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Community Member

Ping

Hi

In my router there are 2 fastethernet.

The fa0/0 is 10.100.253.253/16 and is connected in a linux server with 2 NIC, a with 10.100.253.254/16 and other with 10.0.100.120/16 with various host behind.

My qyestion is. Which is the difference between:

ip route 10.0.0.0 255.255.0.0 fastethernet0/0

and

ip route 10.0.0.0 255.255.0.0 10.100.253.254

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Ping

10.0.0.0 255.255.0.0 fastethernet0/0 --> When this static route is used the router would ARP out fastethernet0/0 int for any packet that has a destination address of 10.0.x.x. Unless the device (linux server) can proxy ARP on behalf of the host (destination address) this packet would fail.

As you can see this kind of static route should be avoided unless there's no other option. Instead of the linux server if you had a Cisco router it would have responded with proxy ARP on behalf of the destination host and pings would have succeeded. Proxy ARP has some other downside as well it can high CPU utilization on the router that does proxy ARP, high number of broadcasts (ARP) on the LAN etc.

With that said you are better off always pointing a static route to a next hop address rather than the egress interface to avoid these problems.

HTH

Sundar

5 REPLIES

Re: Ping

Hi,

The optimum way when dealing with static routes with Ethernet interfaces is to make the route to the outgoing interface + the next-hop - As if only using outgoing interface this will require a layer 3 to layer 2 resolution for all the destinations of all the routed packets through that route - memory (huge amount of entries in the ARP cache, an entry for each destination of routed packets) and CPU (ARP requests), and now since the next-hop is available in the route, the router will have to only do one layer 3 to layer 2 mapping, which is for the next-hop value - and the escense of using the outgoing interface with the next-hop is in order to omit the need of recursive lookups to find the outgoing interface and in order to directly remove the route from the routing table if the interface fails.

And thus i encourage you to use:

ip route 10.0.0.0 255.255.0.0 10.100.253.254 fastethernet0/0

HTH,

Mohammed Mahmoud.

Re: Ping

A very good explanation as usual from Mohammed.

I suggest you have a look at this post which should clear all your doubts

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddf370e

HTH

Narayan

Community Member

Re: Ping

Ok.

But, with both, the linux server it would have to recognize the packet to network 10.0.0.0/16,or not?

Because, in my customer, the linux server just recognized the packages using ip route 10.0.0.0 255.255.0.0 10.100.253.254.

With route

10.0.0.0 255.255.0.0 fastethernet0/0, it's dropped the packet!

The assembly of the package it's same, right?

Re: Ping

10.0.0.0 255.255.0.0 fastethernet0/0 --> When this static route is used the router would ARP out fastethernet0/0 int for any packet that has a destination address of 10.0.x.x. Unless the device (linux server) can proxy ARP on behalf of the host (destination address) this packet would fail.

As you can see this kind of static route should be avoided unless there's no other option. Instead of the linux server if you had a Cisco router it would have responded with proxy ARP on behalf of the destination host and pings would have succeeded. Proxy ARP has some other downside as well it can high CPU utilization on the router that does proxy ARP, high number of broadcasts (ARP) on the LAN etc.

With that said you are better off always pointing a static route to a next hop address rather than the egress interface to avoid these problems.

HTH

Sundar

Community Member

Re: Ping

Great

Very thanks

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