IPv6: Dasar Routing Protocol
Bagian ini akan membahas beberapa hal sekitar routing protocol, seperti,
- Dasar routing procotol.
- Routing Protocol Distance Vector.
- Routing Protocol Link State.
- Interior dan Exterior Gateway Protocol.
- Statik atau Dynamic Routing?
Pada bagian sebelumnya menjelaskan apa yang perlu dikenali oleh router untuk mengalihkan paket dengan benar ke tujuan masing-masing, dan bagaimana ituinformasi dimasukkan ke dalam tabel rute secara manual. Pada bagian ini menunjukkan bagaimana router dapat menemukan informasi ini secara otomatis dan berbagi informasi itu dengan router lain melalui protokol routing dinamis. Protokol routing adalah bahasa yang digunakan router untuk berbicara dengan router lain berbagi informasi tentang jangkauan dan status jaringan.
Protokol routing dinamis tidak hanya melakukan fungsi penentuan rute dan fungsi route-table-update, tetapi juga menentukan jalur terbaik berikutnya jika jalur terbaik ke tujuan menjadi tidak dapat digunakan. Kemampuan untuk mengkompensasi perubahan topologi adalah keunggulan paling penting yang ditawarkan routing dinamis dibandingkan routing statis.
Jelas, agar komunikasi bisa terjadi, komunikator harus berbicara bahasa yang sama. Sejak munculnya IP routing, ada delapan protokol IP routing utama yang dapat dipilih; jika satu router berbicara RIP dan yang lain berbicara OSPF, mereka tidak dapat berbagi informasi routing karena mereka tidak berbicara bahasa yang sama. Bab-bab selanjutnya memeriksa semua protokol routing IP yang digunakan saat ini, dan bahkan mempertimbangkan bagaimana membuat router "bilingual," tetapi pertama-tama perlu untuk mengeksplorasi beberapa karakteristik dan masalah yang umum untuk semua protokol routing IP atau sebaliknya.
Dari delapan protokol ini, BGP membuat obsolete EGP, Cisco Systems EIGRP membuat obsolete IGRP, dan RIPv2 dengan menggantikan RIPv1..
Dasar Protocol Routing
Semua protokol routing dinamis dibangun berbasis pada suatu algoritma. Secara umum, algoritma adalah prosedur step-by-step untuk menyelesaikan masalah. Algoritma routing setidaknya harus menentukan beberapa hal berikut:
- Prosedur untuk meneruskan informasi jangkauan jaringan ke router lain
- Prosedur untuk menerima informasi jangkauan dari router lain
- Prosedur untuk menentukan rute optimal berdasarkan informasi jangkauan yang dimilikinya dan untuk merekam informasi ini dalam tabel rute
- Prosedur untuk bereaksi terhadap, kompensasi, dan perubahan topologi yang disebarkan di jaringan
Beberapa masalah umum untuk protokol routing adalah penentuan jalur, metrik, konvergensi, dan load balancing.
Penentuan jalur
Semua subnet dalam jaringan harus terhubung ke router, dan jika router memiliki interface jaringan, interface itu harus memiliki alamat di jaringan. Alamat ini adalah titik awal untuk informasi keterjangkauan.
Banyak point-to-point link dikonfigurasikan sebagai link "unnumbered" yaitu, tidak ada alamat yang ditetapkan untuk interface point-to-point yang terhubung untuk menghemat alamat. Tetapi link yang tidak bernomor tidak melanggar aturan bahwa setiap antarmuka harus memiliki alamat; mereka menggunakan alamat lain di router, biasanya alamat loopback, sebagai alamat proxy.
Gambar 4-1 menunjukkan jaringan tiga-router sederhana. Router A mengenali jaringan 192.168.1.0, 192.168.2.0, dan 192.168.3.0 karena memiliki interface pada jaringan tersebut dengan alamat yang sesuai dan mask alamat yang sesuai. Demikian juga, Router B mengakui 192.168.3.0, 192.168.4.0, 192.168.5.0, dan 192.186.6.0; Router C mengenali 192.168.6.0, 192.168.7.0, dan 198.168.1.0. Setiap interface mengimplementasikan link data dan protokol fisik jaringan dimana dia tersambung, sehingga router juga mengenali keadaan jaringan (up atau down).
