Cisco Access Control List Guidelines to follow when creating rules.

Cisco Access Control List Guidelines

Just a quick post to remind myself and others ofCisco Access Control List Guidelines. the following Cisco Access Control List Guidelines that we should be aware of. I thought this would be good to post as a quick reference/lookup. This just gives a basic run down of how ACL’s should be implemented as per Cisco CCNA Security.

  • Ensure the last ACE that is processed has a ‘deny any’ or ‘deny any any’
  • ACLs are processed top down, as soon as as an ACE is matched the processing is stopped.
  • Make sure the most specific ACEs are at the top of the list.
  • One ACL per interface, per protocol, per direction.
  • Any new ACE’s that are added to an ACL are added to the bottom by default, unless specified.
  • Router generated traffic is not filtered by outbound ACLs
  • Standard ACLs should be placed as close to the destination as possible.
  • Extended ACLs should be placed as close to the source as possible.

Cisco Router Password Recovery

Cisco Router Password Recovery Steps.

Cisco Router Password Recovery

As a mental note to myself more than anything however some one may also find this useful. These are the steps you need to take in order to recover or reset a Cisco Router Password. This was performed on 2800 series router. This may be different for other router models and the break key sequence may also be different for your favorite terminal software, however these are the outline steps required in order to re-gain access to you Cisco Router.

  1. Connect via the console port.
  2. Power cycle the router.
  3. Issue the break sequence command on startup within 60 seconds of power up to put into ROMmon – ‘Ctrl-Break’ or ‘Break’
  4. Change the config register with ‘confreg 0x2142’ command.
  5. Power cycle the router.
  6. skip the initial startup/setup procedure
  7. move into privilege EXEC mode.
  8. Copy the startup configuration to the running configuration.
  9. Check the config with a show run – and change the password.
  10. Change the config register back to the original setting (usually 0x2102) in Global Configuration mode with ‘config-register 0x2102’.
  11. Save your changes and reboot to set the new config register.

Issue a ‘Show version’ to verify you have the correct config register:

Router Configuration register 0x2102


Configuring IPSec Site-to-Site VPN Tunnels

Configuring a IPSec Site-to-Site VPN Tunnels on a Cisco Router

I thought I would run through configuring IPSec Site-to-Site VPN Tunnels on a cisco router. I’m going to be covering at high level the basic principles needed to configure a IPSec Site-to-Site VPN. The fundemental principals can be used for the Cisco ASA Firewall or Cisco VPN concentrator. The VPN gateways in our example (the routers) are responsible for encapsulating and encrypting the outbound traffic which in a real world example this would from be from a site to a peer gateway at another site. This could be either through an MPLS network from an ISP or more commonly directly over the Internet. When the receiving gateway receives the traffic it strips away the headers, decrypt’s the content with the pre-shared key and forwards on the traffic to a host network on the inside.

In our example we are going to be using 3 networks with an IPSec VPN tunnel being established between two of the routers. I have added the extra network without the tunnel to demonstrate in wireshark the encrypted vs the non encrypted traffic. All three networks are connected to a switch which we are going to image as our either our Internet or MPLS connection.

IPSec VPN Diagram

An IPSec tunnel consists of 5 stages to establish and terminate its connection these are:

  • An ISAKMP tunnel is initiated when the VPN gateway detects ‘interesting traffic’ which is defined by an ACL.
  • IKE Phase 1 is established through negotiating the ISAKMP SA policy that is defined in the config.
  • IKE Phase 2 is established through negotiating the IPSec SA policy.
  • The IPSec tunnel is created and data can begin to be transferred Encrypted.
  • The IPSec tunnel is teared down when either the lifetime of the session expires or the IPSec SA is removed.

Below is the extra config that is used for each router other than the initial config of a standard GNS3 router, this can simple be copied into global configuration mode. The config is split into three main sections the interface configuration, EIGRP configuration and the IPSEC configuration.

I’ve broken down just one of the IPSec configuration below to explain what the various elements are, these need to match at both sites:

ipsec command breakdownBefore generating any interesting traffic if we execute ‘show crypto ipsec sa’ from RouterA we can clearly see there has been no traffic captured by the ACL.

before ipsec traffic

If we now generate some ‘interesting traffic’ such as a ping from PC1 to PC3 which will match our ACL that is specified in our crypto map, then re-issue ‘show crypto ipsec sa’ we will see from the stats that packets have been encrypted. Further to this we will see the inbound and outbound session will be ‘ACTIVE’.

After ipsec traffic

To take this one stage further we will demonstrate the difference in the Encrypted IPSec traffic and ordinary traffic using wireshark. The capture will be taken from the link between RouterA and the core network. We will ping out from PC1 to PC3 (the encrypted traffic caught by our ACL) and PC2 to PC5 (the non-encrypted traffic that doesn’t match our ACL) simultaneously. This is what we see:

IPSec traffic in wireshark.

As we can see from the wireshark capture our encrypted traffic is shown in the Encapsulated Security Payload and our non encrypted traffic in the clear (the ping request and reply).