Explain general network challenges

Exam: Cisco 300-101 - CCNP Implementing Cisco IP Routing (ROUTE v2.0)

In this chapter we will be discussing how to explain the network challenges. You will find questions on this under the main section of Network Principals. This topic is essential to know from the point of view of the exam number 300-101 route. As a network administrator you will come across a lot of challenges in your day today work. In this chapter we will discuss some of these challenges in more details:

Unicast

The LAN switches will be forwarding the layer 2 tables and the content addressable memory tables towards specific ports based on the VLAN number and the MAC address. If there is no entry that corresponds with the destination of the frame’s MAC address in the incoming VLAN then the unicast frame will be sent to all the forwarding ports with the VLAN. This can cause flooding at times. A limited flooding is generally a part of the switching process. If there is continuous flooding it can cause adverse affect on the performance of the network. In this document we will explain the normal issues that can come up due to flooding and also why some traffic may be flooded more often.

The main cause of a flooding is when the destination MAC address is not in the L2 forwarding table. This happens when:

Asymmetric routing - this happens when a large amount of flooded traffic saturates the low bandwidth links. This can cause network issues at times and can also completely remove the connectivity.
This is one of the most common causes of flooding. In this the packets follow different paths depending on the direction that causes asymmetric routing. The flooding that is cause by the asymmetric routing can be limited by using two methods and these are:
- Asymmetric routing with bridge groups on the catalyst 2948G-L3 and 4908G-L3 switches.
- Asymmetric routing and HSRP (hot standby router protocol)
Spanning tree protocol topology changes - this can be another very common cause for flooding. TCN or topology change notification is basically designed to correct the forwarding table once the forwarding topology has changed. This is important in order to avoid connectivity outage.
The TCN follows a simple rule. It shortens the ageing time of the forwarding table. If the address is relearned then it will age out and a flooding will occur. The TCN is a well configured network.
When the port is flapping then there will be repetition of TCN that will lead to flooding. To limit the TCN you can configure portfast on all the end devices.
Forwarding table overflow - there can be another cause of flooding and that is overflow of the switch forwarding table. In this what happens is that the new addresses cannot be learned as a result the packets that are destined to these addresses are flooded till there is some space made available in the forwarding table.
A new address will then be learnt but this is very rare scenario. Another reason for the forwarding table overflow is when there is an attack on the network. This will lead to one host generating frames that are sourced with different MAC addresses.
This will eventually saturate the forwarding tables and lead to flooding. You can limit such attacks by limiting the number of MAC addresses that are learned on the untrusted ports. This can be done using the port security features.

Flooding Situation

We will now move on to how exactly you can understand if there is excessive flooding. Well you can detect a flooding when you capture traces of packets that are seen at the workstation when there is a slowdown or an outage.

You must know that unicast packets that do not involve the workstation should not be seen very often on a port. In case of asymmetric routing you will notice MAC addresses that will not stop flooding even after a reply was received from the destination.

In case of TCN there will be flooding including many different addresses but that should stop eventually and restart again. In L2 forwarding table overflow case you will see some sort of asymmetric routing itself.

However, the difference here is that you will be noticing a high amount of strange packets or you can also see normal packets that will appear in abnormal quantities and from different MAC addresses.

Out-of-order packets

We will now move on to discuss what an out of order packet is. This is another network challenge that you will often find as a network administrator. It is a very common occurrence. In this what happens is that the order of the packets is inverted in the internet.

This can be cause if there was a pre packet load sharing algorithm. This can affect the receiver by making the TCP session slow. This can also affect the re-transmissions in the network. This will cause re-transmissions that are not necessary.

When the TCP receiver gets packets that are not normal it will duplicate the ACks so that it can trigger fast re-transmit algorithm to the sender. These are the ACKs that will make the TCP sender understand that a packet has been lost and as a result it will be transmitted again.

How out of order packets reduce the transmission speed is when there is a fast re-transmission that is triggered by the duplicate ACKs. The TCP sender will assume that this is an indication to the network about congestion. As a result it will reduce the congestion window also known as the cwnd.

This will automatically reduce the transition speed. This will lead to the need to grow large from a slow start all over again. If this recording is happening frequently the congestion window will become of small size and it will hardly grow larger. This will lead to the TCP to transmit packets at a much slower speed and the bandwidth will not be utilised effectively. It also reduces the efficiency of the receiver.

The TCP receiver has to send the data to the upper layer in a particular order. The TCP will buffer all the out of order packets till it can get them in order. While this re ordering is going on the upper layer will get the data in a bulk and not in order as a result it will become inefficient.

These are some of the things that you must know under the topic explain general network challenges. We hope that this chapter will help you to prepare better for the CCNP routing and switching examination.

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