Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 246 Inter-Switch Link (ISL) ISL is Cisco’s proprietary frame-tagging protocol, and supports Ethernet, Token Ring, FDDI, and ATM frames. ISL encapsulates a frame with an additional header (26 bytes) and trailer (4 bytes), increasing the size of an Ethernet frame up to 30 bytes. The header contains the 10 byte VLAN ID. The trailer contains an additional 4-byte CRC for data-integrity purposes. Because ISL increases the size of a frame, non-ISL devices (i.e. non-Cisco devices) will actually drop ISL-tagged frames. Many devices are configured with a maximum acceptable size for Ethernet frames (usually 1514 or 1518 bytes). ISL frames can be as large as 1544 bytes; thus, non-ISL devices will see these packets as giants (or corrupted packets). ISL has deprecated in use over time. Newer Catalyst models may not support ISL tagging. IEEE 802.1Q IEEE 802.1Q, otherwise known as DOT1Q, is the standardized frametagging protocol supported by most switch manufacturers, including Cisco. Thus, switches from multiple vendors can be trunked together. Instead of adding an additional header and trailer, 802.1Q actually embeds a 4-byte VLAN ID into the Layer-2 frame header. This still increases the size of a frame from its usual 1514 bytes to 1518 bytes (or from 1518 bytes to 1522 bytes). However, most modern switches support 802.1Q tagging and the slight increase in frame size. Neither ISL nor 802.1Q tagging alter the source or destination address in the Layer-2 header. Manual vs. Dynamic Trunking ISL or 802.1Q tagging can be manually configured on Catalyst trunk ports. Catalyst switches can also dynamically negotiate this using Cisco’s proprietary Dynamic Trunking Protocol (DTP). * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 247 Configuring Trunk Links To manually configure a trunk port, for either ISL or 802.1Q tagging: Switch(config)# interface fa0/24 Switch(config-if)# switchport trunk encapsulation isl Switch(config-if)# switchport mode trunk Switch(config)# interface fa0/24 Switch(config-if)# switchport trunk encapsulation dot1q Switch(config-if)# switchport mode trunk The first line in each set of commands enters interface configuration mode. The second line manually sets the tagging (or encapsulation) protocol the trunk link will use. Always remember, both sides of the trunk line must be configured with the same tagging protocol. The third line manually sets the switchport mode to a trunk port. The Catalyst switch can negotiate the tagging protocol: Switch(config)# interface fa0/24 Switch(config-if)# switchport trunk encapsulation negotiate Whichever tagging protocol is supported on both switches will be used. If the switches support both ISL and 802.1Q, ISL will be selected. By default, trunk ports allow all VLANs to traverse the trunk link. However, a list of allowed VLANs can be configured on each trunk port: Switch(config)# interface fa0/24 Switch(config-if)# switchport trunk allowed vlan remove 50-100 Switch(config-if)# switchport trunk allowed vlan add 60-65 The first switchport command will prevent the trunk port from passing traffic from VLANs 50-100. The second switchport command will re-allow the trunk port to pass traffic from VLANs 60-65. In both cases, the switchport trunk allowed commands are adding/subtracting from the current list of allowed VLANs, and not replacing that list. Switch(config)# interface fa0/24 Switch(config-if)# switchport trunk allowed vlan all Switch(config-if)# switchport trunk allowed vlan except 2-99 Certain VLANs are reserved and cannot be removed from a trunk link, including VLAN 1 and system VLANs 1002-1005. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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Cisco CCNA Study Guide - Router Alley

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