Posts Tagged ‘features’


This blog is focusing on QoS on the PIX/ASA and is based on 7.2 code to be consistent with the CCIE Security Lab Exam as of the date of this post. I will create a later blog regarding new features to 8.X code for all of you non-exam biased readers :-)

NOTE: We have already seen thanks to our readers that some of these features are very model/license dependent! For example, we have yet to find an ASA that allows traffic shaping. 

One of the first things that you discover about QoS for PIX/ASA when you check the documentation is that none of the QoS tools that these devices support are available when you are in multiple context mode. This jumped out at me as a bit strange and I just had to see for myself. Here I went to a PIX device, switched to multiple mode, and then searched for the priority-queue global configuration mode command. Notice that, sure enough, the command was not available in the CUSTA context, or the system context.

pixfirewall# configure terminal
pixfirewall(config)# mode multiple
WARNING: This command will change the behavior of the device
WARNING: This command will initiate a Reboot
Proceed with change mode? [confirm]
Convert the system configuration? [confirm]
pixfirewall> enable
pixfirewall# show mode
Security context mode: multiple
pixfirewall# configure terminal        
pixfirewall(config)# context CUSTA
Creating context 'CUSTA'... Done. (2)
pixfirewall(config-ctx)# context CUSTA
pixfirewall(config-ctx)# config-url flash:/custa.cfg
pixfirewall(config-ctx)# allocate-interface e2 
pixfirewall(config-ctx)# changeto context CUSTA
pixfirewall/CUSTA(config)# pri?     
configure mode commands/options:
pixfirewall/CUSTA# changeto context system
pixfirewall# conf t
pixfirewall(config)# pr?
configure mode commands/options:

OK, so we have no QoS capabilities when in multiple context mode. :-| What QoS capabilities do we possess on the PIX/ASA when we are behaving in single context mode? Here they are:

  • Policing – you will be able to set a “speed limit” for traffic on the PIX/ASA. The policer will discard any packets trying to exceed this rate. I always like to think of the Soup Guy on Seinfeld with this one – “NO BANDWIDTH FOR YOU!” 
  • Shaping – again, this tool allows you to set a speed limit, but it is “kinder and gentler”. This tool will attempt to buffer traffic and send it later should the traffic exceed the shaped rate.
  • Priority Queuing – for traffic (like VoIP that rely hates delays and variable delays (jitter), the PIX/ASA does support priority queuing of that traffic. The documentation refers to this as a Low Latency Queuing (LLQ).

Now before we get too excited about these options for tools, we must understand that we are going to face some pretty big limitations with their usage compared to shaping, policing, and LLQ on a Cisco router. We will detail these limitations in future blogs on the specific tools, but here is an example. We might get very excited when we see LLQ in relation to the PIX/ASA, but it is certainly not the LLQ that we are accustomed to on a router. On a router, LLQ is really Class-Based Weighted Fair Queuing (CBWFQ) with the addition of strict Priority Queuing (PQ). On the PIX/ASA, we are just not going to have that type of granular control over many traffic forms. In fact, with the standard priority queuing approach on the PIX/ASA, there is a single LLQ for your priority traffic and all other traffic falls into a best effort queue.

If you have been around QoS for a while, you are going to be very excited about how we set these mechanisms up on the security appliance. We are going to use the Modular Quality of Service Command Line Interface (MQC) approach! The MQC was invented for CBWFQ on the routers, but now we are seeing it everywhere. In fact, on the security appliance it is termed the Modular Policy Framework. This is because it not only handles QoS configurations, but also traffic inspections (including deep packet inspections), and can be used to configure the Intrusion Prevention and Content Management Security Service Modules. Boy, the ole’ MQC sure has come a long way.

While you might be frustrated with some of the limitations in the individual tools, at least there are a couple of combinations that can feature the tools working together. Specificaly, you can:

  • Use standard priority queueing (for example for voice) and then police for all of the other traffic.
  • You can also use traffic shaping for all traffic in conjunction with hierarchical priority queuing for a subset of traffic. Again, in later blogs we will educate you more fully on each tool.

Thanks for reading and I hope you are looking forward to future blog entries on QoS with the ASA/PIX.

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The 3560 QoS processing model is tightly coupled with it’s hardware architecture borrowed from the 3750 series switches. The most notable feature is the internal switch ring, which is used for the switch stacking purpose. Packets entering a 3560/3750 switch are queued and serviced twice: first on the ingress, before they are put on the internal ring, and second on the egress port, where they have been delivered by the internal ring switching. In short, the process looks as follows:

[Classify/Police/Mark] -> [Ingress Queues] -> [Internal Ring] -> [Egress Queues]

For more insights and detailed overview of StackWise technology used by the 3750 models, visit the following link:

Cisco StackWise Technology White Paper

Continue Reading

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Sometimes people need to conditionally advertise routes into BGP table based on time of day. Say, we may want to adversite IGP prefix with community 1:100 during daytime and with community 1:200 at the other time. Back in days, the procedure was easy – you had to create time based ACL, and use it in route-map to set communities:

time-range DAY
 periodic daily 9:00 to 18:00

access-list 101 permit ip any any time-range DAY

route-map SET_COMMUNITY 10
 match ip address 101
 set community 1:100
route-map SET_COMMUNITY 20
 set community 1:200

This construct worked fine back in days with 12.2T and 12.3 IOSes up to 12.3(17)T. However, since 12.3(17)T, BGP scanner behavior has changed significally. Up to the new version, redistribution into BGP table was based on BGP scanner periodically polling the IGP routes every scan-interval (one minute by default). With the new IOS code, redistribution is purely event driven: a new route is added/deleted from BGP table based on event, signaled by IGP (e.g. IGP route withdrawn, next-hop change etc). This change in BGP scanner behavior was not clearly documented, unlike the related BGP support for next-hop address tracking feature. Ovbsiously, a change in time-range is not treated as an IGP event, hence the filter does not work anymore.

Still, there is a number of workarounds. Here is one of them: we use time-based ACL to filter or permit ICMP packets, and advertise routers based on that virtual “reachability” info.

First, we create time-range and time-based access-list:

time-range DAY
 periodic daily 9:00 to 18:00
access-list 101 permit ip any any time-range DAY

Next we create a special loopback interface, which is used send ICMP echo packets to “ourself” and attach the ACL to the interface to filter incoming (looped back) packets:

interface Loopback0
 ip address
 ip access-group 101 in

We create a new IP SLA monitor, to send ICMP echo packets over loopback interface. If the time-based ACL permit pings, the monitor state will be “reachable”

ip sla monitor 1
 type echo protocol ipIcmpEcho
 timeout 100
 frequency 1

Next we track our “pinger” state. The first tracker is “on” when the loopback is “open” by packet filter, the second one is active when the time-based ACL filters packets:

track 1 rtr 1 reachability
! Inverse logic
track 2 list boolean and
 object 1 not

The we create two static routes, bound to the mentioned trackets. That is, the static route with tag 100 is only active when loopback is “open”, i.e. time-based ACL permits packets. The other static route is active only when time-range is inactive (the second tracker tells that the destination is “reachable”):

ip route Loopback0 tag 100 track 1
ip route Loopback0 tag 200 track 2

Now we redistribute static routes into BGP, based on tag values, and also set communities based on the tags:

router bgp 1
 redistribute static route-map STATIC_TO_BGP
route-map STATIC_TO_BGP permit 10
 match tag 100
 set community 1:100
route-map STATIC_TO_BGP permit 20
 match tag 200
 set community 1:200

This is also a funny example of how you can tie up together multiple IOS features at the same time.

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