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NE20E-S V800R010C10SPC500 Feature Description - Interface and Data Link 01

This is NE20E-S V800R010C10SPC500 Feature Description - Interface and Data Link

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Null0 Interface

Null0 Interface

Application Scenario

The Null0 interface does not forward packets. All packets sent to this interface are discarded. The Null0 interface is applied in two situations:

  • Loop prevention

    The Null0 interface is typically used to prevent routing loops. For example, during route aggregation, a route to the Null0 interface is always created.

    In the example network shown in Figure 2-8, Device A provides access services for multiple remote nodes.

    Device A is the gateway of the local network that uses the Class B network segment address Device A connects to three subnets through Device B, Device C, and Device D respectively.

    Figure 2-8 Example for using the Null0 interface to prevent routing loops

    Normally, the routing table of Device A contains the following routes:

    • Routes to three subnets, namely,,, and

    • Network segment routes to Device B, Device C, Device D

    • Default routes to the ISP network

    If Device E on the ISP network receives a packet with the destination address on the network segment, it forwards the packet to Device A.

    If the destination address of the packet does not belong to the network segment to which Device B, Device C, or Device D is connected, Device A searches the routing table for the default route, and then sends the packet to Device E.

    In this situation, the packets whose destination addresses belong to the network segment but not the network segment to which Device B, Device C, or Device D is connected are repeatedly transmitted between Device A and Device E. As a result, a routing loop occurs.

    To address this issue, a static route to the Null0 interface is configured on Device A. Then, after receiving the packet whose destination network segment does not belong to any of the three subnets, Device A finds the route whose outgoing interface is the Null0 interface according to exact matching rules, and then discards the packet.

    Therefore, configuring a static route on Device A whose outgoing interface is the Null0 interface can prevent routing loops.

  • Traffic filtering

    The Null0 interface provides an optional method for filtering traffic. Unnecessary packets are sent to the Null0 interface to avoid using an Access Control List (ACL).

    Both the Null0 interface and ACL can be used to filter traffic as follows.

    • Before the ACL can be used, ACL rules must be configured and then applied to an interface. When a router receives a packet, it searches the ACL.

      • If the action is permit, the router searches the forwarding table and then determines whether to forward or discard the packet.

      • If the action is deny, the router discards the packet.

    • The Null0 interface must be specified as the outbound interface of unnecessary packets. When a router receives a packet, it searches the forwarding table. If the router finds that the outbound interface of the packet is the Null0 interface, it discards the packet.

    Using a Null0 interface to filter traffic is more efficient and faster. Using the Null0 interface for packet filtering only requires a route, but using the ACL for packet filtering requires an ACL rule to be configured and then applied to the corresponding interface on a router.

    The Null0 interface can filter only the router-based traffic, whereas the ACL can filter both the router-based and interface-based traffic.

Updated: 2019-01-03

Document ID: EDOC1100055118

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