Monday, 28 November 2011

What is difference between Limited Broadcast address and Directed Broadcast address

We all know broadcast address . It is the address used to address all in the same LAN ( Local Area network).
In broadcast we have two types : Limited broadcast and Directed broadcast. What is the difference between the directed broadcast and the Limited broadcast ?

Limited broadcast is the broadcast limited to a single LAN and which is to be received by all. It is sent to reserved Class E , IP address 255.255.255.255. The destination MAC address for such frames will be FF:FF:FF:FF:FF:FF.  The router simply drop the Limited broadcast address and does not forward it.


Directed Broadcast address is the local subnet broadcast address. If the subnet is 192.168.10.0 , the directed broadcast address will be 192.168.10.255 , which will be heard by all in the same subnet hosts. The router will receive such packet and process it.The  destination mac address will still be FF:FF:FF:FF:FF:FF, as it is to be received by all hosts.

DHCP packets are Limited broadcast packets and NBNS uses directed broadcast packets.



What is the Maximum Ethernet Frame Size supported by Non -VLAN switch

I can across this question and it left me thinking. I searched in Net and could not find any satisfactory answer. What will be the maximum Ethernet frame size allowed to be passed from any Non- Vlan switch? Before answering this question Lets refresh some basics.

We Know that the MTU of the Ethernet is 1500. So anything above 1500 should not be allowed. This does not include header so lets see how much with header .

We have 14 bytes of Ethernet header = 6 byte source MAC address + 6 byte destination Mac address+ 2 Byte protocol type

So total byte excluding CRC will be 1500 + 14 byte ==1514 bytes

Lets say we take example of ICMP packet.

For any ICMP packet this 1500 byte includes === IP header 20 byte + ICMP header 8 bytes + 1472 data

This you can verify by using ping command on your PC

ping 192.168.3.1 -t -l 1472

If your Ethernet interface MTU is 1500 if you give 1473 the packets will be fragmented by the PC . This can be seen in wireshark.

So now total is ==== 14 ( Ethernet header) + IP header 20 byte + ICMP header 8 bytes + 1472 data + 4 byte CRC ===1518 Byte including header


OK this was without VLAN , now we have VLAN 4 bytes so :

Total allowed Ethernet frame with VLAN should be 1518+4 ==1522 bytes

So to accommodate this some changes has been done in standard which is known as 802.1ac-1998. This states that

Section : 4.2.4.2.1 Framing

a) Maximum Frame Size. The receiving CSMA/CD sublayer is not required to enforce the frame size
limit, but it is allowed to truncate frames longer than maxUntaggedFrameSize octets and report this
event as an (implementation-dependent) error. A receiving CSMA/CD sublayer that supports tagged
MAC frames (see 3.5) may similarly truncate frames longer than (maxUntaggedFrameSize + qTag-
PrefixSize) octets in length, and report this event as an (implementation-dependent) error.


This states that if an interface receives a frame more than the size limit it can drop the frame.

So if a switch does not support the vlan it can drop the received frame if the size is above the maxUntaggedFrameSize i.e. 1518 bytes in our case.

And if it supports the VLAN frame it can drop the frame above the (maxUntaggedFrameSize + qTag-PrefixSize) i.e. 1522 bytes.

Also a Non Manageable switch does not mean that it cannot forward or support VLAN . I have seen non manageable switch which can support Jumbo frames also. So it all depends on Vendors how they implement and how much maximum allowed Ethernet size they support.

Thursday, 17 November 2011

Spanning tree Protocol : Difference between Port Priority and Path Cost

When you are working on Spanning tree protocol you know you are trying to make your network more predictable and trying to avoid loops even though redundant links are there.

One of the Questions always asked and is most confusing is " What is difference between STP Port Path cost and Port priority  ? " and " How STP Port path cost and Port Priority should be used ?".

The answer seems difficult to understand but once you do it. you know what it means.

Both the parameters are to be used to determine which of the interface should go in forwarding state.

The behavior and affect of both are different.

Cost is used on local switch to elect the root port of the device. The cost is accumulated up to the root bridge from each interface, and the least cost path to the root bridge becomes the root port on the local bridge.

Here, remember if you change any of the bridges path cost the whole switches in the topology are getting affected, because now again all the local switches has to recalculate the path cost for each port and decide for the designated port and the root ports.

