网络课程笔记

00:00 Hello and welcome to this lecture.

00:01 Now in this lecture,

00:02 let's understand how the routing happens

00:05 for the VPN connection.

00:07 Specifically, there is something called static routing

00:10 and the dynamic routing,

00:11 and let's understand what that means.

00:14 So before we talk about the routing,

00:17 let's understand something called Autonomous Systems or AS.

00:21 So as you know, VPN connects different networks.

00:25 Now these networks would have some unique identifier.

00:29 Now that identifier it is called Autonomous Systems or AS.

00:34 And the unique identifier is an ID.

00:36 So that ID is called ASN Autonomous System Number.

00:41 Now this ASN can be public or private,

00:45 and if it's a public ASN,

00:48 then it is assigned by IANA

00:51 that is Internet Assigned Number Authority,

00:54 which is a standard organization

00:56 that typically oversees the public IP allocation,

01:00 then autonomous system number allocation.

01:02 And in case of DNS,

01:03 it also manage manages the root domain management.

01:07 So this IANA would assign a public AS into your router,

01:13 and then your router can exchange route information

01:16 across the internet.

01:18 However, if these two networks wants to communicate

01:21 with each other,

01:22 as an example, network A and network B

01:25 then it could also be a private ASN.

01:28 And then you can exchange information, Routing information

01:31 with other network over a protocol

01:33 like BGP Border Gateway Protocol.

01:36 And in that case, you can also use your private ASN.

01:39 So here you can say that

01:41 the public ASN ranges from zero to 65525.

01:46 And private ASN number ranges 64512 to 65534.

01:51 So when you work in AWS,

01:53 there is also a default ASN, private ASN,

01:56 which is assigned and which you can use to connect

01:59 to other networks.

02:01 So in short AS is an autonomous system

02:04 and every router which can exchange the route

02:08 with other network,

02:10 you will use this ASN number.

02:13 Now let's talk about the routing.

02:15 So, as I said,

02:16 there are two types of the routing.

02:17 Static routing and Dynamic routing.

02:19 Now let's understand what that means.

02:21 So here we are talking really about the,

02:24 how the route information is shared across two networks.

02:28 And in that sense,

02:29 we are talking about the static routing and dynamic routing.

02:32 So let's take an example.

02:33 Suppose there is a network A with CIDR 10.10.0.0/16

02:37 and network B with CIDR 10.20.0.0/16.

02:40 Now these are two different networks

02:42 and in order for the packet to route

02:44 from network A to network B,

02:46 of course, there should be a route table for router A

02:50 which has an entry for target destination as a network B,

02:54 and the target should be router B.

02:56 Now when this configuration is done,

02:59 your packets can flow from network A to network B.

03:03 Now similarly at network B,

03:04 you should also have the route table,

03:06 but this time the local route would be itself.

03:09 And the,

03:11 for router A, there would be a destination 10.10.0.0/16.

03:15 So that resources from router B

03:18 or network B can communicate with resource in network A.

03:22 So this is given and you understand this.

03:25 Now what happens,

03:27 suppose there is a new new network that comes in

03:31 and suppose you connect router B here with router C,

03:35 or you connect these two network like this.

03:38 Now in this case, of course, in router B,

03:41 you need to modify the route table.

03:43 And then you will add a route saying like,

03:44 if the traffic is going to 10.30.0.0

03:47 which is network C, then it has to go through router C.

03:52 Now this is okay, but at this moment,

03:55 what about a network A Or I say route table of router A?

04:00 Can packet flow from network A to network C?.

04:04 Now we know that there is a route,

04:06 if you go through the router B,

04:08 router B know how to send the traffic to network C,

04:13 but A does not know that.

04:15 So if you send the packet with the destination IP address

04:17 at 10.30.15.16, just an example.

04:22 It will come here and it will see there is no route

04:24 for that destination IP address range.

04:27 So it would just drop that.

04:29 So that's the problem with the static routing.

04:32 These routes needs to be manually added to your route table,

04:38 right?

04:38 So unless you make this entry here again,

04:41 the traffic is not going to flow

04:43 from network A to network C.

04:45 So in static routing, you have to manage these routes.

04:48 You have to modify your route tables

04:50 and add a new routes as the new networks are discovered,

04:55 or your network or your peer network is connected

04:58 to for the network or it expands its own IP address range.

05:03 So that in short is a static routing.

05:04 And as you might have understood, that's a problem.

05:08 And in order to solve that problem,

05:09 there is something called Dynamic routing.

05:12 Now in dynamic routing what happens,

05:16 as your network B router learns the new route

05:19 about network C,

05:21 it would dynamically propagate

05:24 that route to your router A route table.

05:30 So this route will be updated in network A automatically

05:34 as there is a new network discovered on the other side.

05:38 And this routing information is shared over a protocol,

05:43 something like a border gateway protocol.

05:46 And there are other protocols as well,

05:47 but we will learn about how route information is propagated

05:52 to the other networks using protocols

05:54 like border gateway protocol.

05:56 So that's a Dynamic routing in short

05:58 and in the next lecture, we'll understand more about BGP.

06:03 And then we'll see how this dynamic routing happens in AWS.

06:08 So thanks for watching.