rames to be sent over a trunk, both sides must agree to a protocol. Currently, the protocols for trunking supported on Cisco switches are Cisco's Inter- Switch Link (ISL) and the IEEE standard 802.1Q. Not all Cisco switches support both protocols. For example, the Cisco 2950 and 4000 switches only support 802.1Q. To determine whether a switch can use a specific trunking protocol, use the IOS or NX-OS command show interface capabilities, or the CatOS command show capabilities: NX-7K-1-Cozy (config)# sho interface capabilities inc Trunk Ethernet3/1 Trunk encap. type: 802.10 As you can see from the preceding output, the Nexus supports only the open standard, 802.1Q. Here, you can see that a 3750 supports both 802.1Q and ISL: 3750-IOS#sho int g1/0/45 capabilities I inc Trunk Trunk encap. type: 802.10,ISL Trunk mode: off, desirable, nonegotiate ISL differs from 802.1 in a couple of ways. First, ISL is a Cisco-proprietary protocol, whereas 802.10 is an IEEE standard. Second, ISL encapsulates Ethernet frames within an ISL frame, while 802.1Q alters existing frames to include VLAN tags. Furthermore, Chapter 5: Trunking ISL is capable of supporting only 1,000 VLANs, while 802.1Q is capable of supporting 4,096. On switches that support both ISL and 802.1Q, either may be used. The protocol is specific to each trunk. While both sides of the trunk must agree on a protocol, you may configure ISL and 802.1Q trunks on the same switch and in the same network. To add VLAN information to a frame to be sent over a trunk, ISL encapsulates the entire frame within a new frame. An additional header is prepended to the frame and a small suffix is added to the end. The header contains information regarding the VLAN number and some other information, while the footer includes a checksum of the frame. A high-level overview of an ISL frame is shown in Figure 5-3. Header and footer added by ISL ISL header Ethernet header TCP packet Telnet header Figure 5-3. ISL encapsulated frame The frame check sequence (FCS) footer is in addition to the FCS