Now for the Downstream switch the priority works. It is only applicable for the two directly connected switches and if you change priority on the local switch A, the Switch B connected in the down link direction will get affected. Higher the priority the less preferred the port.

Lets say         Switch A-------P1-------Switch B
                                  --------P2-------


Switch A and B are connected to each other with 2 ports so as to form a loop. Lets say Switch A becomes the root bridge, so both its ports will be in forwarding mode. Now for Switch B lets say P2 goes to blocking mode. So to make P1 in blocking mode and P2 in forwarding mode following things can be done:

1) Using Path Cost: Change Path cost in Switch B, P2 port and make it less than the P1 of Switch B..

2) Using Priority : Change priority in Switch A, P2 port and make it less than the P1 of Switch A.

Happy Learning !!!!

Wednesday, 16 November 2011

Test Link : 1.8.5 How to Print Images/ attach images in requirement document or in Microsoft word

We are using TestLink Test Management tool, version 1.8.5  for over a year now and we saw the problem that when we print the document in MSWORD the images are lost. It will not show the images but a blank space will be left in the document.

Again I searched the forum and found one link , i asked the the question and got the answer. these Guys are really good :-)

So i modified as per the suggestion and it worked fine below is the modification code:

Go to

/TestLink_1.8.5.1/lib/results/printDocument.php , insert it on line 249 Before 250 $generatedText .= renderEof();

####################################################################################
# change relative image paths to absolute image paths
# src="/  => src="http(s)://example.com/
####################################################################################

# fetch protocol and domainname
if($_SERVER["SERVER_PORT"]=='443'){
   $url_host = "https://".$_SERVER["HTTP_HOST"];
}elseif($_SERVER["SERVER_PORT"]=='8001'){
   $url_host = "http://".$_SERVER["HTTP_HOST"];
}else{
   $url_host = $_SERVER["HTTP_HOST"].":".$_SERVER["SERVER_PORT"];
}
if(substr($url_host, -1)!='/'){
   $url_host.= '/';
}

$xPattern = "/src=\"\//si";
$xValue = "src=\"".$url_host;
$generatedText = preg_replace($xPattern, $xValue, $generatedText);
####################################################################################
save it .

It should work fine. May be in newer version this bug should be fixed.

Below are links:

Forum Link:



http://www.teamst.org/forum/viewtopic.php?f=11&t=3583&p=11354&hilit=sss&sid=380ad4c03e77d6864e1388fc0cd806c0#p11354










Monday, 14 November 2011

How to change the Test case Title Length in Test Link ( Test Management Tool)

To use Test Link one of the blockers was to increase the Title size of the Test cases. Default Test Case in TestLink Title Size was 100,and as we have to send the Test Plan to external teams/ customers for review, it was a blocking point. No body wants to read the full test case and they wanted everything in title only, so we had to modify it.

If you are happy with the limit dont modify it, as developers dont recommend to modify these settings. Do it at your own risk and according to your requirement.

After digging through forums I have changed following things to increase the TestLink Test case Title Size . It is working in Version 1.8.5.

1) Go to http://localhost/xampp -> Your TestLink Server IP
2) Go to phpmyadmin-> or go to ur database
3) Select you datebase table
4) Search nodes_hierarchy table-> inside that search for "name"
5) Edit the field and increase the value from 100 to 255
6) save
7) Modify testlink/gui/templates/input_dimensions.conf
    Search and increase the value to 255
                   SRS_TITLE_MAXLEN=255
                   REQ_SPEC_TITLE_MAXLEN=255
8) Save

Try giving more title length and it should work fine.

Review of Test Management Tool Testlink

We are using Testlink version 1.8.5 for quite some time (almost a year). We have almost 8000+ test cases. When we started it was like no body wanted to use it and everyone was so comfortable in using excel sheets that even if they were told to use it, they will first update the results in Excel and then unwillingly do it in test link. Even Managers were more comfortable in using excel with all those formulas to know the current status of work done.

But slowly we pushed it, one person by person. When slowly they started using it and once all the test cases are uploaded in the test link, life was easy. Managers got used to immediately knowing the status of work progress and today after one year no body wants Excel, everything  is done in Testlink. Test case Creation, Assignment , execution, report generation, review of test cases , almost everything can be done using Testlink.

I think for small to medium size company who have QA groups can benefit most from this Test Management Tool  Testlink as it is completely free and they have active members in forum who replies almost immediately to the queries.
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