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BGP was introduced to improve upon EGP One of BGP's main features is that it introduces a reliable transport protocol to ensure that route updates are received BGP also implements a keepalive mechanism, ensuring that BGP routers know if neighboring BGP routers fail BGP does not transmit metrics with its route updates, but does transmit a path for each AS that lists the autonomous systems to be visited on the way to the destination AS BGP thus avoids the circulating-packet problem of EGP BGP works on the principle of enforcing policies A policy is manually configured and allows a BGP-enabled router to rate possible routes to other autonomous systems, selecting the best path Configuring BGPWe can use Fig 4-22 to discuss how to configure BGP on a router In this example, let's take router R6 To configure BGP we will perform the following: Define BGP as a routing process Define the networks internal to this AS that are going to be advertised Define the relationships that this router will have with its neighbors Assign administrative weights to paths to control the path selection process This is a basic configuration for BGP There are, however, many further configuration customizations that can be made If you really want to get into complex BGP configuration, I would recommend talking to a Cisco Systems engineer who can help you with customizing BGP for your particular internetwork For a basic configuration, the following commands are entered into router R6: Router6(config)#router bgp 3 Router6(config-router)#network 1473000 Router6(config-router)#network 150100 Router6(config-router)#neighbor 1473011 remote-as 3 Router6(config-router)#neighbor 160455 remote-as 2 The first line in this configuration defines BGP for autonomous system 3 on router 6 The next two lines define the network numbers internal to AS 3 that will be advertised via BGP The fourth line defines an internal neighbor that is in the same AS The BGP process on router 6 will now exchange information with a BGP process defined on R5 The fifth line defines the neighbor in a different AS with which router 6 will exchange information The effect of this configuration is that R6 will share information about networks 1473000 and 150100 with the two specified routers via BGP updatesThe last thing left to do in our basic configuration of BGP is to assign administrative weights to control the path selection process In the following example, a weight of 40,000 is assigned to the path to router R4 Router6(config-router)#neighbor 160455 40000.

The amount of work involved in manually configuring each system on the network is too much for any network administrator The potential for human error is great when inputting the parameters on multiple systems simply due to the fact that it is such a laborious task It is also very easy to assign a duplicate address on the network, which would result in an error It is very hard to implement major changes to your IP infrastructure when you are manually configuring systems For example, a change to a router address would require a lot of time to update the default gateway entry on the clients

figure 4-5

4:

When the IP was developed, it was recognized that hosts would be connected to different networks and that those networks would be interconnected to form an Internet Thus, in developing the IP addressing scheme, it was also recognized that a mechanism would be required to identify a network as well as a host connected to a network This recognition resulted in the development of a two-level addressing hierarchy that is illustrated in Figure 3 3

Router6(config-router)#neighbor 160455 40000 This administrative weight can vary between 0 and 65535, with the default being 32768 The effect of increasing the weight to R4 is to make it less attractive when R6 is calculating which paths to use

Dynamic Host Configuration Protocol (DHCP)

Configuring IP addressing on a large TCP/IP-based network can be a nightmare, especially if machines are moved from one network to another frequently The Dynamic Host Configuration Protocol (DHCP) can help with the workload of configuring systems on a network by assigning addresses to systems on boot-up automatically The process of dynamically assigning IP addresses is managed via a DHCP server The DHCP server is configured with a set of usable IP addresses, called a scope The scope can also include the subnet mask, IP addresses of the default gateway, DNS servers, WINS servers, and other necessary addresses When a PC comes online and is set up to use a DHCP server, it requests an IP address by transmitting a broadcast request packet looking for any DHCP servers on the network (known as DHCP Discovery) The DHCP server responds with an offer containing an IP address that the client can lease (known as the DHCP Offer) The client then accepts the offer by sending a request message for that address from the DHCP server (known as the DHCP Request), and then the server responds with an acknowledgment to the client that it has that address and additional settings for the lease time (known as the DHCP ACK) The DHCP server marks the IP address in its database as being in use so that it is not assigned again When configuring the DHCP server, you will need to configure a scope with the following settings:

The DHCP server issues an IP address to each DHCP client system on the network Each system connected to a TCP/IP-based network is identified by a unique IP address As you learned in this chapter, the IP address consists of four 8-bit octets separated by periods The IP address is normally shown in dotted-decimal notation for example, 192102462 The IP address actually consists of two parts: the network ID and the host ID The subnet mask is used to identify the part of the IP address that is the network ID and the part that is the host ID Subnet masks assign 1s to the network ID bits and 0s to the host ID bits of the IP address A default gateway is required when the client system needs to communicate outside its own subnet Normally, the default gateway is a router connected to the local subnet, which enables IP packets to be passed to other network segments

Under the two-level IP addressing scheme, all hosts on the same network must be assigned the same network prefix but must have a unique host address to differentiate one host from another Similarly, two hosts on different networks must be assigned different network prefixes; however, the hosts can have the same host address This concept is similar to your telephone number No one in your area code can have exactly the same phone number as your own, but it's likely that somewhere the same phone number exists in a different area code