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: - Use the ip
tcp adjust-mss command on the in-facing interface to adjust
the MSS value of the TCP SYN packets to 1452 to help prevent
the router from dropping TCP sessions. The ip tcp
adjust-mss command is effective only for TCP connections
passing through the router.
- Use the ip mtu
command to tell the device to fragment packets going out of the
interface if they are larger than 1492 bytes.
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