802.1q (also known as dot1q) is an IEEE industry standard trunking protocol. Cisco switches supports both ISL (Inter Switch Link - Cisco propietory) and 802.1q.The IEEE 802.1Q standard defines the operation of VLAN Bridges that permit the definition, operation and administration of Virtual LAN topologies within a Bridged LAN infrastructure. The 802.1Q standard is intended to address the problem of how to break large networks into smaller parts so broadcast and multicast traffic would not grab more bandwidth than necessary. The standard also helps provide a higher level of security between segments of internal networks.
The key for the IEEE 802.1Q to perform the above functions is in its tags. 802.1Q-compliant switch ports can be configured to transmit tagged or untagged frames. A tag field containing VLAN (and/or 802.1p priority) information can be inserted into an Ethernet frame. If a port has an 802.1Q-compliant device attached (such as another switch), these tagged frames can carry VLAN membership information between switches, thus letting a VLAN span multiple switches. However, it is important to ensure ports with non-802.1Q-compliant devices attached are configured to transmit untagged frames. Many NICs for PCs and printers are not 802.1Q-compliant. If they receive a tagged frame, they will not understand the VLAN tag and will drop the frame. Also, the maximum legal Ethernet frame size for tagged frames was increased in 802.1Q (and its companion, 802.3ac) from 1,518 to 1,522 bytes. This could cause network interface cards and older switches to drop tagged frames as "oversized."
802.1q adds a 4-Byte header to the frame indicating the VLAN (Virtual LAN) membership as compared to ISL which encapsulates (adds header and trailer) to the frame.
How 802.1q Works
You first designate a port as a trunk port and then decide upon trunking protocol(ISL or dot1q), in mentioned below example interface used for trunking is Fast Ethernet port 0/1.
Hi,i have Cisco ISR C1101-4P router.I have configured it to take DHCP address from ISP on WAN port. Configured VLAN1 IP Address that i receive connecting my PC to LAN Ports.Router can ping IP like 184.108.40.206 or any other and can ping FQDN. But my comput...
Hi, I have just upgraded a C9200L-24T-4X from cat9k_lite_iosxe.16.12.04.SPA.bin to cat9k_lite_iosxe.16.12.05b.SPA.binUsing the steps below. After the upgrade the switch was online and operating the new firmware as expected. Then a...
While I am searching for the Next hop filed behavior of OSPF routes I have found following quote. broadcast” and “non-broadcast” network types will use the next hop IP address of the router advertising the network and the...