Table of Contents

Chapter 1 IP Addressing Configuration. 1-1

1.1 IP Addressing Overview. 1-1

1.1.1 IP Address Classes. 1-1

1.1.2 Special Case IP Addresses. 1-2

1.1.3 Subnetting and Masking. 1-2

1.2 Configuring IP Addresses. 1-3

1.2.1 Assigning an IP Address to an Interface. 1-4

1.2.2 IP Addressing Configuration Example. 1-5

1.3 Displaying and Maintaining IP Addressing. 1-7

Chapter 2 IP Performance Configuration. 2-1

2.1 IP Performance Overview. 2-1

2.2 Enabling Reception and Forwarding of Directed Broadcasts to a Directly Connected Network. 2-1

2.2.1 Enabling Reception of Directed Broadcasts to a Directly Connected Network. 2-1

2.2.2 Enabling Forwarding of Directed Broadcasts to a Directly Connected Network. 2-2

2.2.3 Configuration Example. 2-2

2.3 Configuring TCP Attributes. 2-4

2.3.1 Configuring TCP Optional Parameters. 2-4

2.4 Configuring ICMP to Send Error Packets. 2-4

2.5 Displaying and Maintaining IP Performance. 2-7

 

Chapter 1  IP Addressing Configuration

When assigning IP addresses to interfaces on your device, go to these sections for information you are interested in:

l           IP Addressing Overview

l           Configuring IP Addresses

l           Displaying and Maintaining IP Addressing

1.1  IP Addressing Overview

This section covers these topics:

l           IP Address Classes

l           Special Case IP Addresses

l           Subnetting and Masking

1.1.1  IP Address Classes

IP addressing uses a 32-bit address to identify each host on a network. An example is 01010000100000001000000010000000 in binary. To make IP addresses in 32-bit form easier to read, they are written in dotted decimal notation, each being four octets in length, for example, 10.1.1.1 for the address just mentioned.

Each IP address breaks down into two parts:

l           Net-id: First several bits of the IP address defining a network, also known as class bits.

l           Host-id: Identifies a host on a network.

For administration sake, IP addresses are divided into five classes. Which class an IP address belongs to depends on the first one to four bits of the net-id, as shown in the following figure (in which the blue parts represent the address class).

Figure 1-1 IP address classes

Table 1-1 describes the address ranges of these five classes. Currently, the first three classes of IP addresses are used in quantity.

Table 1-1 IP address classes and ranges

Class	Address range	Description
A	0.0.0.0 to 127.255.255.255	The IP address 0.0.0.0 is used by a host at bootstrap for temporary communication. This address is never a valid destination address. Addresses starting with 127 are reserved for loopback test. Packets destined to these addresses are processed locally as input packets rather than sent to the link.
B	128.0.0.0 to 191.255.255.255	––
C	192.0.0.0 to 223.255.255.255	––
D	224.0.0.0 to 239.255.255.255	Multicast address.
E	240.0.0.0 to 255.255.255.255	Reserved for future use except for the broadcast address 255.255.255.255.

 

1.1.2  Special Case IP Addresses

The following IP addresses are for special use, and they cannot be used as host IP addresses:

l           IP address with an all-zero net ID: Identifies a host on the local network. For example, IP address 0.0.0.16 indicates the host with a host ID of 16 on the local network.

l           IP address with an all-zero host ID: Identifies a network.

l           IP address with an all-one host ID: Identifies a directed broadcast address. For example, a packet with the destination address of 192.168.1.255 will be broadcasted to all the hosts on the network 192.168.1.0.

1.1.3  Subnetting and Masking

Subnetting was developed to address the risk of IP address exhaustion resulting from fast expansion of the Internet. The idea is to break a network down into smaller networks called subnets by using some bits of the host-id to create a subnet-id. To identify the boundary between the host-id and the combination of net-id and subnet-id, masking is used. (When subnetting is not adopted, a mask identifies the boundary between the host-id and the host-id.)

