Each sub-interface is configured with IP subnet, VLAN-id and type of encapsulation (802.1q standard or proprietary Cisco ISL). Physical interface of router connected to switch is divided to multiple logical sub-interfaces. On opposite side of trunk there is a Router-on-a-Stick which needs to understand tagged 802.1q frames coming from trunk port of switch. VLAN-id field inside 802.1q tag is determined according to VLAN configured on switchport where a frame originally enters switch. CISCO ROUTER ON A STICK MACThe VLAN-ID information is placed in to 802.1 tag and a tag is added to original Ethernet frame right after source MAC address. This switch port is called a trunk port and frames sent through the trunk port must carry information about VLAN they are coming from. Instead of transferring frames from particular VLAN on dedicated switch port it exists a method which allows to send frames from multiple VLANs through one port of switch. This device is known as router and when it is connected with its only one interface to L2 switch this concept is called Router-on-a-Stick configuration. Layer 2 switch can't do routing between subnets and frames destined outside of VLAN must be sent to device which is capable of routing between VLAN. Each VLAN should have assigned its own IP address space - subnet. If they are hosts belonging to different VLANs it is not connectivity between hosts so it must be a mechanism which provides connectivity - routing between hosts on different VLANs. Hosts are grouped in to VLANs according to switch port configuration. Layer 2 switch is a network device which provides network access for hosts connected to its ports.
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