default route as interface

[Mary Poppins]

Hi,

I examine the ping in a test config in the packet tracer. The config:

PC1 - Switch - (Fa0/1) RouterA (Fa0/0)  - (Fa0/0) RouterB (Fa0/1) - PC2

I configured the routers default gateway as follows:

RouterA : ip router 0.0.0.0 0.0.0.0 fastethernet 0/0

RouterB : ip router 0.0.0.0 0.0.0.0 fastethernet 0/0

I pinged the PC1 from PC2, but the ping is dropped at the routerB, in the process when it wanted to create the output pdu at layer2.

When I changed the default gateway style as

on routerA : ip router 0.0.0.0 0.0.0.0 200.200.200.2

on routerB : ip router 0.0.0.0 0.0.0.0 200.200.200.1

where routerA Fa0/0 ip address is 200.200.200.1 and routerB Fa0/0 ip is 200.200.200.2

I think in the first case the process can't get the ip address of the gateway so there is no gateway mac address for the layer 2 frame. While in the second case the gateway exact ip address is known.

My question is: if am I right, in what situation is the "ip route <addr> <mask> <interface>" format used for?

thank you

[liudongbo · Accepted solution]

I think that you can try to enable arp proxy on RouterA and RouterB, this is the reason for packets drop on the Router.

in what situation is the "ip route " format used for?

narmally, it is for P2P interface or for interface detection.

[ShivamNaik08815]

should we use  a fully specified static route for point-to-point serial connections ?

[Richard Burts]

If the outbound interface is a point to point serial interface then you have choices. It is certainly ok to use a fully specified static route. To the extent that a fully specified static route makes it easier to analyze and to troubleshoot it might be preferable. But it is not an issue if you use a static route that specifies only the outbound interface when the outbound interface is point to point serial. The real question is whether the router needs a layer 2 address (in addition to the layer 3 address) to forward an IP packet. For point to point serial no layer 2 address is required so the simple static route specifying the outbound interface is satisfactory. But for multipoint or broadcast interfaces the layer 2 address is required and a static route specifying only the outbound interface is problematic.

Note that I describe this as problematic and not as impossible. As noted in previous responses enabling proxy arp can allow the simple static route to operate successfully. But it makes the router work harder, uses more bandwidth on the outbound interface, creates a larger arp table which uses more memory and will require more cpu cycles to maintain the arp table. So the preferable implementation on multipoint or broadcast interfaces is a static route that specifies next hop (or specifies both next hop and outbound interface).

[Joseph W. Doherty]

BTW, one of the issues that Rick mentions, i.e. ". . . creates a larger arp table which uses more memory . . ." can be so severe, on some routers, it can exhaust all available memory.  (Usually happens on "small" routers doing this with an Internet facing interface.)

[Harold Ritter]

Hi,

I addition to what Dongbo wrote, it is generally not a good thing to configure the default route using only the physical interface as a next hop is this interface is a broadcast interface. In more recent IOS versions, we issue the following warning message if you do so.

R1(config-if)#ip route 0.0.0.0 0.0.0.0 e1/0

%Default route without gateway, if not a point-to-point interface, may impact performance

Regards

[Mary Poppins]

Ohh I see,

Because in the p2p environment there is only 2 participant, so it is obvious which interface on the segment is the gateway.

as far as i can see in packet tracer there is no proxy arp

thank you for your help

have a nice day

[ashirkar]

Hello Mary,

Hope your doing well,Please go through this documet "Static Routes With Next Hop As An Exit Interface Or An IP Address".It may help you.

Regards,

Ashish Shirkar

Community Manager - NI

[Mary Poppins]

I have just read it

"2) If you configured static route point to outgoing interface, forwarding router assume destination address is directly connected to that interface and router will try to find the L2 address of the destination by sending ARP request out of the interface to the destination address in case of Ethernet or looking for a static/dynamic map entry in the mapping table in case of frame-relay."

So if I undersatnd it well, this kind of routing can't be work without proxy arp in the other side, because the router think that every forwarding traffic destination is the on its outgoing interface segment, and try to find its MAC address with an apr request on that outgoing interface. Hmm.
The page you linked said in this scenario (with proxy arp enabled) there will be a large amount of arp request and a larger arp table in that router. So I think it should be usable in an environment where the router (on which the default gateway given with the interface format) doesn't have to forward to a lot of different destination. Or is it a wrong hypothesis? Is there environment where this is useful?

thank you

[ashirkar]

Hello Mary,

It is recommended to use next hop as ip address than interface for multi-access interface like Ethernet, Frame-relay point to multipoint interfaces .let's assume if you configure default route on internet router pointing interface next-hop to interface no than ip address, So for every destination you will have an arp entry in table ,you can think how many arp entries will be there .Adding ARP for Ethernet /or static L2 mapping in case of FR will be just a workaround in lab topology .I just Provided info for user better understanding. Hope that clears to you, Let me know if you have anything in your mind

Regards,

Ashish Shirkar

Technical Community Manager -NI

[Mary Poppins]

yes, it is completly clear

thank you very much

have a nice weekend for everybody