Table of Contents

1 IP Performance Configuration· 1-1

IP Performance Overview· 1-1

Enabling Reception and Forwarding of Directed Broadcasts to a Directly Connected Network· 1-1

Enabling Reception of Directed Broadcasts to a Directly Connected Network· 1-1

Enabling Forwarding of Directed Broadcasts to a Directly Connected Network· 1-2

Configuration Example· 1-2

Configuring TCP Attributes· 1-3

Configuring TCP Optional Parameters· 1-3

Configuring ICMP to Send Error Packets· 1-4

Displaying and Maintaining IP Performance· 1-6

 

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

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 TCP timers

l          Configuring the TCP buffer size

l          Enabling ICMP error packets sending

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 switch to receive and forward directed broadcasts to a directly connected network will give hackers an opportunity to attack the network. Therefore, the switch is disabled from receiving and forwarding directed broadcasts by default. However, you should enable the feature when using the Wake on LAN function to forward directed broadcasts to a host on the remote network.

Enabling Reception of Directed Broadcasts to a Directly Connected Network

If a switch is enabled to receive directed broadcasts, the switch 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:

 

Enabling Forwarding of Directed Broadcasts to a Directly Connected Network

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

 

 

Configuration Example

Network requirements

As shown in Figure 1-1, 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.

Network diagram

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

 

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 cannot be received by the VLAN-interface 2 of Switch B.

Configuring TCP Attributes

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:

 

 

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.

Advantages 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, telling it to reselect a correct next hop 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 starts 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 that 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.

Disadvantages of sending ICMP error packets

Although sending ICMP error packets facilitates 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 of ICMP error packets:

 

 

Displaying and Maintaining IP Performance