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Overload bit is special bit in the IS-IS LSP used to inform the network that the advertising router is not yet ready to forward transit traffic.  The overload bit was first intended for signaling overload or resource shortage on specific router for the rest of the network.

You can use the command set-overload-bit intentionally on specific router to signal other routers not to use it as a transit hop in their SPF calculations. Typically this is done for a temporary situation like an overloaded router due to memory or processing shortage and released when the router recovers from the problematic situation.

Overload bit is a simple and handy technique that can be used whenever you want to isolate a specific router in your network before a maintenance, attack or to avoid problematic path.

we usually use overload bit in the following situations:

• To verify operation of new installed routers before allowing them to forward transit traffic.
• Preventing control plane routers like (Route Reflectors) from being used accidentally in the forwarding path.
• On routers’ start-up to avoid traffic black-holes until routing protocols are fully converged.
• To isolate a specific router before decommissioning or a maintenance operation.

I advice you to try it in the lab and to review its options  if you had not yet and fell the beauty of the simple networking tricks.

In my post about out of band management networks I mentioned console servers as a mean of providing centralized remote access to network devices collocated in the same site. This post is a complementary post for the previous one if you are planning to use a console server in your out of band management network. Below are 7 questions to ask before you buy a console server:

1. What access methods does this console server support?     ” Dial up, GSM, IP connectivity”
2. How many serial console ports does the console server provide?    “No. of managed devices”
3. How many Ethernet ports does it provide?    “if required”
4. Does it provide DC or AC power management for your devices?    “if required”
5. How is the console server managed and configured?
6. What authentication methods does the console server provide?
7. What security features and protocols does this console server provide?

Some devices provide a lot other features but I thought these are the main points you need to know about console server before you buy. Share your experience with us..

Building a robust out of band management network is a critical part of any service provider or large enterprise backbone. Although failures may not occur frequently but believe me if you are not prepared when they happen you will know how OoB management is important and critical.

Network failures do happen for reasons ranging from human errors to power or hardware failures. One of your main objectives as a network designer is to keep your network available and achieve the highest possible up-time. There are a lot of things you can do to achieve these objectives in terms of redundancy and design best practices.

Out of band management network should be in your priorities; specially when it is your last resort in case of big failures that need to be handled as fast as possible.

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Last Wednesday Cisco announced its fourth fiscal quarter financial reports. Cisco reported a 79% jump in profit however its stock price fell down by 8% in after hours trading as sales were slightly below analysts’ expectations. The question is Why this strong negative reaction?

Lets take a closer look; the world is being very cautious recently about financial and economic indicators, no one really knows what is going to happen as economic and financial indicators are so confusing and showing mixed signals. Some indicators warn of  the possibility of a double dip recession while others indicate that things is still going fine but may be we are just slowing down. There is no clear direction and most of the markets are pausing or moving sideways for the last few months.

Although Cisco’s profit was up by 79% which is great, sales were slightly lower than expectations and the market has been recently very reactive  and sensitive to bad or unexpected news than it is for good news.

Have a look at this article for the full story Cisco Profit shares fell on revenue mess.

The article is mentioning some numbers and I can see most of the numbers are improving except the days sales outstanding that rose from 39 to 41 days which is also in the safe side.

Some indicators about the US economy will be announced this week and will be deeply analyzed  by economists for signs whether the economy is going to a double dip or just slowing down. Lets hope for the best and if you are  investing keep your eyes open and keep some cash aside.

P.S. This site is still about networking topics but I thought some of you might be interested in economics or finance like me and the post is still about Cisco the tech giant so we have not gone so far

ADSL !!!

I can hear you say “I know everything about ADSL, its a mature technology and I already know it inside out, why is he  writing about ADSL now?”

To be honest I am also surprised by writing about ADSL after working with it for about 7 years now and the rest of the world is talking about FTTx technologies.However, I found it a good start for a series of posts about broadband technologies. Although ADSL is a mature technology, it is still growing even after the spread of fiber technologies and I believe it has the potential to grow more in the coming years.

ADSL was born of the need for speed internet access coupled with the desire for low cost dedicated connections. ADSL is still satisfying both needs for both data carriers and subscribers. ADSL data rates are sufficient for almost  all internet applications available today at low cost compared to other technologies and using the same infrastructure that was built 100 years ago (PSTN).

Lets start by covering DSL technology without getting into much telecommunication details that I forgot myself few years after my graduation from college.

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In this post we’ll be covering a couple of topics from the Inside Out of the link-state protocols that have always been ambiguous and full of details, we’ll try to make them as crystal clear as we can.

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In this post we are going to cover the protocol packets and database structure for both routing protocols.

To start let’s first highlight a couple of facts. OSPF runs on top of IP, that is it uses IP packets to exchange its messages (and thus it is vulnerable to spoofing and DoS attacks, and accordingly the use of authentication is strongly recommended), while on the other hand IS-IS runs directly over layer 2, it creates its own packet (or PDU (Protocol Data Unit) to be more specific) and then encapsulates it directly inside the layer 2 frame, this leverages IS-IS a point of strength that we’ll cover later (and thus it is more difficult to spoofing and DoS attacks).
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In our previous post we started consolidating the endless story of OSPF vs IS-IS, in this post we will cover the historical part of the story, it might not be interesting for some people, but I do believe that the history is what makes the future, so please bare with me through this post.

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In the previous post IS-IS Neighbor Discovery we have discussed how IS-IS automatically discovers neighbors, in this post we will discuss the DIS role in broadcast networks.

After the adjacency state reached the UP state the DIS election process take place, the router with the highest priority value (0-127 specified in the Priority field of the IIH PDU) win the election, if multiple routers have the same priority which is the case in most scenarios due to the default value “64″ of most implementations, the router with the highest SNPA (MAC address discovered from the MAC header of the received IIH PDUs) win the election and become the DIS for this segment (in Some circumstances the System-ID is also used in the election process.

On a frame-relay network the L2 address is DLCI which is the same in both sides so in this case the System-ID of the origin routers (discovered from the Source-ID of the received IIH PDUs) is used instead and the router with highest System-ID win the election.

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Like other routing and signaling protocols IS-IS has built-in automatic neighbor discovery mechanism which is known by IIHs (IS-IS Hello PDUs), because IS-IS is not IP based protocol the IIH PDUs and all other IS-IS PDUs are directly encapsulated on the data-link layer.

IS-IS has two hierarchical levels (L1 and L2) and two network types (Point-to-Point and Broadcast) so it’s essential to have different types of hello messages or PDUs for serve these networks since the data-link layer for broadcast and P2P links have different style of addressing so they need different treatment, due to this differences  there are three IIH PDUs and they are  identified in the IS-IS PDU Type field in the IS-IS 8-bit common header that is attached to all the IS-IS messages as the following:

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