The Complete IS-IS Routing Protocol- P7

The Complete IS-IS Routing Protocol- P7:IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, itswell-structured data formats, its flexibility and easy extensibility are all appealing – IS-ISepitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choicein some of the world’s largest networks. Thus, if one is at all interested in routing, it is wellworth the time and effort to learn IS-IS.
Nội dung trích xuất từ tài liệu:
The Complete IS-IS Routing Protocol- P7 Flooding 167 Seattle Chicago San Francisco New York Los Angeles Atlanta rx LSP tx LSPFIGURE 6.13. ATM overlay networks and flooding stress Each router in the Figure 6.13 setup forms an adjacency with the other routers, effectivelyforming a full-mesh. So far, so good. Now, consider the following scenario: the ATM vir-tual circuit between Seattle and Los Angeles breaks for some reason, as indicated by thedotted gray line. Both Seattle and LA notice the break and therefore generate a new LSP(incrementing the Sequence Number and removing the adjacency between Seattle andLA). The new LSP is sent according to the flooding rule on all interfaces where there areadjacencies in the Up state. Thus, both Seattle and LA send four copies (gray arrows) oftheir new LSPs into the network. Next, the four other routers will receive the two LSPs(white arrows). Here is where the trouble starts: because the flooding algorithm is so sim-ple, the algorithm does not yet know that all the other routers already been have updatedand know that the adjacency between Seattle and LA is down. What follows is a multi-plication of LSPs due to the simplicity of the flooding algorithm. All of the routersreceive the two new LSPs and re-send the LSP to all the logical interfaces except on theones on which they got the LSP (gray arrows). What results is that 32 LSPs are sent fora single broken ATM VC. This does not sound too stressful for a modern router’s controlplane; however, just think if there are not six routers, but 100 routers in the network. Theproblem is that the number of LSPs grows by the square of the number of routers, or inmathematical speak O(N 2). Thus, a single failing VC in the network may generate up to10,000 LSP updates, all flying around in a relatively short amount of time. This is anawful lot of stress for the control plane of a router, no matter how powerful.168 6. Generating, Flooding and Ageing LSPs Things get even worse with another failure scenario: what if not a single VC, but anentire router is going down (due to a reboot, for example)? The amount of LSPs growsby O(N 3). In a network of 100 routers spanning a full-mesh, this means that a single fail-ing router generates up to 1,000,000 LSPs in a short amount of time. Ironically, 99 percent of the LSPs hold information that is already known by some other neighbour. Sowhat can be done to mitigate the dark side of flooding? The answer to this is discussed inthe next section.6.4.2 Mesh-GroupsLet’s go back to the basic flooding algorithm and change it a little bit. Now the ruleis: Do not send out a received LSP on all the links where we have an adjacency in the Upstate. Rather, send out the LSP on some of these links. Figure 6.14 shows a router thatis not sending out an LSP on all of the possible links. Instead, some links have been prunedoff the flooding topology. The result is that all routers still see LSP updates, but the exces-sive multiplication of LSPs is avoided. The official name for this kind of functionality isknown as Mesh-Groups and has been documented in RFC 2973. The Mesh-Group pruningis done based on the topology of the network and is not automatic. There are two basic concepts behind Mesh-Groups. The first concept is blocking aninterface entirely, as shown in Figure 6.14. Here, one or a set of interfaces is removedfrom the flooding list. It is also very straightforward to configure on IOS and JUNOSsoftware, as shown in the following two configuration snippets. Both vendors share thesame spirit in their implementation of the Mesh-Group functionality. The LSP floodingin both vendors’ implementations is an interface property. In IOS, you configure everythingat the physical/logical interfaces prepended by the keyword isis. In JUNOS software,all the logical interfaces can be referenced directly under the protocols isisinterface configuration branch, which is very practical, as the relevant informationis then at one place. Pruned flooding links rx LSP tx LSPFIGURE 6.14. Mesh-Group blocks remove certain links from the flooding topology Flooding 169IOS configurationIn IOS, LSP flooding can be reduced using the isis mesh-group blocked configurationcommand in interface-configuration mode, as shown in the following:London# show running-config[ … ]interface atm 1/2.1 ip router isis isis mesh-group blocked[ … ] In JUNOS the configurations statement is very similar. The first flavour of Mesh-Groups can be enabled by use of the mesh-group blocked config-uration directiveunder the protocols isis interface configurationhierarchy, as shown in the following:JUNOS software configurationhannes@Frankfurt> show configuration[ … ]protocols { isis { interface at-4/0/0.200 { mesh-group blocked; } }}[ … ] You may ask why the word Group is contained in Mesh-Group. So far we have not con-figured a Group number. What is the Group number related to? This number is related to therefined version of Mesh-Groups where the flooding is not turned off entirely for an interface.Some LSPs are still sent. How is this second flavour of Mesh-Groups configured? First, allthe logical interfaces on an IS-IS router have to be organized in groups of interfaces. InFigure 6.15 ...