VLANs Overview

One  should be familiar with the concepts related to TCP/IP traffic flow and switch operation. You should also feel confident about how to diagnose basic layer 2 connectivity issues.  In this one, I am going to extend your understanding of layer 2 technologies by introducing Virtual LANs (VLANs).

Problem With Switching
   Each port of a switch creates its own collision domain . In addition to that a switch can use FULL DUPLEX connectivity when connecting other devices to its ports (computers, printers, switches, routers). That allows the ports to SEND and RECEIVE streams of bits at the SAME time. This is due to the special design of a switch. Thus, the efficiency of transmission is radically increased when compared to its older cousin a hub using half-duplex connections (sending or receiving but not both at the same time).

However, switches still maintain ONE BROADCAST DOMAIN. This means that in some situations they flood frames out of all active interfaces except the one that receives the frame. The flooding occurs if either of these are true:

1. The destination MAC address of the arriving frame is unknown.
2. The destination MAC address of the arriving frame is broadcast.
3. The destination MAC address of the arriving frame is multicast.
4. A switch reaches its limit of MAC addresses learned on a port. Then all other MAC addresses can no longer be learned.

Pic. 1 - Switches maintain one broadcast domain (bottom left computer sends broadcast).

Icons designed by: Andrzej Szoblik - http://www.newo.pl

In a flat network like the one depicted above (Pic. 1), imagine a thousand computers sending broadcast traffic (e.g. ARP requests). They will be propagated everywhere as per rules described earlier. Imagine another situation in which a broken NIC (Network Interface Card =  Network Adapter) sends thousands of broadcast frames per second. Those will be flooded to all hosts interrupting them as they need to process broadcast frames. In those situations not only do we interrupt all hosts by sending frames to them, but also saturate links with garbage data unnecessarily. Why would my computer have to listen to broadcast traffic sent by HR server if I work in IT department? I do not use HR server's resources at all. Exactly!

VLANs Are Broadcast Domains

Virtual LANs are the method of creating multiple broadcast domains of smaller size in a switching infrastructure. They are commonly used solution to the above mentioned problems. By configuring VLANs on the switches you create multiple broadcast domains which are treated as separate, isolated LANs which CANNOT communicate with one another by default. This allows us to contain the broadcast/multicast/unicast traffic WITHIN a boundary of a given VLAN. 

Pic. 2 - VLANs Are Broadcast Domains

If you consider traffic in the Pic. 2, the computers in red transmit their bits onto the wire, switches will send those only to computers that are in the same VLAN, that is red in this case. For instance, if the bottom right red computer sends layer 2 broadcast (destination MAC address = FFFF.FFFF.FFFF), only computers in red VLAN are going to receive this transmission. Computers located in turquoise VLAN will NOT receive those frames anymore. This way we can segment the traffic between different hosts based on criteria such as groups of interests (workgroups), type of traffic (e.g. VoIP), type of the application used, user location, etc. So, the major benefits of using VLANs are: 

1. Broadcast/multicast traffic propagation is limited to a given VLAN (broadcast domain) where it originated.
2. Security is increased, as hosts located in different VLANs CANNOT communicate at all. The only way for them to communicate is to allocate different network/subnet addresses for VLANs and use a layer 3 device (router) to move the packets between them. The routers offer some control as to who can transmit to whom (ACLs, firewalls etc.). How to accomplish routing between VLANs I will explain in my next post.

I hope the above description sheds enough light on what VLANs are used for. Now, is the time to look at some details regarding their configuration.

VLAN Port Types

In order to segment the traffic, the hosts generating it must be assigned to the appropriate VLAN since all ports of the switch are members of VLAN 1 by default. The process of configuring that usually involves three major steps:

1. Configuring VLAN number in the switch database (optionally name of the VLAN and/or other parameters).
2. Assigning hosts to VLANs defined in step 1. There are two ways of doing that: either MAC address can be assigned to a VLAN (dynamic method), or port of the switch can be assigned to a VLAN (manual method).
3. Configuring VLAN Trunk connections between the switches. Even though, this step is optional, the majority of designs out there will need it.

