ISL and 802.1Q frames

The ISL frame consist of three primary fields: the encapsulation frame (original frame), which is encapsulated by the ISL header, and the FCS at the end:

ISL Header (26 bytes) | Encapsulation Frame (Original Data) | FCS (4 bytes)

In ISL, the original frame is encapsulated and an additional header is added before the frame is carried over a trunk link. Also, a FCS is generated based on some fields in the ISL Header and the Encapsulation Frame and added to the end of the frame. At the receiving end, the header and FCS are removed and the frame is forwarded to the assigned VLAN. The FCS field consists of 4 bytes and contains a 32-bit CRC value.

802.1Q is the IEEE standard for tagging frames on a trunk and supports up to 4096 VLANs. In 802.1Q, the trunking device inserts a 4-byte tag into the original frame and recomputes the frame check sequence (FCS) before the device sends the frame over the trunk link. At the receiving end, the tag is removed and the frame is forwarded to the assigned VLAN. 802.1Q does not tag frames on the native VLAN. IEEE 802.1Q uses an internal tagging mechanism which inserts a 4-byte tag field in the original Ethernet frame itself.

802.1Q modifies the FCS field inside the original Ethernet frame while ISL leaves the original FCS field inside the Ethernet frame unchanged, it just adds another FCS field outside the original Ethernet frame.

(Reference: http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml)

IP Service Level Agreements (SLAs)

Cisco IOS IP Service Level Agreements (SLAs) allow users to monitor network performance between Cisco routers or from either a Cisco router to a remote IP device. Cisco IOS IP SLA has been the most popular way to measure performance statistics (i.e: latency, jitter, packet loss and MOS). Cisco IOS IP SLAs Responder is a component embedded in the destination Cisco router whose functionality is to respond to Cisco IOS IP SLAs request packets. The responder adds timestamps to the echoed packets to allow unidirectional packet loss, latency, and jitter measurements to a Cisco device. The accuracy of the measurements is improved significantly if the responder is used.

Cisco IOS IP SLAs Benefits
1. Measure end-to-end IP layer network
2. Deploy new applications and services with complete confidence
3. Verify and monitor quality of service (QoS) and differentiated services.
4. Increase end user confidence and satisfaction
5. Implement SLA measurement metrics
6. Notify users about network issues proactively
7. Measure network performance continuously, reliably, and predictably

Reference
http://www.cisco.com/en/US/technologies/tk648/tk362/tk920/technologies_white_paper09186a00802d5efe.html