CCNA -A Short Notes - 6 - VLANs (Virtual LANs)

Part VI – VLANs (Virtual LANs)

VLANs: logical grouping of network users and resources connected to administratively defined ports on a switch. The segmentation into VLAN creates smaller collision and broadcast domains and enhances security. Layer 3 switches or routers are needed to route packets between VLANs. 

Switch Fabric: group of interconnected switches. 

Dynamic vs static VLANs: Dynamic VLAN determine a host’s VLAN assignment automatically from a MAC address table, protocols, or applications. VMPS (VLAN Management Policy Server) can be used to set up a database of MAC address-to-VLAN mappings. A static VLAN is one in which the administrator manually configured the port VLAN membership.

Access vs Trunk Links: Links that are part of one VLAN are access links. Devices attached to an access link are unaware of their VLAN membership. Trunk links can carry up to 1005 VLANs. A scheme is needed to identify what VLAN a frame belongs to (called frame tagging). ISL and IEEE 802.1q are two standards of frame tagging supported by Cisco switches. 

Trunk Protocol: used with ISL or 802.1q to allow VLAN trunking. 

ISL (Inter-Switch Link): proprietary to Cisco switches, and is used for FastEthernet or Gigabit Ethernet links only, on a switch port, router interface or a compatible server NIC. The server will then be able to be on multiple VLANs. The original frame is encapsulated with a 26-byte header and a 4-byte Frame Check Sequence (FCS) footer rather than modified. The ISL frames are up to 1522 bytes, which is over the Ethernet maximum of 1518. 

802.1q: IEEE standard for frame tagging, required when using non-Cisco equipment. Inserts a field into the frame to identify the VLAN. 

LANE (LAN Emulation): Used to communicate multiple VLANs over ATM. 

802.10 (FDDI): used to send VLAN information over FDDI. Uses a SAID field in the frame header to identify the VLAN. 

VTP (VLAN Trunk Protocol): Protocol created by Cisco to manage all the configured VLANs across a switched internetwork and to maintain consistency throughout the network. VTP allows an administrator to add, delete and rename VLANs which is then propagated to all the switches in the switch fabric. A VTP server must be created (default on switches). The other switches client or transparent (forward VTP information but do not accept updates) and must be on the same domain name to share information. Only the client does not store its configuration in NVRAM. The clients will update their information when a packet with a higher revision number is received. Updates are sent every 5 minutes or when a change occurs. Clients switches cannot make any changes, and transparent switches can make changes but the changes will remain local and not be broadcasted. 

VTP Pruning: in order to reduce bandwidth, the VTP information will only be sent through trunk links which require the information. It is disabled by default on all switches. Once pruning is enabled on a VTP server, it is enabled for the whole domain. VLAN 1 is the administrative VLAN and is not eligible for pruning.

Router Switching Modes:

Mode	Description	Mode	Description
Process Switching	Frame copied on the router’s process buffer. The router then performs a router performs a lookup on the Layer 3 address with the routing table, forwards the packet to the exit interface. The processor is very busy with routing.	Optimum Switching	Faster than fast switching because all processing is carried out on the interface processor including CRC.
Fast Switching	The first packet of a session is compared against the fast-switching cache then if no entry is found, packets are examined by the routing processor. Each interface processor calculates the CRC. Other packets from the same session will follow the same path.	Distributed Switching	Happens on Versatile Interface Processor (VIP) cards, which have a switching processor onboard, so very efficient. All required processing is done right on the VIP processor, which maintains a copy of the router’s routing cache.
Autonomous Switching	Packets are compared to the autonomous switching cache on the interface processor, without interrupting the route processor.	Netflow Switching	Collects detailed data for use in conjunction with circuit accounting and application utilization information, but increases the overhead.
Silicon Switching	Only on 7000 Series routers equipped with a Silicon Switching Processor (SSP) Packets are compared to the silicon-switching cache on the silicon switching engine (SSE). Packets must still traverse the backplane of the router to get to the SSP and then back to the exit interface.	Cisco Express Forwarding (CEF)	Switching function designed for high-end backbone routers. It functions on Layer 3 and its biggest asset is the ability to remain stable in a large network. More efficient than both fast and optimum default switching paths. Doesn’t rely on cached information, bur refers to two alternate resources: the Forward Information Base which is duplicated from the routing table, and the adjacency table, a Layer 2 MAC address table of connected routers.