Each subnet mask comprises 32 bits related to the corresponding bits in an IP address. In a subnet mask, the part containing consecutive ones identifies the combination of net-id and subnet-id whereas the part containing consecutive zeros identifies the host-id.

Figure 1-2 shows how a Class B network is subnetted.

Figure 1-2 Subnet a Class B network

While allowing you to create multiple logical networks within a single Class A, B, or C network, subnetting is transparent to the rest of the Internet. All these networks still appear as one. As subnetting adds an additional level, subnet-id, to the two-level hierarchy with IP addressing, IP routing now involves three steps: delivery to the site, delivery to the subnet, and delivery to the host.

In the absence of subnetting, some special addresses such as the addresses with the net-id of all zeros and the addresses with the host-id of all ones, are not assignable to hosts. The same is true of subnetting. When designing your network, you should note that subnetting is somewhat a tradeoff between subnets and accommodated hosts. For example, a Class B network can accommodate 65,534 (2¹⁶ – 2. Of the two deducted Class B addresses, one with an all-one host-id is the broadcast address and the other with an all-zero host-id is the network address) hosts before being subnetted. After you break it down into 512 (2⁹) subnets by using the first 9 bits of the host-id for the subnet, you have only 7 bits for the host-id and thus have only 126 (2⁷ – 2) hosts in each subnet. The maximum number of hosts is thus 64,512 (512 × 126), 1022 less after the network is subnetted.

Class A, B, and C networks, before being subnetted, use these default masks (also called natural masks): 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively.

1.2  Configuring IP Addresses

Besides directly assigning an IP address to an interface, you may configure the interface to obtain one through BOOTP or DHCP as alternatives. If you change the way an interface obtains an IP address, from manual assignment to BOOTP for example, the IP address obtained from BOOTP will overwrite the old one manually assigned.

 

 

This section includes:

l           Assigning an IP Address to an Interface

l           IP Addressing Configuration Example

1.2.1  Assigning an IP Address to an Interface

You may assign an interface multiple IP addresses, one primary and multiple secondaries, to connect multiple logical subnets on the same physical subnet.

Follow these steps to assign an IP address to an interface:

To do…	Use the command…	Remarks
Enter system view	system-view	––
Enter interface view	interface interface-type interface-number	––
Assign an IP address to the interface	ip address ip-address { mask | mask-length } [ sub ]	Required No IP address is assigned by default.

 

 

1.2.2  IP Addressing Configuration Example

I. Network requirements

As shown in Figure 1-3, VLAN-interface 1 on Switch is connected to a LAN comprising two segments: 172.16.1.0/24 and 172.16.2.0/24.

To enable the hosts on the two network segments to access the external network through the switch, and enable the hosts on the two network segments to communicate with each other, do the following:

l           Assign a primary IP address and a secondary IP address to VLAN-interface 1 on the switch.

l           Set the switch as the gateway on all hosts.

II. Network diagram

Figure 1-3 Network diagram for IP addressing configuration

III. Configuration procedure

# Assign a primary IP address and a secondary IP address to VLAN-interface 1.

<Switch> system-view

[Switch] interface vlan-interface 1

[Switch-Vlan-interface1] ip address 172.16.1.1 255.255.255.0

[Switch-Vlan-interface1] ip address 172.16.2.1 255.255.255.0 sub

# Set the gateway address to 172.16.1.1 on the PCs attached to the subnet 172.16.1.0/24, and to 172.16.2.1 on the PCs attached to the subnet 172.16.2.0/24.