The above mentioned configuration steps define two different port types VLANs can use:

1. Access Port - this type of port can be member of ONE VLAN ONLY. If a static port-to-vlan configuration is used, the port interprets all incoming frames as belonging to this specific VLAN. In case of using mac-address-to-vlan configuration the port will determine VLAN number (ID) for transmission based on the MAC address which is mapped to a specific VLAN.
2. Trunk Port - which by default belongs to ALL VLANS (1-4094). In other words, this port is capable of sending and receiving a traffic coming from different VLANs.

When is the trunk (multi VLAN) port required?

The below picture (Pic. 3) illustrates the need for it.

Pic. 3 - VLAN Port Types

The grey rectangles symbolize two switches. The colors, represents different ports assigned to different VLANs. Of course, VLANs in practice use numbers, not colors, to distinguish between themselves. When any bottom computer sends broadcast (or unicast towards another computer in the same VLAN/color connected to the upper switch), the port connecting the two switches must be trunk (multi-vlan port). In such situation w must allow all VLAN members to communicate with their peers in the same VLAN, irrespective where they are located. Both switches have yellow, red and blue members here! And according to the rules, red computers must be able to talk to all red computers located on the same and all other switches as well (yellow-to-yellow, and blue-to-blue).They are members of the same Virtual LAN after all.

In such design, in which members of the same logical network (VLAN) or broadcast domain are connected to different physical switches, the connection between them must be a trunk. Trunk ports run a special protocol called IEEE 802.1q (Cisco have also their own protocol called ISL, details of which are beyond the scope of this tutorial). This protocol is responsible for 'tagging' the frames (injecting extra information into their headers), while sending them out the trunk port.

Why?

Let me explain. Look carefully at the Pic. 3 and imagine that the computer connected  to yellow VLAN is sending broadcast towards all computers that are in the same, yellow, VLAN. The port between the switches is trunk, and as such allows ALL VLANs in and out. But the problem is that the receiving port on the upper switch gets the Ethernet frame on the port working as trunk as well. So, this port is also a MULTI-VLAN port! How does this upper, receiving, switch know which VLAN the frame is coming from? Well, it does NOT know whether the VLAN sending this broadcast was yellow, red or blue. This is where the sending (bottom) switch, using the trunk as outbound port, is going to inject extra 4 bytes into the Ethernet frame while transmitting it out. The tag will contain VLAN ID (number) of the sender. This way, the broadcast frame will have an extra information allowing the receiving switch (upper one) to recognize which VLAN it is coming from and forward this broadcast to ALL computers in the same VLAN (here yellow VLAN).

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NOTICE!
The TAG  is stripped off on the outbound ports configured as ACCESS ones. The tag is useful only on trunk ports.

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Before we finish this VLAN overview lesson, let me show you what information this TAG contains.

Pic. 4 - 802.1q TAG

The 802.1q tag is injected between the source MAC address and the type field in the Ethernet II header (pic. 4). It consist of two fields taking two bytes each:

1. First two byte field contains a signature of 802.1q protocol using value of 0x8100.
2. Second two byte field  contains:

• PRI - Class of Service 3 bits used by QoS, 
• Canonical bit for token ring support, 
• VLAN ID value that takes up 12 the least significant bits in the tag.

802.1q Native VLAN
There is one more thing I need to touch upon that is related to the 802.1q trunk port. That is the concept of Native Vlan. The designers of the protocol decided to send frames coming from so called 'native VLAN' out the trunk as UNTAGGED. In other words this frame does not have any tag inserted into the Ethernet header. So, frame coming from 'native VLAN' is a regular Ethernet frame. As long as the switches agree on the trunk link which VLAN is their 'native VLAN' for this trunk, a frame arriving on the trunk port without the tag is assumed to be coming from the same native VLAN the sender was transmitting. The default  'native VLAN' is VLAN 1, since this one cannot be removed from the switch. Probably the reason VLAN 1 is the 'native VLAN' by default is becuase switches originate frames such CDP, VTP, STP from this VLAN and there is no need to tag them as they are switch-to-switch communication only.

