is-islayer2layer3osirouting
IS-IS runs natively on layer 2 of the OSI reference model. How will the routing be done in that?
My doubt is that layer 3 has the ability to do the routing. But, ISIS does not run in layer 3 rather it natively runs in layer-2. In that, how does the IS-IS perform routing in layer 2 of the OSI model?
There are two important facts about the OSI model to remember:
It is a conceptual model. That means it describes an idealized, abstract, theoretical group of networking functions. It does not describe anything that someone actually built (at least nothing that is in use today).
It is not the only model. There are other models, most notably the TCP/IP protocol suite (RFC-1122 and RFC-1123), which is much closer to what is currently in use.
A bit of history: You’ve probably all heard about the early days of packet networking, including ARPANET, the Internet’s predecessor. In addition to the U.S. Defense Department’s efforts to create networking protocols, several other groups and companies were involved as well. Each group was developing their own protocols in the brand new field of packet switching. IBM and the telephone companies were developing their own standards. In France, researchers were working on their own networking project called Cyclades.
Work on the OSI model began in the late 1970s, mostly as a reaction to the growing influence of big companies like IBM, NCR, Burroughs, Honeywell (and others) and their proprietary protocols and hardware. The idea behind it was to create an open standard that would provide interoperability between different manufacturers. But because the ISO model was international in scope, it had many competing political, cultural and technical interests. It took well over six years to come to consensus and publish the standards.
In the meanwhile, the TCP/IP model was also developed. It was simple, easy to implement, and most importantly, it was free. You had to purchase the OSI standard specifications to create software for it. All the attention and development efforts gravitated to TCP/IP. As a result, the OSI model was never commercially successful as a set of protocols, and TCP/IP became the standard for the Internet.
The point is, all of the protocols in use today, the TCP/IP suite; routing protocols like RIP, OSPF and BGP; and host OS protocols like Windows SMB and Unix RPC, were developed without the OSI model in mind. They sometimes bear some resemblance to it, but the OSI standards were never followed during their development. So it’s a fools errand to try to fit these protocols into OSI. They just don’t exactly fit.
That doesn’t mean the model has no value; it is still a good idea to study it so you can understand the general concepts. The concept of the OSI layers is so woven into network terminology, that we talk about layer 1, 2 and 3 in everyday networking speech. The definition of layers 1, 2 and 3 are, if you squint a bit, fairly well agreed upon. For that reason alone, it’s worth knowing.
The most important things to understand about the OSI (or any other) model are:
Dividing the protocols into layers allows us to talk about their different aspects separately. It makes the protocols easier to understand and easier to troubleshoot. We can isolate specific functions easily, and group them with similar functions of other protocols.
Each “function” (broadly speaking) encapsulates the layer(s) above it. The network layer encapsulates the layers above it. The data link layer encapsulates the network layer, and so on.
Layers abstract the layers below it. Your web browser doesn’t need to know whether you’re using TCP/IP or something else at at the network layer (as if there were something else). To your browser, the lower layers just provide a stream of data. How that stream manages to show up is hidden from the browser. TCP/IP doesn’t know (or care) if you’re using Ethernet, a cable modem, a T1 line, or satellite. It just processes packets. Imagine how hard it would be to design an application that would have to deal with all of that. The layers abstract lower layers so software design and operation becomes much simpler.
Decoupling: In theory, you can substitute one specific technology for another at the same layer. As long as the layer communicates with the one above and the one below in the same way, it shouldn’t matter how it’s implemented. For example, we can remove the very well-known layer 3 protocol, IP version 4, and replace it with IP version 6. Everything else should work exactly the same. To your browser or your cable modem, it should make no difference.
The TCP/IP model is what TCP/IP protocol suite was based on (surprise!). It only has four layers, and everything above transport is just “application.” It is simpler to understand, and prevents endless questions like “Is this session layer or presentation layer?” But it too is just a model, and some things don’t fit well into it either, like tunneling protocols (GRE, MPLS, IPSec to name a few).
Ultimately, the models are a way of representing invisible abstract ideas like addresses and packets and bits. As long as you keep that in mind, the OSI or TCP/IP model can be useful in understanding networking.
Try to issue command #ip router isis
edit: Cisco Packet Tracer doesn't support ISIS.
Cisco Packet Tracer Supported Protocols:
Application • FTP , SMTP, POP3, HTTP, TFTP, Telnet, SSH, DNS, DHCP, NTP, SNMP, AAA, ISR VOIP, SCCP config and calls ISR command support, Call Manager Express
Transport • TCP and UDP, TCP Nagle Algorithm & IP Fragmentation, RTP
Network • BGP, IPv4, ICMP, ARP, IPv6, ICMPv6, IPSec, RIPv1/ v2/ng, Multi-Area OSPF, EIGRP, Static Routing, Route Redistribution, Multilayer Switching, L3 QoS, NAT, CBAL , Zone-based policy firewall and Intrusion Protection System on the ISR, GRE VPN, IPSec VPN
Network Access/ Interface • Ethernet (802.3), 802.11, HDLC, Frame Relay, PPP, PPPoE, STP, RSTP, VTP, DTP, CDP, 802.1q, PAgP, L2 QoS, SLARP, Simple WEP, WPA, EAP
Best Answer
ISIS works quite differently than most of the protocols you know. In the original OSI networking, ISIS would make adjacencies with it's neighbors, and tell them about the adresses of any ES it knows. IS means intermediate system, so besically a router. ES means end system, so any host. ISIS has different logic with different levels of routing. The levels used in ISIS are*:
Basically, the logic is as follows: For level 1: flood all the connected and redistributed routes you know to all the level 1 neighbors in this area. Set default route to any routers with level 2 capability you know. For level 2: Import all routes from level 1 routers you know, flood to all level 2 neighbors in any area. (Please note that you need routers that are both level 1 and 2 to make the translations)
Every router floods any packets they receive to their neighbors according to those two rules. The information in these packets are in the format Type,Length,Value (TLV). Using TLV's means the following:
ISIS, just like OSPF, runs a Dijkstra Algorithm to find the shortest path. All information for this algorithm is contained in the TLV's. L2 Communication with neighbors is done in two ways: -When connected to a shared segment, use the Layer 2 protocol of that segment to communicate. This is done to be able to traverse switches that might not be able to speak the OSI layer 2 protocol CLNS. -When connected to a point to point network, communication on Layer 2 is CLNS.
This doesn't mean that there is no L3 communication. ISIS routers still need a layer 3 address. Since we are talking OSI, and not IP, the address is slightly different. In this case, the address is called a Network Entity Title (NET), which is one address for the entire system (so not one per interface). The NET has many forms, but for most ISIS adjacencies, works as follows:
49.<4 char area>.<12 char system identifier>.00. It doesn't really matter what you use as identifier, but say we have router 192.0.2.5 in area 52, we could make the NET 49.0052.1920.0000.2005.00 Since the addresses are in HEX you prefer 49.0052.0000.B00B.0000.00 (The addresses are hexadecimal).*) There are other levels in OSI routing, but those aren't significant for the current operation of ISIS
A short illustration of routing