1596 CHAPTER 81: GRE C ONFIGURATION■ If a device needs to reference a link aggregation group for the tunnel interfaceto receive and send packets, the link aggregation group must already exist.Otherwise, the tunnel interface cannot work properly and the tunnel cannotcommunicate. For devices that do not support link aggregation, no linkaggregation group ID needs to be configured on the tunnel interface.■ You can enable or disable the checksum function at both ends of the tunnel asneeded. If the checksum function is enabled at the local end but not at theremote end, the local end calculates the checksum of a packet to be sent butdoes not check the checksum of a received packet. Contrarily, if the checksumfunction is enabled at the remote end but not at the local end, the local endchecks the checksum of a received packet but does not calculate the checksumof a packet to be sent.■ When you configure a route through the tunnel, you can configure a staticroute, whose destination address is the destination address of the packet notencapsulated in GRE and next hop is the address of the tunnel interface at theremote end. Or, you can enable the dynamic routing protocol on both thetunnel interface and the router interface connecting the private network sothat the dynamic routing protocol can establish a routing entry that allows thetunnel to forward packets through the tunnel. It is not allowed to set up astatic route whose destination address is in the subnet of the tunnel interface.Displaying andMaintaining GREGRE over IPv4 TunnelConfigurationExampleNetwork requirementsRouter A and Router B are interconnected through the Internet. Two private IPv4subnets Group 1 and Group 2 are interconnected through a GRE tunnel betweenthe two routers.Network diagramFigure 444 Network diagram for a GRE over IPv4 tunnel (on routers)Configuration procedure1 Configure Router A# Configure an IPv4 address for interface Ethernet 1/0. system-view[RouterA] interface ethernet 1/0To do... Use the command... RemarksDisplay information about a specified orall tunnel interfacesdisplay interface tunnel[ number ]Available in any viewDisplay IPv6 information about a tunnelinterfacedisplay ipv6 interfacetunnel numberAvailable in any viewIPv4Group2IPv4Group 1Eth1/010.1.1.1/24Eth1/010.1.3.1/24Tunnel010.1.2.1/24S2/01.1.1.1/24S2/12 .2.2.2/244 networkGRE tunnelTunnel 010 .1.2.2/24Router A Router BIPv