# Use the ping command to verify the connectivity between the switch and the hosts on the subnet 172.16.1.0/24.

<Switch> ping 172.16.1.2

  PING 172.16.1.2: 56  data bytes, press CTRL_C to break

    Reply from 172.16.1.2: bytes=56 Sequence=1 ttl=255 time=25 ms

    Reply from 172.16.1.2: bytes=56 Sequence=2 ttl=255 time=27 ms

    Reply from 172.16.1.2: bytes=56 Sequence=3 ttl=255 time=26 ms

    Reply from 172.16.1.2: bytes=56 Sequence=4 ttl=255 time=26 ms

    Reply from 172.16.1.2: bytes=56 Sequence=5 ttl=255 time=26 ms

 

  --- 172.16.1.2 ping statistics ---

    5 packet(s) transmitted

    5 packet(s) received

    0.00% packet loss

    round-trip min/avg/max = 25/26/27 ms

The information shown above indicates the switch can communicate with the hosts on the subnet 172.16.1.0/24.

# Use the ping command to verify the connectivity between the switch and the hosts on the subnet 172.16.2.0/24.

<Switch> ping 172.16.2.2

  PING 172.16.2.2: 56  data bytes, press CTRL_C to break

    Reply from 172.16.2.2: bytes=56 Sequence=1 ttl=255 time=25 ms

    Reply from 172.16.2.2: bytes=56 Sequence=2 ttl=255 time=26 ms

    Reply from 172.16.2.2: bytes=56 Sequence=3 ttl=255 time=26 ms

    Reply from 172.16.2.2: bytes=56 Sequence=4 ttl=255 time=26 ms

    Reply from 172.16.2.2: bytes=56 Sequence=5 ttl=255 time=26 ms

 

  --- 172.16.2.2 ping statistics ---

    5 packet(s) transmitted

    5 packet(s) received

    0.00% packet loss

    round-trip min/avg/max = 25/25/26 ms

The information shown above indicates the switch can communicate with the hosts on the subnet 172.16.2.0/24.

# Use the ping command to verify the connectivity between hosts on the subnet 172.16.1.0/24 and hosts on subnet 172.16.2.0/24. Ping Host B on Host A to verify that the ping operation is successful.

1.3  Displaying and Maintaining IP Addressing

To do…	Use the command…	Remarks
Display information about a specified or all Layer 3 interfaces	display ip interface [ interface-type interface-number ]	Available in any view
Display brief information about a specified or all Layer 3 interfaces	display ip interface brief [ interface-type interface-number ]

 

Chapter 2  IP Performance Configuration

When configuring IP performance, go to these sections for information you are interested in:

l           IP Performance Overview

l           Enabling Reception and Forwarding of Directed Broadcasts to a Directly Connected Network

l           Configuring TCP Attributes

l           Configuring ICMP to Send Error Packets

l           Displaying and Maintaining IP Performance

2.1  IP Performance Overview

In some network environments, you need to adjust the IP parameters to achieve best network performance. IP performance configuration includes:

l           Enabling the device to receive and forward directed broadcasts

l           Configuring the maximum TCP segment size (MSS) of the interface

l           Configuring TCP timers

l           Configuring the TCP buffer size

l           Enabling ICMP error packets sending

2.2  Enabling Reception and Forwarding of Directed Broadcasts to a Directly Connected Network

Directed broadcasts refer to broadcast packets sent to a specific network. In the destination IP address of a directed broadcast, the network ID is a network-specific number and the host ID is all ones. Enabling the device to receive and forward directed broadcasts to a directly connected network will give hackers an opportunity to attack the network. Therefore, the device is disabled from receiving and forwarding directed broadcasts by default. You should however enable the feature when:

l           Using the UDP Helper function to convert broadcasts to unicasts and forward them to a specified server.

l           Using the Wake on LAN function to forward directed broadcasts to a PC on the remote network.

2.2.1  Enabling Reception of Directed Broadcasts to a Directly Connected Network

If a device is enabled to receive directed broadcasts, the device will determine whether to forward them according to the configuration on the outgoing interface.

Follow these steps to enable the device to receive directed broadcasts:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable the device to receive directed broadcasts	ip forward-broadcast	Required By default, the device is disabled from receiving directed broadcasts.

