TAILIEUCHUNG - The Complete IS-IS Routing Protocol- Part 4

The Complete IS-IS Routing Protocol- P4: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | Routing and Forwarding Tables 17 Relay DLCIs or ATM VCs since each can have an IP address associated with it. If there is no IP address assigned to a logical interface then any traffic arriving on that interface will be discarded. Once traffic arrives on the input interface there is typically a lookup engine that tries to determine the next-hop for a given IP address prefix the prefix is the network portion of the IP address . The next-hop information consists of an outgoing interface plus Layer 2 data link framing information. Since the outgoing interface is not enough for multiaccess networks like Ethernet LANs the router needs to prepend the destination Media Access Control MAC address of the receiver as well. Next the packet is transported inside the router chassis by any form of switch fabric. Common switch fabric designs are crossbars shared memory shared bus and multistage networks. The last stage before final sending of a packet to the next-hop router is the queuing stage. This buffers packets if the interface is congested schedules and deliver packets to an outgoing interface. Routing and Forwarding Tables Just what is the difference between a routing and a forwarding table The short answer is size and amount of origin information. The routing table of a well-connected Internet core router today uses dozens of megabytes MB of memory to store complete information about all known Internet routes. Figure shows why such a massive amount of memory is needed. A router needs to store all the routes that it receives from each neighbour. So for each neighbour an Input Routing Information Base RIB-in is kept. Due to path redundancy in network cores a prefix will most likely be known by more than one Transit traffic RIB-in 1 Figure . Internet core routers need to store what routes have been learned and advertised on a per neighbour basis 18 2. Router Architecture path. What the routing software does is to determine the best path for a given prefix .

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