TAILIEUCHUNG - The Complete IS-IS Routing Protocol- Part 11

The Complete IS-IS Routing Protocol- P11: 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. | Levels 87 Figure . The OSPF constraint that one interface can only be in one area can cause sub-optimal routing sub-optimal routing. Figure shows how OSPF routes inter-area versus intra-area traffic. Consider traffic flowing between the two leaf-sites S source and D destination . Traffic arrives at the ABR and OSPF has two routes available to route that traffic - one direct route the intra-area over two low-speed T1 circuits and another route that leads over the backbone the inter-area route which has one T1 segment less and plenty of bandwidth available as there is a Gigabit Ethernet segment in the path. But just like any other hierarchical routing protocol OSPF prefers to get inter-area backbone traffic to intra-area routes as soon as possible. So ultimately the traffic takes the path indicated by the gray arrow. Common practice to fix that problem in OSPF is to spend money to put another link between the two Area Border Routers as indicated by the thick black dotted line. This link is configured to run in Area 52 and produces a lot of new low-cost paths to avoid the slower T1 hopping of traffic. In IS-IS the problem is solved similarly except that you do not have to expense two Gigabit Ethernet router ports Figure shows how IS-IS avoids this expense by the level between the routers that were OSPF Area Border Routers IS-IS L1L2 capable. Now over the same physical circuit the Gigabit Ethernet Segment IS-IS forms adjacencies on aper-level basis and both Level-1 and Level-2 adjacencies form on the same link. Therefore the Gigabit Ethernet link is an integral part of Area 52 and preferred when traffic travels from S to D. Route Leaking Between Levels Every routing protocol passes a certain amount of routing information up the routing hierarchy and other routing information is passed down the routing hierarchy. There is a bi-directional flow of routing information known as route leaking. To better understand how IS-IS leaks routes between levels .

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