gt 0 any gt 0 log
access-list 102 deny   ip any any
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ARP entries are periodically refreshed if you use CEF switching

In a previous post I've been writing about the inability to clean the ARP cache due to cached CEF adjacencies. As it turns out, this behavior has another side effect: the router will automatically refresh all ARP entries (and CEF adjacencies) as they expire from the ARP cache. This might become a problem on high-end devices with a lot of directly connected hosts if you set the arp timeout to a low value.

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Router Responses to Port Scans

Recently I was trying to figure out what the various port states reported by Nmap really mean. This is what's actually going on:

  • If a packet is intercepted by a router's access-list, the router sends back an ICMP administratively prohibited packet. This is reported as filtered port by Nmap (and probably as stealth port by some other scanners).
  • If you do a TCP SYN scan of a router and the scanned port is not active, the router sends back TCP RST packet. This is reported as closed port.
  • If you perform a UDP scan of a router, the router sends back ICMP port unreachable message if the UDP application is not active. This is reported by Nmap as filtered port (even though in most cases it should be equivalent to closed TCP port).
  • In some cases, the router simply doesn't reply to UDP scans (for example, if you scan the discard service). This is reported as Open¦Filtered (as the scanner cannot reliably determine whether the probe was dropped due to a filter or simply not replied to).

Note: In any case, UDP scans are way more unreliable than TCP scans due to connectionless nature of UDP.

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Default interface configuration command

The easiest way to remove all settings from an interface is to use the default interface configuration command. For example, if you've configured Frame Relay interface with subinterfaces ...
interface Serial0/0/0
 no ip address
 encapsulation frame-relay
 load-interval 60
!
interface Serial0/0/0.100 point-to-point
 bandwidth 2000
 ip address 172.16.1.1 255.255.255.252
 ip load-sharing per-packet
 ip ospf cost 50
 frame-relay interface-dlci 100
... and have erase all interface-specific configuration, the ...

rtr(config)#default interface serial 0/0/0
Building configuration...

Interface Serial0/0/0 set to default configuration
... gets you there. As you can see, after the configuration change, the main interface has no IP address and the subinterface is deleted.
a1#show ip interfaces brief
Interface          IP-Address  OK? Method Status     Protocol

... non-relevant lines deleted ...

Serial0/0/0        unassigned  YES TFTP   up         up
Serial0/0/0.100    unassigned  YES manual deleted    down
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Inspect router-generated traffic

A while ago a reader has asked me whether you could modify an IP access-list when the interface IP address changes. While that's definitely doable with Tcl and Embedded Event Manager, it's not a trivial task, so I've tried to understand why he would need such a functionality.

The answer was quite interesting: he's running NTP on his firewall router and thus needs to accept incoming NTP responses from an external NTP server. While that could be easily achieved with the following configuration (only the relevant bits-and-pieces are shown), he didn't want to make the access-list too generic (allowing NTP from the external server to any IP address).
ip inspect name DEFAULT100 tcp
ip inspect name DEFAULT100 udp
!
interface Dialer0
 description $FW_OUTSIDE$
 ip access-group 102 in
 ip inspect DEFAULT100 out
!
access-list 102 remark #### Dialer0 incoming ####
access-list 102 remark #### non-relevant lines deleted
access-list 102 permit udp host 1.2.3.4 eq ntp any eq ntp
This problem nicely illustrates a broader issues: the router does not inspect it's own traffic and thus does not prepare conduits for the return packets; you have to specify all the return traffic you're expecting in the incoming access list. This drawback has been fixed in IOS release 12.3(14)T with the introduction of the Inspection of Router-Generated Traffic feature. In our scenario you only need to change the inspect rules:
ip inspect name DEFAULT100 tcp router-traffic
ip inspect name DEFAULT100 udp router-traffic
... and the router synchronizes to an external NTP server:
sp#show ip inspect sessions
Established Sessions
 Session 474032B4 (192.168.1.3:123)=>(10.0.0.1:123) udp SIS_OPEN
sp#
01:04:34: %NTP-5-PEERSYNC: NTP synced to peer 10.0.0.1
01:04:34: %NTP-6-PEERREACH: Peer 10.0.0.1 is reachable
Note: This article is part of You've asked for it series.
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DHCP Response Sets the Default Route

It makes perfect sense in hindsight, but I was nonetheless pleasantly surprised: when the router acting as a DHCP client (configured with the ip address dhcp interface configuration command) receives the DHCP reply packet containing the default gateway option (option #3), it installs a static default route toward that next-hop.

Even better, the default route is installed with the administrative distance 254 (floating static route), making sure that the default route you’ve configured manually or the default route received via a routing protocol are not overwritten.

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Fix the IOS quiet mode for the IOS HTTP(S) server

The IOS documentation claims that the quiet mode the router enters after a series of login failures blocks all telnet (or ssh) sessions as well as HTTP requests. Unfortunately the latter is wrong; you can execute any HTTP request on the router during the quiet mode.

If you want to block HTTP requests during the quiet mode, you can use EEM applets to change the HTTP server configuration when the quiet mode is started and completed.
First you need to configure a standard numbered IP access list that will be used to block HTTP requests during the quiet mode (the ip http access-class command accepts only numbered ACLs), for example:
access-list 95 deny any log
Then you define two EEM applets: one that triggers when the router enters the quiet mode (matching the SEC_LOGIN-1-QUIET_MODE_ON syslog message) and another that runs when the quiet mode is finished (triggered with the SEC_LOGIN-5-QUIET_MODE_OFF). Both applets modify the router configuration, changing the access-list used in ip http access-class configuration command.
event manager applet EnterQuietMode
 event syslog occurs 1 pattern "SEC_LOGIN-1-QUIET_MODE_ON" period 1
 action 1.0 cli command "configure terminal" 
 action 1.1 cli command "ip http access-class 95" 
 action 2.0 syslog msg "Entered Quiet mode on HTTP server"
!
event manager applet ExitQuietMode
 event syslog occurs 1 pattern "SEC_LOGIN-5-QUIET_MODE_OFF" period 1
 action 1.0 cli command "configure terminal"
 action 1.1 cli command "ip http access-class 70"
 action 2.0 syslog msg "Exiting Quiet mode on HTTP server"
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