![]() I ran across some obscure document back in 1999 or 2000 that mentioned in passing the UDP wrapping and thought that curious but but tracert took the ping utility and added the UDP portion to collect socket info. RE: CLARIFICATION - PING LOOPBACK OSI LAYERS burtsbees (Programmer) 12 Feb 09 21:44ĭon't know any vendor that does wrap the ping in UDP. Firewalling is considered L1-7 for CCNA.īut you gotta start somewhere. And a router can now do firewalling with the right code. However, can't a router do access lists? Those go up to L4. Remember, for CCNA, Cisco puts routers at 元. OSI is a very old model and it really isn't a very good reference for much of what we do. I've had many discussions over the years with Cisco, students, instructors and vendors about where ARP fits. ARP is carried over a L2 frame, but it uses an ethertype of 0806xh rather than the usual 0800xh for IP packets. ARP is a binding protocol that glues l2 and l3 addresses for communication within an ethernet LAN. ![]() The IEEE Ethernet standards do not included ARP as part of the standards. The RFCs for arp don't indicate what layer of OSI it lives at. This actually makes it more difficult for someone with lots of experience and knowledge in the "real world" to pass CCNA exams because you may know too much!!!! Realistically, as you do more research, you find many of the things that are taught as black or white are actually shades of gray. Because the Application layer functionality for the TCP/IP stack is included in the loopback check and because the top three OSI layers map roughly to the Application layer of TCP/IP, for CCNA level work, Cisco courseware and classes teach ping loopback as testing all OSI layers.ĬCNA tries to make complicated concepts black or white. ![]() Vendors also link the test to the NIC so L2 and L1 can be checked with loopback circuitry. Because it is used to check the TCP/IP protocol stack install, vendors implement that check with a ping loopback command or a ping 127.0.0.1. The loopback test was considered so important that the designers of the IP address space "accidently" blew away an entire Class A block just to implement the test. For instance, some vendors put icmp packets (layer 3) in a UDP wrapper when implementing ping. Vendors implement ping with slight differences. Great discussion however, Burt, ICMP is the packet type (echo and echo-reply) that the UTILITY ping uses. Http ://m/free/t_I PReservedP rivateandL oopbackAdd resses.htmĬCNA R&S, ETA FOI RE: CLARIFICATION - PING LOOPBACK OSI LAYERS burtsbees (Programmer) 12 Feb 09 09:30 127.0.0.1 is the address most commonly used for testing purposes. Since the lower layers are short-circuited, sending to a loopback address allows the higher layers (IP and above) to be effectively tested without the chance of problems at the lower layers manifesting themselves. The purpose of the loopback range is testing of the TCP/IP protocol implementation on a host. In essence, this represents a "short-circuiting" of the normal protocol stack data is sent by a device's layer three IP implementation and then immediately received by it. Instead, they "loop back" to the source device at the IP level. IP datagrams sent by a host to a 127.x.x.x loopback address are not passed down to the data link layer for transmission. However, one special range of addresses is set aside for loopback functionality. That datagram then passes down to the data link layer of the device's physical network for transmission to the next hop, on the way to the IP destination. Normally, when a TCP/IP application wants to send information, that information travels down the protocol layers to IP where it is encapsulated in an IP datagram. This would suggest that either layer 3 alone (OSI) is tested or 3-7! LoL Let me further confuse and complicate matters. LoL So simple a topic such as an ICMP echo request yet so much controversy! LoL Please let me know what you guys think and thanks for your view Tad!ĬCNA R&S, ETA FOI RE: CLARIFICATION - PING LOOPBACK OSI LAYERS maczen (Instructor) ![]() ![]() This is becoming a very interesting topic and I am quite curious to see where it goes. I still feel that a regular ping would test 1-3 and a loopback 1-4 (due to the TCP stack test) but I have been wrong before. (6)) is occurring nor anything at the application layer! Furthermore, no presentation (ASCII/EBCDIC/encryption etc. Regardless, a session would not be established (5) although the TCP software was tested. oad.wikime dia.org/wi kipedia/co mmons/thum b/3/3b/UDP _encapsula tion.svg/3 50px-UDP_e ncapsulati on.svg.png Tad, that is the logic that I was following with the exception that I considered the TCP/IP stack more of a layer 4 hence my (personal view) of a loopback ping testing 1-4 OSI. Sorry, said can not ping but meant can not capture ping traffic with or without the loopback adapter installed. ![]()
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