Contents

Configuring IP forwarding basic settings························································ 1

About FIB table······························································································································· 1

Keeping the TTL unchanged in packets passing through forwarding devices to a specific destination·· 2

Keeping the HopLimit unchanged in IPv6 packets passing through forwarding devices to a specific destination    2

Displaying and maintaining FIB table································································································ 2

Configuring IP forwarding basic settings

About FIB table

A device uses the FIB table to make packet forwarding decisions.

A device selects optimal routes from the routing table, and puts them into the FIB table. Each FIB entry specifies the next hop IP address and output interface for packets destined for a specific subnet or host.

For more information about the routing table, see Layer 3—IP Routing Configuration Guide.

Use the display fib command to display the FIB table. The following example displays the entire FIB table.

<Sysname> display fib

Destination count: 4 FIB entry count: 4

Flag:

  U:Usable   G:Gateway   H:Host   B:Blackhole   D:Dynamic   S:Static

  R:Relay     F:FRR

Destination/Mask   Nexthop         Flag     OutInterface/Token       Label

10.2.0.0/16        10.2.1.1        U        GE1/0/1                  Null

10.2.1.1/32        127.0.0.1       UH       InLoop0                  Null

127.0.0.0/8        127.0.0.1       U        InLoop0                  Null

127.0.0.1/32       127.0.0.1       UH       InLoop0                  Null

A FIB entry includes the following items:

·     Destination—Destination IP address.

·     Mask—Network mask. The mask and the destination address identify the destination network. A logical AND operation between the destination address and the network mask yields the address of the destination network. For example, if the destination address is 192.168.1.40 and the mask 255.255.255.0, the address of the destination network is 192.168.1.0. A network mask includes a certain number of consecutive 1s. It can be expressed in dotted decimal format or by the number of the 1s.

·     Nexthop—IP address of the next hop.

·     Flag—Route flag.

·     OutInterface—Output interface.

·     Token—MPLS Label Switched Path index number.

·     Label—Inner label.

Keeping the TTL unchanged in packets passing through forwarding devices to a specific destination

About this task

With this task performed, the device keeps the value unchanged in the TTL field of an incoming packet if the packet is destined for the specified IPv4 address.

Software version and feature compatibility

This feature is available only in Release 5111 and later.

Procedure

1.     Enter system view.

system-view

2.     Prevent the device from changing the TTL in an incoming packet destined for a specific destination IPv4 address.

forwarding ttl-unvaried destination [ vpn-instance vpn-instance-name ] ip-address

By default, the device decrements the TTL in an incoming packet by one.

Keeping the HopLimit unchanged in IPv6 packets passing through forwarding devices to a specific destination

About this task

With this task performed, the device keeps the value unchanged in the HopLimit field of an incoming IPv6 packet if the packet is destined for the specified IPv6 address.

Software version and feature compatibility

This feature is available only in Release 5111 and later.

Procedure

1.     Enter system view.

system-view

2.     Prevent the device from changing the hop limit in an incoming IPv6 packet destined for a specific destination IPv6 address.

forwarding hop-limit-unvaried destination [ vpn-instance vpn-instance-name ] ipv6-address

By default, the device decrements the hop limit in an incoming IPv6 packet by one.

Displaying and maintaining FIB table

To display FIB entries, execute the following command in any view:

display fib [ vpn-instance vpn-instance-name ] [ ip-address [ mask | mask-length ] ]