Gambar 4-1. Setiap router tahu tentang jaringan yang terhubung langsung dari alamat dan mask yang dikonfigurasikan.
Sepintas lalu, prosedur berbagi informasi tampak sederhana. Lihatlah Router A:
- Router A memeriksa IP address dan mask terkait dan menyimpulkan bahwa ia menempel ke jaringan 192.168.1.0, 192.186.2.0, dan 192.168.3.0.
- Router A memasukan jaringan terkait ke dalam tabel routing, bersama dengan semacam flag yang menunjukkan bahwa jaringan terhubung langsung.
- Router A menyebarkan informasi ke dalam sebuah paket: "Jaringan yang terhubung langsung dengan saya adalah 192.168.1.0, 192.186.2.0, dan 192.168.3.0."
- Router A mengirimkan copy informasi routing ini, atau memperbarui routing, ke Router B dan C.
Router B dan C, setelah melakukan langkah yang sama, telah mengirim pembaruan dengan jaringan yang terhubung langsung ke A. Router A memasukkan informasi yang diterima ke dalam tabel rutenya, bersama dengan alamat sumber router yang mengirim paket pembaruan. Router A sekarang mengenali semua jaringan dan alamat router yang tersambung.
Prosedur ini sepertinya cukup sederhana. Jadi mengapa protokol routing jauh lebih rumit dari ini? Lihat kembali Gambar 4-1:
What should Router A do with the updates from B and C after it has recorded the information in the route table? Should it, for instance,
pass B's routing information packet to C and pass C's packet to B? If Router A does not forward the updates, information sharing might
not be complete. For instance, if the link between B and C does not exist, those two routers would not recognize each other's networks.
Router A must forward the update information, but this step opens a new set of problems.
If Router A hears about network 192.168.4.0 from both Router B and Router C, which router should be used to reach that network? Are they both valid? Which one is the best path? What mechanism will be used to ensure that all routers receive all routing information while preventing update packets from circulating endlessly through the network?
The routers share certain directly connected networks (192.168.1.0, 192.168.3.0, and 192.168.6.0). Should the routers still advertise these networks?
These questions are almost as simplistic as the preceding preliminary explanation of routing protocols, but they should give you an indication for some of the issues that contribute to the complexity of the protocols.for some of the issues that contribute to the complexity of the protocols.
Each routing protocol addresses these questions one way or another, which will become clear in following sections and chapters.
Metrics
When there are multiple routes to the same destination, a router must have a mechanism for calculating the best path. A metric is a variable assigned to routes as a means of ranking them from best to worst or from most preferred to least preferred. Consider the following example of why metrics are needed.
Assuming that information sharing has properly occurred in the network of Figure 4-1, Router A might have a route table that looks like Table 4-1. Table 4-1. Rudimentary route table for Router A of Figure 4-1.
Network Next-Hop Router 192.168.1.0 Directly connected 192.168.2.0 Directly connected 192.168.3.0 Directly connected 192.168.4.0 B, C 192.168.5.0 B, C 192.168.6.0 B, C 192.168.7.0 B, C
This route table says that the first three networks are directly connected and that no routing is needed from Router A to reach them, which is correct. The last four networks, according to this table, can be reached via Router B or Router C. This information is also correct. But if network 192.168.7.0 can be reached via either Router B or Router C, which path is the preferable path? Metrics are needed to rank the alternatives.
Different routing protocols use different metrics. For example, RIP defines the "best" route as the one with the least number of router hops; EIGRP defines the "best" route based on a combination of the lowest bandwidth along the route and the total delay of the route. The following sections provide basic definitions of these and other commonly used metrics. Further complexitiessuch as how some routing protocols such as EIGRP use multiple parameters to compute a metric and deal with routes that have identical metric valuesare covered later, in the protocol-specific chapters of this book.
Hop Count
A hop-count metric simply counts router hops. For instance, from Router A, it is one hop to network 192.168.5.0 if packets are sent out interface 192.168.3.1 (through Router B) and two hops if packets are sent out 192.168.1.1 (through Routers C and B). Assuming hop count is the only metric being applied, the best route is the one with the fewest hops, in this case, A-B.
But is the A-B link really the best path? If the A-B link is a DS-0 link and the A-C and C-B links are T-1 links, the two-hop route might actually be best, because bandwidth plays a role in how efficiently traffic travels through the network.
