16-dynamic-routing-BGP-v1

N1 r1 as1 which advertises that you can reach network

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Unformatted text preview: h will allow you to enter AS1. • R2 receives that message, updates its own routing table, and then adds itself to the path vector for N1 as follows… ITM440/540 Carol Davids © 2008 12 Explanation of diagram • R2 sends this path vector: N1 R2 AS2, AS1 • This means that you can reach the network N1, if you hop to R2 on AS2, which will lead you to AS1 where N1 is located. • When R3 receives this message, it updates its routing table and adds itself to the path vector as follows… ITM440/540 Carol Davids © 2008 13 Explanation of diagram • R3 sends the path vector message N1 R3 AS3, AS2, AS1. • This means that you can reach N1 by hopping to R3, traversing AS3, AS2 and finally AS1 where N1 is located. ITM440/540 Carol Davids © 2008 14 Initialization • In Figure 22.30, we see four AS’s. • Each speaker node only knows about the destination networks within the AS • Each speaker node sends its routing table to its neighbors ITM440/540 Carol Davids © 2008 15 Initialization • In Figure 22.30 of edition 4 ITM440/540 Carol Davids © 2008 16 Stabilized tables • In illustration 22.31, the neighbors have exchanged their routing tables. • Here, if D1 in AS4 receives an IP datagram destined for A2 in AS1, the D1 routing table tells D1 to use the path: AS4-AS3-AS1. ITM440/540 Carol Davids © 2008 17 Advantages of path vector routing • Prevents loops • When the router receives a path vector message it checks to see if its AS is in the path list to a destination network • If it is, the message is not used to update the routing table ITM440/540 Carol Davids © 2008 18 Advantages of path vector routing • Enables policy routing • The policy of a particular AS may include things like “do not traverse any AS that does not implement security precautions”. • If a router receives a path vector message that includes an AS on the ‘banned’ list, the router can choose not to include the information in the message. ITM440/540 Carol Davids © 2008 19 Advantages of path vector routing • Enables policy routing (continued) • This means that you don’t always take the shortest path, but you do take only paths that your policy allows. ITM440/540 Carol Davids © 2008 20 Advantages of path vector routing • Path vector provides attributes • The information contained in the path is actually more complex than simply the AS identification • It describes attributes (characteristics) of the path • The ORIGIN attribute, for example tells what method was used to derive the route. Its value could be RIP or OSPF, for example. ITM440/540 Carol Davids © 2008 21 Advantages of path vector routing • Path vector provides attributes (continued) • AS_PATH attribute defines the list of autonomous systems that must be traversed to reach the destination network. • NEXT_HOP attribute identifies the next router to which the packet should be sent • The attributes can be compared to the policy and the best route can then be chosen and placed...
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