Cisco Specific Protocols
Over the years, Cisco Systems has developed a number of proprietary protocols that have been used to perform different functions on an L2 network. Most of these protocols use an IEEE 802.3 frame format with an 802.2 SNAP encapsulation. Most have a Logical Link Control (LLC) of 0xAAAA03 (indicating SNA and the Cisco Organizational Unit Identifier (OUI) 0x00000c. The majority use a multicast destination M address to communicate. This is generally a variation on 0100.0ccc.cccc. The SNAP protocol type varies and generally is included in each protocol discussion where appropriate. Knowing the specifics of these protocols should make it easier to identify them on the network when troubleshooting using a sniffer. Figure 6-3 shows in detail the frame format of most Cisco L2 protocols.
Figure 6-3. 802.3 with 802.2 SNAP Frame Format
|
DST SRC MAC MAC Lerglîl |
S Type DATA CRC |
|||||
|
i i i i |
r i 2 ! 33 -1492 Bytes ! 4' Bytes |
|||||
|
DSAP |
* DST MAC: Generally a râriartlol ÛiÛO.Occc.ccct * SRC MAC: Placed irom a pool In Ihe &witcn EPFKîm '$02,2 LLC Fields DSAP-AA + SSAP:AA * CNTRL:03 - SNAP -602.2 SNAP Fields Org Cade:ta00000c [Cisco) Protocol Type: Varies SSAf> * DST MAC: Generally a râriartlol ÛiÛO.Occc.ccct * SRC MAC: Placed irom a pool In Ihe &witcn EPFKîm '$02,2 LLC Fields DSAP-AA + SSAP:AA * CNTRL:03 - SNAP -602.2 SNAP Fields Org Cade:ta00000c [Cisco) Protocol Type: Varies Of special note is the unique relationship two of these protocols have with VLAN 1 on Cisco switches. Cis Discovery Protocol (CDP) and VLAN trunking protocol (VTP) are discussed in more detail later. Both of these protocols communicate over VLAN 1 only. Even if VLAN 1 is not used on a trunk port, these protocols continue to pass information on VLAN 1. For this reason, and the fact that VLAN 1 cannot be deleted, it is not recommended to use VLAN 1 for user or trunk ports. More information on this topic cai be found here: http://www.cisco.com/warp/public/473/103.html. |
|||||
Continue reading here: VLAN Trunking Protocol VTP
Was this article helpful?