 

2.2.2  Enabling Forwarding of Directed Broadcasts to a Directly Connected Network

Follow these steps to enable the device to forward directed broadcasts:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enter interface view	interface interface-type interface-number	—
Enable the interface to forward directed broadcasts	ip forward-broadcast [ acl acl-number ]	Required By default, the device is disabled from forwarding directed broadcasts.

 

 

2.2.3  Configuration Example

I. Network requirements

As shown in Figure 2-1, the host’s interface and VLAN-interface 3 of Switch A are on the same network segment (1.1.1.0/24). VLAN-interface 2 of Switch A and VLAN-interface 2 of Switch B are on another network segment (2.2.2.0/24). The default gateway of the host is VLAN-interface 3 (IP address 1.1.1.2/24) of Switch A. Configure a static route on Switch B to enable the reachability between host and Switch B.

II. Network diagram

Figure 2-1 Network diagram for receiving and forwarding directed broadcasts

III. Configuration procedure

l           Configure Switch A

# Enable Switch A to receive directed broadcasts.

<SwitchA> system-view

[SwitchA] ip forward-broadcast

# Configure IP addresses for VLAN-interface 3 and VLAN-interface 2.

[SwitchA] interface vlan-interface 3

[SwitchA-Vlan-interface3] ip address 1.1.1.2 24

[SwitchA-Vlan-interface3] quit

[SwitchA] interface vlan-interface 2

[SwitchA-Vlan-interface2] ip address 2.2.2.2 24

# Enable VLAN-interface 2 to forward directed broadcasts.

[SwitchA-Vlan-interface2] ip forward-broadcast

l           Configure Switch B

# Enable Switch B to receive directed broadcasts.

<SwitchB> system-view

[SwitchB] ip forward-broadcast

# Configure a static route to the host.

[SwitchB] ip route-static 1.1.1.1 24 2.2.2.2

# Configure an IP address for VLAN-interface 2.

[SwitchB] interface vlan-interface 2

[SwitchB-Vlan-interface2] ip address 2.2.2.1 24

After the above configurations, if you ping the subnet broadcast address (2.2.2.255) of VLAN-interface 2 of Switch A on the host, the ping packets can be received by VLAN-interface 2 of Switch B. However, if you disable the ip forward-broadcast command, the ping packets can not be received by the VLAN-interface 2 of Switch B.

2.3  Configuring TCP Attributes

2.3.1  Configuring TCP Optional Parameters

TCP optional parameters that can be configured include:

l           synwait timer: When sending a SYN packet, TCP starts the synwait timer. If no response packets are received within the synwait timer timeout, the TCP connection is not successfully created.

l           finwait timer: When the TCP connection is in FIN_WAIT_2 state, finwait timer will be started. If no FIN packets are received within the timer timeout, the TCP connection will be terminated. If FIN packets are received, the TCP connection state changes to TIME_WAIT. If non-FIN packets are received, the system restarts the timer from receiving the last non-FIN packet. The connection is broken after the timer expires.

l           Size of TCP receive/send buffer

Follow these steps to configure TCP optional parameters:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Configure TCP synwait timer’s timeout value	tcp timer syn-timeout time-value	Optional By default, the timeout value is 75 seconds.
Configure TCP finwait timer’s timeout value	tcp timer fin-timeout time-value	Optional By default, the timeout value is 675 seconds.
Configure the size of TCP receive/send buffer	tcp window window-size	Optional By default, the buffer is 8 kilobytes.

 

 

2.4  Configuring ICMP to Send Error Packets

Sending error packets is a major function of ICMP protocol. In case of network abnormalities, ICMP packets are usually sent by the network or transport layer protocols to notify corresponding devices so as to facilitate control and management.

I. Advantage of sending ICMP error packets

There are three kinds of ICMP error packets: redirect packets, timeout packets and destination unreachable packets. Their sending conditions and functions are as follows.

