February 9, 2010
I was asked today how to calculate the Bc values. The known formulas always add confusion. So the aim of this article is not to add more confusion, but offer an easy alternate way to calculate the Bc values used with shaping.
First lets review some basic shaping definitions.
CIR (Committed Information Rate)
- Dictates the output rate one aims to average per second on the circuit/interface.
- Book formula : CIR = Bc / (Tc/1000)
Tc (Time-Interval)
- It is the time in milliseconds into which a second is divided for transmission intervals.
- The Tc can’t be adjusted directly, but it can be changed by setting the Bc to a specific value..
- The maximum value of Tc is 125ms (8 intervals per second) and the minimum value is 10ms (100 intervals per second).
- Actually 8ms (125 intervals per second) on distributed platforms. On distributed platforms, the Tc must be defined in 4-ms increments. The nearest multiple of 4 ms within the 10-ms target is 8 ms.
- Book formula : Tc = (Bc / CIR) x 1000
Bc (Committed Burst Rate)
- Bc is the number of committed bits allowed to be sent per interval (Tc) to conform with the target-rate (CIR) per second.
- If Bc worth of bits are sent every interval in a second, the output rate is the CIR.
- Book formula : Bc = CIR x (Tc/1000)
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Posted in General info, QOS | Tagged Shaping | Leave a Comment »
February 5, 2010
Ever busy with a scheduled change, and the configuration all of a sudden differs from what you configured five minutes ago?
Normal IOS (not XR) behaviour allows multiple users to make instant changes to the running configuration. Occasionally two users make changes to the same config portion at the same time. One overwriting the others. ONLY the last commands entered will take effect.
The Configuration Lock feature allows a one to have exclusive change access to the Cisco IOS running configuration, preventing multiple users from making concurrent configuration changes.
There are two modes:
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Posted in General info | Tagged IOS | Leave a Comment »
February 1, 2010
Thanks to my iPhone, publishing this was made easy.
Feel free to download 3 of the 15 chapters here.
Then if you are interested in obtaining a full copy of Short-Notes V4.1 just click on the BUY link on the right-hand side under pages.
Ping me if any questions.
Posted in CCIE R&S, General info | Tagged CCIE Study Guide, Short Notes | 1 Comment »
January 31, 2010
Short-Notes is the result of an unplanned book I wrote during my studies for the CCIE Routing & Switching lab.
It is a good source for theory. It is a great reference guide for the practical commands. It also presents complete configuration examples, in a completely new way, to easily see how the technologies are implemented. Most topics include the DOC-CD locations, so more info is at hand if needed. Ultimately this is the only CCIE study guide, (I have found) which can be used to review all the work start to finish the day before the lab.
A deeper look at the origins…
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Posted in CCIE R&S, General info | Tagged CCIE Study Guide, Short Notes | Leave a Comment »
January 19, 2010
I have updated the free Switching and OSPF study guides.
Feel free to download them at these posts:
01- Switching
07- OSPF
Posted in CCIE R&S | Tagged CCIE Study Guide, Short Notes | Leave a Comment »
January 19, 2010
I came across a really good Performance Routing document, that I thought should assist R&S v4 candidates. It has really great examples of the different scenarios along with implementations.
Here is a depiction of the PfR process flow for OER configuration:
Source: Cisco Design Land
Posted in CCIE R&S | Tagged CCIE R&S, pfr | Leave a Comment »
January 7, 2010
I had a interesting question from a friend today.
Assume the following scenario:
Im going to exclude any MPLS connectivity, as it is not relevant.
The PE (Router1) connects the CE (Router3) with two links, one serial and one wireless.
This particular ISP runs mostly static routes to client sites (within the VRF’s) or alternatively eBGP.
On a wireless link it is always good practise to run BGP to detect when connectivity with the remote end is lost in the underlying Layer2 network. (Preventing a blackhole)
Regarding routing on the Serial Link, there as a default route out from Router 3 and a static route to 10.33.33.0/24 on Router1 pointing to Router3.
The client wants to load-balance traffic across both links. And the Admin Distance of the static route was set to 20 to match eBGP. (this is the scenario)
So the question : Why does Router1 not install both routes (the eBGP route and the Static), both with an prefix-length of /24, a Admin Distance of 20, and metric of 0 into the RIB??
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Posted in General info | Tagged Routing | 1 Comment »
January 7, 2010
Inspired by the flow chart that Mr Richard Bannister did for the BGP route path selection, I did one for conventional route-selection in the RIB.
Posted in General info | Tagged Routing | 1 Comment »
December 24, 2009
Often knowing the necessary show commands is not enough, you need to understand the output.
