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Virtual LANs And Trunking

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Virtual LANs And Trunking

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents

Virtual LANs and Trunking

A LAN includes devices that are on the same broadcast domain. With VLANS, the switch creates multiple broadcast domains on a per-interface basis.

VLAN: Broadcast domain created by one or more switches. Based on interface configuration, different VLANs exist.

Motivations for using VLANs include:

1. Group users by department, or by groups that work together, instead of physical location

2. Reduce overhead by reducing broadcasts

3. Enforce better security by placing sensitive devices on separate VLANs

4. Separate specialized traffic from mainstream traffic

Creating VLANs: Interface 0/2 is on VLAN 1; Interface 0/4 is on VLAN 4, etc.

- > Rarely used alternative is to group VLANS according to MAC address, so if port switched, VLAN remains. This requires too much administration typically.


Trunking with 802.1q and ISL

Trunking is used when you have VLANs on more than one (interconnected) switch. When sending from one switch to another, need a way to identify which VLAN frame came from.

Inter-Switch Link (ISL): Proprietary to Cisco, fully encapsulates original frame in ISL header/trailer. Header is 26 bytes, includes VLAN number and MAC addresses of sending/receiving switches.

802.1q: IEEE standard, doesn't encapsulate, but rather adds extra 4-byte header after MAC destination address.

- > Since contents changed, a new FCS is needed (recalculated)

**BOTH use 12-bit long VLAN ID, BOTH support separate instance of STP**

- > Different interfaces block for different VLANs


Passing Traffic between VLANs

Devices in same VLAN = same subnet

Layer 2 Switching: Switch receive a frame, processes it, and forwards it. Has MAC Address table per VLAN.

Layer 3 Forwarding (using Router): In order to get different VLANS to communicate with each other, a router is needed. Need separate interface for each VLAN.

- > Can use router with Fast Ethernet port that supports Trunking, and use single physical connection at 100 Mbps

Layer 3 Forwarding (using L3 Switch): Uses a switch that also has routing features, uses router internal to the switch. Only difference is internal processing (IP routing protocols, builds IP routing tables).

Layer 4 Switching: Analyzes port numbers, forwards/analyzes (accounting) data sent. To perform this, switch must keep track of every Layer 4 flow (1000 TCP connections = 1000 L4 entries in table).

- > Accounting feature called NetFlow switching

Layer 5-7 Switching: Looks at application layer headers; Cisco calls Content Delivery Networks (CDN).


Multilayer Switching: Switches often perform functions at different layers (Layer 3 switches are most always multilayer because they perform layer 2 functions as well).

Type Description
Layer 2 Switching Forward frames based on MAC address
External router connected to L2 Switch Router forwards like always, based on destination IP
L3 Switch Forwards based on destinations IP for traffic destined for another VLAN
L4 Switch Forward on port numbers, also do accounting
Layer 5-7 Switch Forwards based on application layer header, known as CDN
Multilayer Switching Switching based on multiple layers, L3/L2 functions



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