Bandwidth
A bandwidth metric would choose a higher-bandwidth path over a lower-A bandwidth metric would choose a higher-bandwidth path over a lower- bandwidth link. However, bandwidth by itself still might not be a goodcmetric. What if one or both of the T1 links are heavily loaded with otherctraffic and the 56K link is lightly loaded? Or what if the higher-bandwidthclink also has a higher delay?
Load
This metric reflects the amount of traffic utilizing the links along the path. The best path is the one with the lowest load.
Unlike hop count and bandwidth, the load on a route changes, and, therefore, the metric will change. Care must be taken here. If the metric changes too frequently, route flapping the frequent change of preferred routes might occur. Route flaps can have adverse effects on the router's CPU, the bandwidth of the data links, and the overall stability of the network.
Delay
Delay is a measure of the time a packet takes to traverse a route. A routing protocol using delay as a metric would choose the path with the least delay as the best path. There might be many ways to measure delay. Delay might take into account not only the delay of the links along the route, but also such factors as router latency and queuing delay. On the other hand, the delay of a route might not be measured at all; it might be a sum of static quantities defined for each interface along the path. Each individual delay quantity would be an estimate based on the type of link to which the interface is connected.
Reliability
Reliability measures the likelihood that the link will fail in some way and can be either variable or fixed. Examples of variable-reliability metrics arethe number of times a link has failed, or the number of errors it has received within a certain time period. Fixed-reliability metrics are based on known qualities of a link as determined by the network administrator. The path with highest reliability would be selected as best.
Cost
This metric is configured by a network administrator to reflect more- or less-preferred routes. Cost might be defined by any policy or link characteristic or might reflect the arbitrary judgment of the network administrator. Therefore, "cost" is a term of convenience describing a dimensionless metric.
The term cost is often used as a generic term when speaking of route choices. For example, "RIP chooses the lowest-cost path based on hop count." Another generic term is shortest, as in "RIP chooses the shortest path based on hop count." When used in this context, either lowest-cost (or highest-cost) and shortest (or longest) merely refer to a routing protocol's view of paths based on its specific metrics.
Convergence
A dynamic routing protocol must include a set of procedures for a router to inform other routers about its directly connected networks, to receive and process the same information from other routers, and to pass along the information it receives from other routers. Further, a routing protocol must define a metric by which best paths might be determined.
A further criterion for routing protocols is that the reachability information in the route tables of all routers in the network must be consistent. If Router A in Figure 4-1 determines that the best path to network 192.168.5.0 is via Router C and if Router C determines that the best path to the same network is through Router A, Router A will send packets destined for 192.168.5.0 to C, C will send them back to A, A will again send them to C, and so on. This continuous circling of traffic between twoor more destinations is referred to as a routing loop.
The process of bringing all route tables to a state of consistency is called convergence. The time it takes to share information across a network and for all routers to calculate best paths is the convergence time. Figure 4-2 shows a network that was converged, but now a topology change has occurred. The link between the two left-most routers has failed; both routers, being directly connected, know about the failure from the data link protocol and proceed to inform their neighbors of the unavailable link. The neighbors update their route tables accordingly and inform their neighbors, and the process continues until all routers know about the change.
Figure 4-2. Reconvergence after a topology change takes time. While the network is in an unconverged state, routers are susceptible to bad routing information.Notice that at time t 2 the three left-most routers recognize the topology change but the three right-most routers have not yet received that information. Those three have old information and will continue to switch packets accordingly. It is during this intermediate time, when the network is in an unconverged state, that routing errors might occur. Therefore, convergence time is an important factor in any routing protocol. The faster a network can reconverge after a topology change, the better.
Load Balancing
Recall from Chapter 3, "Static Routing," that load balancing is the practice of distributing traffic among multiple paths to the same destination, so as to use bandwidth efficiently. As an example of the usefulness of load balancing, consider Figure 4-1 again. All the networks in Figure 4-1 are reachable from two paths. If a device on 192.168.2.0 sends a stream of packets to a device on 192.168.6.0, Router A might send them all via Router B or Router C. In both cases, the network is one hop away. However, sending all packets on a single route probably is not the most efficient use of available bandwidth. Instead, load balancing should be implemented to alternate traffic between the two paths. As noted in Chapter 3, load balancing can be equal cost or unequal cost, and per packet or per destination.