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Silver

Exit Interface vs Next-hop

Book suggests to use exit interface instead of using next-hop ip address. But while preparing for exam i noticed next-hop address is configured especially when configuring default route on edge router towards ISP. In what other situations we prefer next-hop ip address than exit interface?

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Posted by WebUser Haroon David Peter

5 REPLIES
VIP Super Bronze

Exit Interface vs Next-hop

You should always try to use the IP address for the next-hop and not the exit interface:

a router performs Address Resolution       Protocol (ARP) on the Ethernet for every destination the router finds through       the default route because the router considers all of these destinations as       directly connected to Ethernet 0.

This kind of default route, especially if it is used by a lot of       packets to many different destination subnets, can cause high processor       utilization and a very large ARP cache (along with attendant memory allocation       failures).

Specifying a numerical next hop on a directly connected interface       prevents the router from performing ARP or each destination address. However,       if the interface with the next hop goes down and the numerical next hop is       reachable through a recursive route, you should specify both the next hop IP       address and the interface through which the next hop should be found. For       example,       ip       route 0.0.0.0 0.0.0.0 Serial 3/3 192.168.20.1.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800ef7b2.shtml

HTH

Hall of Fame Super Silver

Exit Interface vs Next-hop

David

Reza makes some good points and I would like to re-inforce them - especially on Ethernet (or other multi access interfaces) it is much better to specify the next hop than just the output interface. The need to ARP for every outbound destination is the main reason to prefer static routes with next hop and the dependency on proxy arp is another reason.

And I agree with Reza that in some circumstances it is better to specify both the next hop and the outbound interface in a static route.

But I will note an exception to this advice. If the static route is using a point to point serial interface (HDLC, or PPP, or PTP Frame Relay subinterface) then a static route specifying just the outbound interface does not have these issues and should work just fine.

HTH

Rick

Silver

Exit Interface vs Next-hop

Thank you for explaining this.

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Posted by WebUser Haroon David Peter

Hall of Fame Super Silver

Exit Interface vs Next-hop

You are welcome. I am glad that our discussion has been helpful.

HTH

Rick

Exit Interface vs Next-hop

Hi,

To clear you more in the creation of route with next hope interface or IP address and which is preferable.

So, there are two methods to create static routes:

The ip route 10.1.1.1 255.255.255.0 eth 0/0 --> This generates an ARP broadcast that looks for the next-hop IP address.

The ip route 10.1.1.1 255.255.255.0 172.16.1.1 --> This does not generate an ARP request. It keeps Layer 2 out of the routing process.


So which is the better way to define a route? It must be using next hope IP address as we doesn't want to generate to much of broadcast in our Network...


Please rate the helpfull posts.
Regards,
Naidu.

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