1)         Sending ICMP redirect packets

A host may have only a default route to the default gateway in its routing table after startup. The default gateway will send ICMP redirect packets to the source host and notify it to reselect a correct next hop router to send the subsequent packets, if the following conditions are satisfied:

l           The receiving and forwarding interfaces are the same.

l           The selected route has not been created or modified by ICMP redirect packet.

l           The selected route is not the default route of the device.

l           There is no source route option in the packet.

ICMP redirect packets function simplifies host administration and enables a host to gradually establish a sound routing table to find out the best route

2)         Sending ICMP timeout packets

If the device received an IP packet with a timeout error, it drops the packet and sends an ICMP timeout packet to the source.

The device will send an ICMP timeout packet under the following conditions:

l           If the device finds the destination of a packet is not itself and the TTL field of the packet is 1, it will send a “TTL timeout” ICMP error message.

l           When the device receives the first fragment of an IP datagram whose destination is the device itself, it will start a timer. If the timer times out before all the fragments of the datagram are received, the device will send a “reassembly timeout” ICMP error packet.

3)         Sending ICMP destination unreachable packets

If the device receives an IP packet with the destination unreachable, it will drop the packet and send an ICMP destination unreachable error packet to the source.

Conditions for sending this ICMP packet:

l           If neither a route nor the default route for forwarding a packet is available, the device will send a “network unreachable” ICMP error packet.

l           If the destination of a packet is local while the transport layer protocol of the packet is not supported by the local device, the device sends a “protocol unreachable” ICMP error packet to the source.

l           When receiving a packet with the destination being local and transport layer protocol being UDP, if the packet’s port number does not match the running process, the device will send the source a “port unreachable” ICMP error packet.

l           If the source uses “strict source routing" to send packets, but the intermediate device finds the next hop specified by the source is not directly connected, the device will send the source a “source routing failure” ICMP error packet.

l           When forwarding a packet, if the MTU of the sending interface is smaller than the packet but the packet has been set “Don’t Fragment”, the device will send the source a “fragmentation needed and Don’t Fragment (DF)-set” ICMP error packet.

II. Disadvantage of sending ICMP error packets

Although sending ICMP error packets facilitate network control and management, it still has the following disadvantages:

l           Sending a lot of ICMP packets will increase network traffic.

l           If receiving a lot of malicious packets that cause it to send ICMP error packets, the device’s performance will be reduced.

l           As the redirection function increases the routing table size of a host, the host’s performance will be reduced if its routing table becomes very large.

l           If a host sends malicious ICMP destination unreachable packets, end users may be affected.

To prevent such problems, you can disable the device from sending ICMP error packets.

Follow these steps to disable sending ICMP error packets:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Disable sending ICMP redirection packets	undo ip redirects	Required Enabled by default.
Disable sending ICMP timeout packets	undo ip ttl-expires	Required Enabled by default.
Disable sending ICMP destination unreachable packets	undo ip unreachables	Required Enabled by default.

 

 

2.5  Displaying and Maintaining IP Performance

To do…	Use the command…	Remarks
Display current TCP connection state	display tcp status	Available in any view
Display TCP connection statistics	display tcp statistics
Display UDP statistics	display udp statistics
Display IP packets statistics	display ip statistics
Display ICMP flows statistics	display icmp statistics
Display socket information	display ip socket [ socktype sock-type ] [ task-id socket-id ]
Display FIB forward information	display fib [ | { begin | include | exclude } string | acl acl-number | ip-prefix ip-prefix-name ]
Display FIB forward information matching the specified destination IP address	display fib ip-address1 [ { mask1 | mask-length1 } [ ip-address2 { mask2 | mask-length2 } | longer ] | longer ]
Display statistics about the FIB items	display fib statistics
Clear statistics of IP packets	reset ip statistics	Available in user view
Clear statistics of TCP connections	reset tcp statistics
Clear statistics of UDP flows	reset udp statistics