Here is a good example and breakdown of each of the fields with the command:
show traffic-shape
VC = 'DLCI's'
Access List = 'Used to shape traffic of common type for separation'
Target Rate = 'CIR in bits'
Byte-Limit = 'Bc+Be ie the size the token bucket, express in BYTES'
Sustain bits/int = 'Bc value per Tc, (int is short for interval or Tc)'
Excess bits/int = 'Be value'
Interval (ms) = 'Tc value'
Increment (bytes) = 'How many bytes of token replenished each Tc, ie Bc value in bytes'
Adapt Active = 'Shows Adaptive shaping has been enabled. If a BECN is received, the flow is throttled back'
What else can be set about the configuration here?
The interface have 3 DLCI’s defined.
DLCI’s 413 and 405 have a CIR of 56k. This was not configured. This is default behaviour. When ‘frame-relay traffic-shaping’ is enabled each DLCI on that interface will be allocated a 56k CIR unless changed. Here it is clear that DLCI 403 has a map-class policy applied.
Oh and Merry Christmas guys 
Posted in Frame-Relay, OUTPUT-101, QOS | Tagged FRTS, OUTPUT-101 | Leave a Comment »
December 21, 2009
While searching for CCIE jokes, I found an old forum thread at Cisco Learning network containing jokes about the man, the legend, the hex-translator, the missing E-bit(evil) : Scott Morris.
Here are some of the jokes I think is pretty funny:
- Scott Morris once planned a cross-country trip using a Route Map!
- Scott Morris plays a rather unique instrument called the ISAKMP!
- Every VPN is an EasyVPN for Scott Morris!
- When Scott Morris was four years old he was putting together OSI models!
- Scott Morris’ home wireless network runs on brain waves!
- Scott Morris slayed the Kerberos daemon.
- Scott Morris’s driver’s license is a PDF!
- If you doubt Scott Morris just sh Scott | s certification
- There are no hidden IOS commands. Only those Scott Morris chooses not to look at!
- Scott Morris has counted to pi.. twice!
- Normal people teach their dogs to fetch. Scott Morris taught his dog to route.
- Morpheus was searching for Scott Morris!
- Scott Morris doesn’t have a steering wheel in his car. He has a CLI!
- Scott Morris found Waldo in an extended access control list!
- Scott Morris is actually an undercover SNMP Agent!
My favourite three are :
- Scott Morris ran track in high school and always won the 100 meter frame relay!
- He taught his dog to ARP! arp, arp, arp, arp.
- MD5 : Morris Digests 5 CCIE’s for breakfast!
Posted in Humour | Leave a Comment »
December 17, 2009
This chapter provides perfect insight about the concise content of each technology section covered in Short-Notes V4.
Short-Notes is the definitive CCIE R&S Study guide.
Feel free to download, but please let me know you views and comments : blog@ru.co.za . Alternative please rate this post (click on title, then rate below). 
OSPF Short-Notes
Posted in OSPF | Tagged CCIE R&S, CCIE Study Guide, OSPF | 3 Comments »
December 15, 2009
Serialization/Access-Rate is the physical clocking speed of the interface (ie 64-kbps/128-kbps etc), which determines the amount of data that can be encapsulated on to the wire.
Serialization Delay or Serialization Rate is a constant based on the access rate of the interface. It is the time needed to place data on the physical wire.
These values are set in hardware and cannot be changed.
A data frame can be sent onto the physical wire ONLY at the serialization rate of the interface. Thus serialization delay is the size of the frame in bits divided by the clocking speed of the interface.
Serialization Delay = Frame Size/Link Speed
For example, a 1500-byte frame (12000-bites/64000-bites) will take 187.5ms to serialize (put on the wire) on a 64-kbps circuit.
|
Link- |
Frame Size (Bytes) |
|
Speed |
64 |
128 |
256 |
512 |
1024 |
1500 |
|
64 kbps |
8 ms |
16 ms |
32 ms |
64 ms |
128 ms |
187 ms |
|
128 kbps |
4 ms |
8 ms |
16 ms |
32 ms |
64 ms |
93 ms |
|
256 kbps |
2 ms |
4 ms |
8 ms |
16 ms |
32 ms |
46 ms |
|
512 kbps |
1 ms |
2 ms |
4 ms |
8 ms |
16 ms |
23 ms |
|
768 kbps |
0.640 ms |
1.28 ms |
2.56 ms |
5.12 ms |
10.4 ms |
15 ms |
For low-speed WAN connections (those with a clocking speed of 768kbps or below), it might be necessary to provide a mechanism for Link Fragmentation and Interleaving (LFI) when running delay sensitive application like voice.
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Posted in QOS | Tagged LFI, QOS | Leave a Comment »
