Medianet Reference Guide - Cisco

Network Availability Technologies
Chapter 3
Medianet Availability Design Considerations
for a total of 30 bytes of encapsulation. ISL trunking can be configured on a switch port interface,
as shown in Example 3-1. The trunking mode is set to ISL, and the VLANs permitted to traverse the
trunk are explicitly identified; in this example, VLANs 2 and 102 are permitted over the ISL trunk.
Example 3-1
ISL Trunk Example
Switch(config)#interface GigabitEthernet8/3
Switch(config-if)# switchport
Switch(config-if)# switchport trunk encapsulation isl
Switch(config-if)# switchport trunk allowed 2, 102
In contrast with ISL, 801.1Q does not actually encapsulate the Ethernet frame, but rather inserts a 4-byte
tag after the source address field, as well as recomputes a new FCS, as shown in Figure 3-13. This tag
not only preserves VLAN information, but also includes a 3-bit field for class of service (CoS) priority
(which is discussed in more detail in Chapter 4, “Medianet QoS Design Considerations”).
Figure 3-13
IEEE 802.1Q Tagging
Original Ethernet Frame
DA SA TYPE/LEN DATA FCS
Original Frame
DA SA TAG TYPE/LEN DATA FCS
Tagged Frame
Inserted 4-Byte IEEE 802.1Q Tage
Recomputed FCS
228674
IEEE 802.1Q also supports the concept of a native VLAN. Traffic sourced from the native VLAN is not
tagged, but is rather simply forwarded over the trunk. As such, only a single native VLAN can be
configured for an 802.1Q trunk, to preserve logical separation.
Note
Because traffic from the native VLAN is untagged, it is important to ensure that the same native VLAN
be specified on both ends of the trunk. Otherwise, this can cause a routing blackhole and potential
security vulnerability.
IEEE 802.1Q trunking is likewise configured on a switch port interface, as shown in Example 3-2. The
trunking mode is set to 802.1Q, and the VLANs permitted to traverse the trunk are explicitly identified;
in this example, VLANs 3 and 103 are permitted over the 802.1Q trunk. Additionally, VLAN 103 is
specified as the native VLAN.
Example 3-2
IEEE 802.1Q Trunk Example
Switch(config)# interface GigabitEthernet8/4
Switch(config-if)# switchport
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport trunk allowed 3, 103
Switch(config-if)# switchport trunk native vlan 103
Trunks are typically, but not always, configured in conjunction with EtherChannels, which allow for
network link redundancy, and are described next.
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Medianet Reference Guide
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