TAILIEUCHUNG - The Complete IS-IS Routing Protocol- P17

The Complete IS-IS Routing Protocol- P17: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. Thus, if one is at all interested in routing, it is well worth the time and effort to learn IS-IS. | 472 15. Troubleshooting The IS-IS configuration looks alright - all interfaces are referenced. At the top there is a pointer to an export policy which we will examine closer. JUNOS configuration On first sight the static-to-isis policy looks good however once you check the indentation of the terms and accept statements you will find out that the policy does not do what the network operator wanted it to do. hannes@Munich show configuration policy-options . policy-statement static-to-isis term reject_management from route-filter 8 orlonger then reject term static from protocol static then accept At first sight this policy looks good. However once we start to compare the indentation of the then part we realize that the term static does not have a valid then statement. Due to a misconfiguration it got inserted at the wrong level in the policy. What the standalone then accept term does is accept every unicast route in the routing tables and mark it for export into the IS-IS link-state database. Because there is no from statement at the same indentation level as the final then accept statement we have an unconditional export of the entire Internet routing table into IS-IS. The final then logic is executed when no terms match the routes. The logic is here Is the route 10 8 or longer No that s a private address. Is the route static No it s an Internet route. Okay then unconditionally accept the route into IS-IS. The distributed storage space that each node may allocate is 1492 -27 256 375 Kbytes. How many IPv4 prefixes do fit in those 375 Kbytes Figure in Chapter 12 IP Reachability Information illustrates the structure and storage requirements of the Extended IP Reachability TLV 135. Worst case the TLV consumes 9 bytes and best case 5 bytes due to variable prefix length packing. For the average Internet route we can assume a prefix length between 16 and 24 and safely assume a total storage requirement of 8 bytes per prefix. In a single TLV on average

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