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Knowledge Base

FAQ - Difference Between Configuring Only the Next Hop and Both Next Hop and Outbound Interface for a Static Route

Publication Date:  2013-09-24  |   Views:  222  |   Downloads:  0  |   Author:  92825  |   Document ID:  ETC0001084669


Issue Description

What is the difference between configuring only the next hop address and configuring both the next hop address and outbound interface for a static route on NE40E (VRP5)? 


When configuring a static route, you can either specify the outbound interface or the next hop address as required. Actually, every routing entry requires a next-hop address. When sending a packet, the device first searches for the matched route in the routing table according to the destination address. The link layer cannot find the associated link layer address to forward the packet unless the next-hop address of the packet is specified.

1. For a point-to-point interface, the next hop address is specified when you specify the outbound interface. That is, the address of the peer interface is the next hop address.

For example, the protocol used to encapsulate POS is the Point-to-Point protocol. The remote IP address is obtained through PPP negotiation. You need specify only the outbound interface rather than the next hop address.

2. The interface (such as ATM interface) of the NBMA type supports the point to multi-point network. You need to configure the IP route and the mapping from IP address to the link-layer address. This requires configuring of the next hop address.

3. You must specify the next hop address for an Ethernet or virtual-template (VT) interface.

This is because the Ethernet interface is a broadcast interface whereas the VT interface can be associated with several virtual access (VA) interfaces. Therefore, if the Ethernet interface or the VT interface is specified as the outbound interface, multiple next hops exist and the system cannot decide which next hop is to use.

In actual applications, when specifying a broadcast interface (such as an Ethernet interface), a VT interface, or an NBMA interface as the outbound interface, you must specify the associated next-hop address.

A static route with only the next hop address specified can participate in route selection only when its next hop is successfully iterated. A static route with both the next hop address and outbound interface specified can directly participate in route selection. Routes that are selected can be delivered to the FIB and guide packet forwarding. 

For example:

ip route-static Pos1/0/0 <--This route is with an outbound interface and route iteration is not required, with the iteration depth being 0. 

ip route-static <----This route is with no outbound interface and one time of iteration is required, with the iteration depth being 1. As a result, since BGP routes are iterated based on the next hop addresses, the static route with the smallest iteration depth is selected.