![]() 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. Otherwise, R1 will have no way to know how to reach R2, and vice-versa.1. The crucial point is that the IP addresses on the link between R1 and R2 have to be in the same subnet (or you need to configure static routes or other less elegant tricks). Success rate is 100 percent (5/5), round-trip min/avg/max = 20/32/48 ms Sending 5, 100-byte ICMP Echos to 72.40.40.1, timeout is 2 seconds: Neighbor ID Pri State Dead Time Address InterfaceĢ.2.2.2 1 FULL/DR 00:00:38 72.40.43.1 GigabitEthernet0/0ġ.1.1.1 1 FULL/BDR 00:00:37 72.40.42.1 GigabitEthernet0/0Īnd you are able to check the reachability of the OSPF-learned network by pinging : R2#ping 72.40.40.1 Your OSPF comes up now : R1#sh ip ospf nei (Please note that 72.40.42.1/23 and 72.40.43.1/23 are in the same subnet) << as pointed out above, this is the crucial point Your OSPF adjacency is not coming up because the IP addresses on the facing interfaces between R1 and R2 are not in the same subnet.Īs soon as you use two IP addresses that are in the same subnet (as in the drawing above), you will have IP connectivity between R1 and R2, and your OSPF adjacency can be established : (I imagine your setup to be something like this)
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