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NOTICE!
As of the time of writing this tutorial, all ports of Cisco switches belong to VLAN 1 by default which is also the (untagged) 'native vlan'. That VLAN is not going to tag frames on trunk-to-trunk connections.

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I am sure you realize what can happen if the two ports connecting switches use different VLAN ID for their 'native VLAN'. Yes, that can cause leaking frames between VLANs. And this is a serious security issue. So keep the same 'native VLAN' on trunk paired ports between switches.

With courtesy:

http://ciscoiseasy.blogspot.in

 

 

Securing Networks: Access Control List (ACL) Concepts

This article is the start of a new series centered in IT security, but focused on securing networks with access control lists, commonly referred to as ACLs. Access control lists, their function, and proper implementation are covered in Cisco exams, but the concepts and deployment strategies are also covered in certifications like Security + and CISSP. In this article, we will investigate and define the different types of access control lists and examine some deployment concepts, especially the “why” we use them and the “when”. Future articles will focus on their implementation on Cisco routers, specific designs for permitting and denying services, and venture into the world of firewalls.

What are Access Control Lists?

ACLs are a network filter utilized by routers and some switches to permit and restrict data flows into and out of network interfaces. When an ACL is configured on an interface, the network device analyzes data passing through the interface, compares it to the criteria described in the ACL, and either permits the data to flow or prohibits it.
  Packet filtering can help limit network traffic and restrict network use by certain users or devices. ACLs can filter traffic as it passes through a router and permit or deny packets at specified interfaces. An ACL is a sequential collection of permit and deny conditions that apply to packets. When a packet is received on an interface, the switch compares the fields in the packet against any applied ACLs to verify that the packet has the required permissions to be forwarded, based on the criteria specified in the access lists. It tests packets against the conditions in an access list one by one. The first match determines whether the switch accepts or rejects the packets. Because the switch stops testing conditions after the first match, the order of conditions in the list is critical. If no conditions match, the switch rejects the packets. If there are no restrictions, the switch forwards the packet; otherwise, the switch drops the packet.
   Switches traditionally operate at Layer 2 only, switching traffic within a VLAN, whereas routers route traffic between VLANs.
 

You configure access lists on a router or switch to provide basic security for your network. If you do not configure ACLs, all packets passing through the switch could be allowed onto all parts of the network. You can use ACLs to control which hosts can access different parts of a network or to decide which types of traffic are forwarded or blocked at router interfaces. For example, you can allow e-mail traffic to be forwarded but not Telnet traffic. ACLs can be configured to block inbound traffic, outbound traffic, or both. However, on Layer 2 interfaces, you can only apply ACLs in the inbound direction.

An ACL contains an ordered list of access control entries (ACEs). Each ACE specifies permit or deny and a set of conditions the packet must satisfy in order to match the ACE. The meaning of permit or deny depends on the context in which the ACL is used.

The switch supports two types of ACLs:

•IP ACLs filter IP traffic, including TCP, User Datagram Protocol (UDP), Internet Group Management Protocol (IGMP), and Internet Control Message Protocol (ICMP).

•Ethernet or MAC ACLs filter non-IP traffic

Why Do We Use Access Control Lists?

There are a variety of reasons we use ACLs. The primary reason is to provide a basic level of security for the network. ACLs are not as complex and in depth of protection as stateful firewalls, but they do provide protection on higher speed interfaces where line rate speed is important and firewalls may be restrictive. ACLs are also used to restrict updates for routing from network peers and can be instrumental in defining flow control for network traffic.

When do we use Access Control Lists?

