mtu commands. Figure shows the command syntax for these two commands. When a host (usually a PC) initiates a TCP session with a server, it negotiates the IP segment size by using the maximum segment size (MSS) option field in the TCP SYN packet. The value of the MSS field is determined by the maximum transmission unit (MTU) configuration on the host. The default MSS value for a PC is 1500 bytes. However, the PPPoE standard only supports an MTU of 1492 bytes.(PPPoE uses eight bytes of the data segment to encapsulate the IP packet into a PPP stream, effectively reducing the MTU over standard Ethernet to 1500.) In most cases, the optimum value for the max-segment-size argument is 1452 bytes. This value plus the 20-byte IP header, the 20-byte TCP header, and the 8-byte PPPoE header fill the 1500-byte packet that matches the MTU size for the Ethernet link. Disparities between the host and PPPoE MTU size cause the router in between the host and the server to drop 1500-byte packets and terminate TCP sessions over the PPPoE network. Even if the path MTU (which detects the correct MTU across the path) is enabled on the host, sessions may be dropped because system administrators sometimes disable the ICMP error messages that must be relayed from the host in order for path MTU to work. As a result of these issues, it is necessary to limit the packet sizes going in either direction: on the outward facing dialer interface and on the inward facing E0/0 interface. On the in-facing interface, you just need to set the MSS to a lower value, but on the out-facing interface, you need to set the MTU to lower values: Figure shows an example of configuring the MTU and MSS on the router interfaces.
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.6 Configuring PAT PAT is a feature of a network device that translates TCP or UDP communications made between a host and port on an outside network and a host and port on an inside network. PAT allows a single IP address to be used for many internal hosts. A PAT device transparently and automatically modifies the IP packets' destination or source host IP and port fields that belong to its internal hosts. PAT is related to NAT and is sometimes referred to as NAT overload. Like NAT, port address translation makes changes to the sender’s address or recipient’s address on data packets. However, any IP address change involves the PAT device’s outside IP address while changes that NAT makes involve a pool of addresses. PAT translates both the IP and port fields wherever those values belong to an internal host. Port numbers on packets coming from the external network, rather than destination IP addresses, are used to identify and designate traffic to different computers on the inside network. Figure displays an example of static PAT. You can translate several internal addresses using NAT into just one or a few external addresses using PAT. PAT uses unique source port numbers on the inside global IP address to distinguish between translations. Because the port number is encoded in 16 bits, the total number of internal addresses that NAT can translate into one external address is, theoretically, as many as 65,536. PAT attempts to preserve the original source port. If the source port is already allocated, PAT attempts to find the first available port number. PAT starts from the beginning of the appropriate port group, 0–511, 512–1023, or 1024–65,535. If PAT does not find a port that is available from the appropriate port group and if more than one external IP address is configured, PAT moves to the next IP address and tries to allocate the original source port again. PAT continues trying to allocate the original source port until it runs out of available ports and external IP addresses. PAT, PPP, and IPCP are popular techniques used to scale limited addresses. Figure displays how to use PAT to share the one registered IP address of the public interface for all the devices behind the PAT router to access the Internet. Figure illustrates a sample PAT configuration on the Cisco router. The access list will match any source address in the 10.0.0.0/8 network. In this example, the Dialer0 interface is the outside interface, and the Ethernet0/0 interface is the inside interface. PAT translates the 10.x.x.x source addresses to the Dialer0 IP address. The Dialer0 interface receives its IP address from the service provider aggregation router using IPCP.
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.7 Configuring DHCP to Scale DSL The Cisco IOS DHCP Server feature is a full implementation that assigns and manages IP addresses from specified address pools within the router to DHCP clients. After a DHCP client has booted, it sends a DHCP broadcast to obtain an IP address. Once it has an address, the client begins sending packets. The IP address of the default router should be on the same subnet as the client device. With the Cisco IOS DHCP Server, configuration information can be updated automatically. Network administrators can configure one or more centralized DHCP servers to update specific DHCP options within the DHCP pools. The remote servers can request or “import” these option parameters from the centralized servers. To configure a DHCP address pool on a Cisco IOS DHCP Server and enter DHCP pool configuration mode, use the ip dhcp pool global configuration command. To import DHCP option parameters into the Cisco IOS DHCP Server database, use the import all DHCP pool configuration command.To configure the subnet number and mask for a DHCP address pool on a Cisco IOS DHCP Server, use the network DHCP pool configuration command.To specify the default router list for a DHCP client, use the default-router DHCP pool configuration command. Note that the DHCP server excludes this address from the pool of assignable addresses. Figure describes how to configure the Cisco router as the DHCP server for the end-user PCs behind the router Ethernet interface.In this example, a DHCP address pool with the name MyPool is configured. The CPE router acts like a DHCP server to the hosts, connected to the Ethernet 0/0 interface. Hosts obtain IP addresses from range 10.0.0.2 to 10.255.255.254 with the subnet mask 255.0.0.0. The IP address 10.0.0.1 is excluded from this range, because this address is already used on the router interface. Hosts receive a default route pointing to the router interface IP address 10.0.0.1 and other parameters that the router receives from the aggregation router, such as Domain Name System (DNS) and Windows Internet Naming Service (WINS).
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.8 Configuring a Static Default Route You can configure a static default route on a Cisco router to allow the router to reach all unknown destinations toward the dialer interface. In most DSL installations, the CPE does not run a dynamic routing protocol to the aggregation router of the service provider. Therefore, a static default route is required on a Cisco router. When a PPPoE session is established between a Cisco router and the aggregation router of the service provider, the dialer interface IP address is assigned from the service provider aggregation router via IPCP. The service provider aggregation router automatically builds a /32 host route to reach the Cisco router dialer interface. To configure a static default route on a Cisco router, enter global configuration mode and use the ip route 0.0.0.0 0.0.0.0 command. Figure shows an example describing how to configure a static default route on a Cisco router. In this example, a static