Oct 15

IOS Upgrade on a Cisco ASR

I was recently tasked with upgrading all the IOS on our ASRs as we move over to running PFRv3.  Like most any other cisco router upgrade the process is pretty straight forward but there are a few caveats in the syntax used on the ASRs.  For those that may have never performed a IOS upgrade I will run through the entire process.

Check the current IOS Version

Like any upgrade you need to know where you are starting to see what is going to be needed to get you to where you want to be.  If you are jumping several versions of major code then you may need to upgrade to an intermediate code in between to get to the final version you wish to run.

Run a show version

Running the “show version” command will provide you the current system image file that your ASR is running.

Show Version Continue reading

Jan 23

VPN Troubleshooting – Phase 1 – ISAKMP (IKE) Status Messages MM_WAIT_MSG#

When troubleshooting Phase 1 of a VPN tunnel the MM_WAIT_MSG state can be a great clue as to why your tunnel is not forming. If your firewall hangs at a certain state then this will show you where in the path your VPN is failing.Phase1

MM_WAIT_MSG2

What is happening

                Initiator sends its hashed IKE policy details to the receiver to create the initial contact.  Initiator will stay in this state until it receives a response back from the remote peer.

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Dec 17

How to perform a pcap packet capture and copy the file off of a Cisco ASA

Cisco uses a different way to run and save packet captures on its ASA firewall than a popular Linux tcpdump/Wireshark tools. Below is a quick guide to capture and then copy out a pcap file from the firewall for offline analysis.

Setting up your Packet Capture

The basic syntax is:

#capture <Name for capture> type raw-data match ip  <source IP/Network> <Network Mask> <destination IP/Network> <Network MAsk>
#capture <Name for capture> packet-length 1522 buffer 524288
#capture <Name for capture> interface <Name of interface to capture on>

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Sep 20

Looking up a VPN PSK on a Cisco ASA

When you have VPN tunnels out to 3rd party customers there comes a time when something is going to go wrong and at least one end of the tunnel is going to have to be rebuilt.  More often than not commanddocumentation got lost or was not updated the last time things changed and now you have no idea what the PSK was that you used on that tunnel.  You quickly look at the configuration on the other end only to find that the PSK is stared out in the running config. Continue reading

Sep 20

The Use of Prefix Lists for Route Filtering

Using IP prefix lists has several advantages over using an ACL.  Prefix lists match on more than just an IP address, they also match on the prefix length of the route.  This as we have seen maybe an advantage but can also cause problems with routes being filtered that we didn’t intend.  Finally, the internal processing of the IP prefix lists uses an internal tree structure that results in faster matching of routes than with ACLs.

Much like ACLs,  Prefix configuration commands using the same name end up being in the same list.  As with named ACLs, each ip prefix-list command has a sequence number to allow later deletion of individual commands and insertion of commands into a particular sequence position.  Prefix lists are read in order, top to bottom and once a match is found the rest of the list does not get read. Each command has a permit or deny action, but because it is used only for matching routes, and not for packet filtering, the permit or deny keyword just implies whether a route is matched (permit) or not (deny).

Creating a Prefix list uses the following command syntax.

ip prefix-list list-name [seq seq-value] {deny | permit prefix/prefixlength}[ge gevalue] [le le-value]

EX.  Ip prefix-list FILTER_ADV_ROUTES seq 10 permit 172.31.26.0/23 ge 24 le 32

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