As I mentioned before, ACLs for routers are not as complex or robust as stateful firewalls, but they do offer a significant amount of firewall capability. As an IT network or security professional, placement of your defenses is critical to protecting the network, its assets and data. ACLs should be placed on external routers to filter traffic against less desirable networks and known vulnerable protocols.
One of the most common methods in this case is to setup a DMZ, or de-militarized buffer zone in your network. This architecture is normally implemented with two separate network devices.
An example of this configuration is given in Figure 1.

The most exterior router provides access to all outside network connections. This router usually has less restrictive ACLs, but provides larger protection access blocks to areas of the global routing tables that you wish to restrict. This router should also protect against well known protocols that you absolutely do not plan to allow access into or out of your network. In addition, ACLs here should be configured to restrict network peer access and can be used in conjunction with the routing protocols to restrict updates and the extent of routes received from or sent to network peers.
The DMZ is where most IT professionals place systems which need access from the outside. The most common examples of these are web servers, DNS servers, and remote access or VPN systems.
The internal router of a DMZ contains more restrictive ACLs designed to protect the internal network from more defined threats. ACLs here are often configured with explicit permit and deny statements for specific addresses and protocol services.

What Does an Access Control List Consist Of?

Regardless of what routing platform you utilize, all have a similar profile for defining an access control list. More advanced lists have more distinct control, but the general guidelines are as follows:

• Access control list name (depending on the router it could be numeric or combination of letters and numbers)
• A sequence number or term name for each entry
• A statement of permission or denial for that entry
• A network protocol and associated function or ports
• Examples include IP, IPX, ICMP, TCP, UDP, NETBIOS and many others
• Destination and Source targets
• These are typically addresses and can be defined as a single discrete address, a range or subnet, or all addresses
• Additional flags or identifiers
• These additional statements request additional functions when a match is found for the statement. These flags vary for each protocol but a common flag added to statements is the log feature that records any match to the statement into the router log

What Types of Access Control Lists Are There?

There are several types of access control lists and most are defined for a distinct purpose or protocol. On Cisco routers, there are two main types: standard and extended. These two types are the most widely used ACLs and the ones I will focus on in this and future articles, but there are some advanced ACLs as well. Some of the advanced ACLs include reflexive ACLs and dynamic ACLs and they are defined as follows. Reflexive ACLs, also known as IP Session ACLs, are triggered from an outbound ACL for traffic initiated from the internal network. The router will identify this new traffic flow and create an entry in a separate ACL for the inbound path. Once the session ends, the entry in the reflexive ACL is removed.
Dynamic ACLs or lock-and-key ACLs are created to allow user access to a specific source/destination host through a user authentication process. Cisco implementations utilize IOS Firewall capabilities and do not hinder existing security restrictions.

Implementation of ACLs on a Router Interface

Placement and understanding of the traffic flow is important to understand up front before you configure an ACL on a router interface. Understanding of the placement and impact of ACLs are frequent questions in CCNA and CCNP exams and mistakes in ACL placement are some of the most common ones network administrators make during security implementation. Trust me, it happens to us all and I am not immune to that one. Figure 2 provides a good example of the traffic flow when it comes to ingress and egress on a router network interface.

As you can see from this diagram, ingress traffic flows from the network into the interface and egress flows from the interface to the network. IT network and security professionals must pay close attention here. ACLs start with a source address first in their configuration and destination second. As you configure an ACL on the ingress of a network interface it is important to recognize that all local network or hosts should be seen as sources here, and the exact opposite for the egress interface.
What makes this most confusing is the implementation of ACLs on the interface of a router that faces an external network. Look back at Figure 1. In that example, the ingress side is coming from the outside network and those addresses are considered to be sources, while all internal network addresses are destinations. On the egress side, your internal network addresses are now source addresses and the external addresses are now destinations.
As you add ports in extended ACLs, confusion can mount. The best advice I have before any implementation is to document your flows and note your source/destination addresses. We will cover more of these implementations later in ACL configuration articles.

Summary

Access control lists are a principle element in securing your networks and understanding their function and proper placement is essential to achieving their best effectiveness.

With courtesy:
http://blog.pluralsight.com/access-control-list-concepts