Extended IP Access-list

As we all have learned in our pursuit to CCNA there are many different types of access-list. In reality most commonly used are ip access lists.

Major difference between ip standaard access-list an ip extended access-list is the ability to insert and remove entries within the ACL. When an entry has to be removed with in an standard ACL the entire ASL need to be removed and added again. The latter off course without the ACE which needs removal.

A extended ACL has slightly more steps but has some major benefits. Let say you have the following ACL;

access-list 130 permit ip host host
access-list 130 permit ip host any
access-list 130 deny ip any any

And the entry “permit ip host any” must be replace with “permit ip host host”

The following steps need to be taken are;

  • Obtain information about the ‘rule number’
  • Insert new ACE
  • Remove wrong ACE
  • Resequence rule numbers

Obtain rule number information

R4#sh access-lists 130
Extended IP access list 130
 10 permit ip host host
 20 permit ip host any
 30 deny ip any any

Insert new ACE

Rule 11 until 19 are free to use. Same goes for 21 until 29. Let use number 15.

*Feb 19 13:13:35.871: %SYS-5-CONFIG_I: Configured from console by console
R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#ip access-list extended 130 
R4(config-ext-nacl)#15 permit ip host host
*Feb 19 13:14:24.206: %SYS-5-CONFIG_I: Configured from console by console
R4#sh access-lists 130 
Extended IP access list 130
 10 permit ip host host
 15 permit ip host host
 20 permit ip host any
 30 deny ip any any

Remove wrong ACE

ACE that needs removal has rule number 20. Removal is as easy as “no 20” within the access-list.

R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#ip access-list extended 130 
R4(config-ext-nacl)#no 20
*Feb 19 13:18:32.452: %SYS-5-CONFIG_I: Configured from console by console
R4#sh ip access-lists 130 
Extended IP access list 130
 10 permit ip host host
 15 permit ip host host
 30 deny ip any any

The rule numbers are somewhat borked. This needs a little repair.

Resequence rule numbers

Repair command is;

ip access-list resequence

*Feb 19 13:22:42.838: %SYS-5-CONFIG_I: Configured from console by console
R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#ip access-list resequence 130 10 10
*Feb 19 13:23:12.473: %SYS-5-CONFIG_I: Configured from console by console

R4#sh access-list 130 
Extended IP access list 130
 10 permit ip host host
 20 permit ip host host
 30 deny ip any any

Naturally the most important benefit is that the access-list is applied all the time. In regard of the standard ip access-list; a slight modification results in removing the ACL and re-instating the ACL again. You will never know which traffic passes the interface while working on the access-list.

The same applies for named access-list.


Cisco NEXUS devices have a slight different syntax to resequence
NEXUS-R3(config)# resequence ip access-list some_vlan 10 10
Posted in CCNP, Commands | Tagged , , , ,

EIGRP neighbors

Cisco EIGRP protocol allows different methods to form neighborship relations. Default method is form neighborship automatically via multicast (multicast address of is used)

After an adjacency is formed ‘hellos’ are  sent using IP protocol 88 (eigrp). This means that;

  1. an interface need to be IP enabled
  2. both interfaces to a link need to have an ip address within the same subnet.

Alternative method of creating an neighborship relation is setting the neighborship ‘hard-coded’. This is done withing the eigrp routing context. Setting a neighborship ‘over’ an interface implicitly disables the ability to form adjacencies using multicast. Inbound EIGRP multicast packets are discarded and outbound are not send out that particular interface.

This alternative method can be used when you want to prevent a rogue router in your network inject all sorts of garbage subnets.

Off course a better way to set a md5 key on eigrp update exchanges.

Posted in Commands

Cisco Express Forwarding

An topic that entertained my imagination is Cisco’s IP CEF. So I had to take a dive into whichever forwarding methods there are for Cisco routers.

Currently there are three methods of packet forwarding;

  • Processor based forwarding
  • Fast forwarding (reactive)
  • Cisco Express Forwarding (proactive)

Processor based forwarding is as the name describes processor intensive. When for each packet the CPU has to harvest information from various tables (sources)  this would delay packet forwarding significantly. Or you would need to upgrade a router chassis with a more powerful routing engine. Routers have various tasks at hand; forwarding packets, running routing protocols, maintaining an arp table and handling other processes e.g. ntp, snmp, syslog. Just to name a few.

At some point Cisco and other vendors decide to walk another path. Here is the Cisco solution.

