Routing - 8 : Layer 3 Protocols
We talked about routers and routing, and by now we should have an understanding about the processes involved. Now let's talk about some of the protocols that allow routing to occur.
When we discussed the bridging functions occurring at layer 2 of the OSI model, we also looked at the protocols involved. Layer 2 protocols include Ethernet, token ring, FDDI, and Spanning Tree. Well, it makes sense to assume that routers also have protocols that support communication between themselves and other routers, and these are the layer 3 protocols.
There are two main types of protocols that operate at layer 3: routable protocols and routing protocols. They sound almost identical, but this is not as confusing as it sounds.
A routable protocol is a protocol that can be forwarded by a router, and a routing protocol is used by routers to communicate with other routers.
Let's look at routable protocols first.
Take a minute and think about how a router works. A router receives and forwards a packet based on the network address in the packet. However, what if the packet did not contain a network address? Would the router forward it? Nope. That packet is un-routable, and the router discards it.
This brings us back to routable protocols. A routable protocol must contain layer 3 information. Basically, what this means is there must be a network address in the packet.
The most common layer 3 protocol by far is IP, the Internet Protocol.
Additional routable protocols include XNS, used by Xerox. Care to guess what XNS stands for? Xerox Network Systems - good guess. Another protocol is IPX from Novell, and it stands for Internet Packet Exchange. Another is AppleTalk - you can probably guess what kind of computers use this. And there are others: BanyanVines, DecNet and many more.
IP is the most common routable protocol and must be used for communication across the Internet. The others were developed by companies to support their own equipment or to try to compete with IP for global presence. Today, IP is the primary layer 3 protocol, and all others are gradually becoming less prevalent. Essentially, Ethernet dominates layer 2, and IP dominates layer 3.
So, if a routable protocol allows communication to be routed across the network, what does a routing protocol do?
A routing protocol is designed to allow routers to share what is called reachability information. Reachability is just a long word that defines which networks can be reached through a specific router. For example, if router A is connected to networks 1, 2, 3 and 4, it tells other routers that these networks are reachable through it. This is the reachability information it advertises to the other routers.
Because routers have been around for a while, there are several different protocols to choose from.
The first, and still most common, is called the Routing Information Protocol, or RIP. RIP is a very basic protocol and is only used in small networks. There are actually two versions of this protocol, RIP and RIP II.
The next most common protocol is called Open Shortest Path First, or OSPF. This protocol is more complex and is used for larger networks than those supported on RIP.
Finally there is Border Gateway Protocol, or BGP. BGP is not really a protocol like RIP or OSPF. RIP and OSPF are designed to be used in a corporation's network and allow the routers to share reachability information. BGP, on the other hand, is used as a way to share information between networks, not just routers.
For example, if you have a group of networks, some running RIP and some running OSPF, how can you share information between routers on the different networks? You need to use BGP. The most obvious place BGP is used is the Internet. However, you can also use BGP in large corporate networks to allow multiple networks to communicate.
RIP, OSPF and BGP are the most common of all the layer 3 routing protocols; however, there are probably over a hundred more. Most are proprietary to a specific company or are older and no longer in use, but they are still there.
Later in this course we look into the specifics of each of these protocols.
#internet #router #subnetworks