Fast Forwarding uses a specially created table fastswitching-cache. When a packet arrives at a router for which the destination is not in the cache. The router invokes the IP Input process (cpu intentive). IP input gathers information about how to forward the packet to its destination. This information is stored in the fastswitching-cache. And the packet is forwarded accordingly. At this point there is no advantage for this single packet. But for the next packet going to the same destination there is a advantage.

The CPU is used only once in determining the destination information. However there is a downside; if the outgoing interface would change, for example by a network failure, then this information is NOT passed through to the fastswitching-cache. The entry in the fastswitching-cache is only cleared after a time-out is reached.

Cisco Express Forwarding is an entire different beast. The table used is called  forwarding information base or FIB for short. When a packet arrives it is handled bij the IP input process. Same process as processor switching and fast switching. But when all relevant information is gathered is it stored into the FIB. At this point no major difference to Fast Forwarding. CEF also , proactively, stores information from network protocols into the FIB as well. This enables shorter forwarding delay times. And while  CEF uses specific information (FIB) in a specific process (CEF) the processing can be done in specialized hardware; Application Specific Integrated Circuit or ASIC for short. For starters this relieves that main CPU from packet forwarding tasks. And secondly this also eliminates any forwarding delay.

In the next post I will dive into how to read the output of show ip cef.






Posted in CCNP, Commands | Tagged ,

Virtual Lab setup

Today I want to talk about the lab setup I have made. But first a slight warning; I will not discuss any use of illegal software nor will I tell you how to get IOS/IOU files.

For quite some years I used Windows as an operating system. In 2009 I made a radical change; I installed Ubuntu as a main operating system. Gradually Ubuntu was installed on the PC’s in my house-hold. As Ubuntu has a less demanding hardware requirement list the usage span of the hardware was also stretched. Spending less money on computer hardware is big and very welcome bonus. Ubuntu can be found on https://www.ubuntu.com/

Virtual LAB software

For network simulation/lab software I use GNS3. GNS3 also helped my decision to run Ubuntu. As GNS3 is much more stable on a linux environment. GNS3 can be found on https://www.gns3.com/ Create a free account and search for the installation manual.

The standard Ubuntu telnet client doesn’t support tabbed views. That is why Konsole was added to the installation. The GNS3 preferences need some adjustment, add the following; konsole –new-tab -p tabtitle=”%d” -e “telnet %h %p”. To the console application command for telnet setting.


My laptop with an Intel I5, carries 16Gb of RAM. And my desktop also an Intel I5 is fully loaded with 32Gb of RAM. Lots of RAM make simulations run smooth as silk. The laptop runs with an SSD drive. Remember that each virtual router has its own memory footprint e.g. more virtual router equals to more memory usage. Cisco csr1000v gobbles up a staggering 3Gb memory per virtual router and if I remember correctly Juniper olive still eats a modest 2Gb of memory. IOS and IOU virtual routers have much smaller memory footprint.


  • Software:
    • Ubuntu
    • GNS3
    • Konsole
    • Network OS image files of your liking. (Cisco IOS/Cisco IOU/Juniper Olive …)
  • Hardware:
    • Intel I5
    • Memory slots completely filled up and maxed out.


You dont have to be brave to make a setup with GNS3. Although  little persistence is needed and some help from your friend McGoogle.


UPDATE: 2017-feb-26 Today I wrote a little article about a short-coming I found in GNS3. GNS3 caveat – port status I am sure some more will follow. GNS3 is a fantastic tool for emulating a netwerk. But it is still an emulation. Nothing beats the real thing; a stack of hardware.





Posted in GNS3 | Tagged , , ,

CCNP Books have arrived

The 300-115 (switch)  book was already in my possession.  This week the last two books arrived; 300-101 (route)  and 300-135 (tshoot).
For some time I was cramming labs from whichever tutorial I could lay my hands on. Now the study will be more structured using the official cert guides.


Posted in CCNP | Tagged , , ,

T-Mobile – Evernote course

evernote_smallLast week I received an email with en invite to attend a free Evernote course. Free is good and Evernote is great tool to store and organize brainfarts. Immediately I jumped to the occasion to enroll to the course.

I am very  curious to the new things I am about to learn in using Evernote. The course will be given in an auditorium of T-Mobile head quarters in The Hague.

Enabling initiative like this, T-Mobile is giving something back to the community.Well done….. very well done.

Posted in Lifehacking | Tagged ,