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PROS/CONS of IP routing using the Interface instead of IP address

Posted on 2003-11-10
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Last Modified: 2012-06-21
We use static routes, no routing protocols.

Rather than setting up a bunch of /30 bit subnets I use "ip unnumbered".

Then route to the interface.

ip route 0.0.0.0 0.0.0.0 e1/0

Is there any benefit that I am missing to using subnets and routing to IP addresses intead of interfaces?

Would it make any differecnce if we were using RIP or OSPF?

Thanks.
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Question by:praflik
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6 Comments
 
LVL 79

Expert Comment

by:lrmoore
ID: 9719488
Yes, there are detrimental effects of using the interface.

From Cisco documentation:
If you point a static route to a broadcast interface, the route will be inserted into the routing table only when the broadcast interface is up. This configuration is not recommended because when the next hop of a static route points to an interface, the router considers each of the hosts within the range of the route to be directly connected through that interface. For example, ip route 0.0.0.0 0.0.0.0 Ethernet0.

With this type of configuration, a router will perform Address Resolution Protocol (ARP) on the Ethernet for every destination the router finds through the default route because the router will consider 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 will prevent 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.

ReF:
http://www.cisco.com/en/US/tech/tk365/tk80/technologies_tech_note09186a00800ef7b2.shtml

My preference is to always use /30 subnets for WAN links. You can use ODR without running any dynamic protocol such as RIP or OSPF or EIGRP:
http://www.cisco.com/en/US/tech/tk365/tk810/technologies_white_paper09186a0080093fde.shtml
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Expert Comment

by:NicBrey
ID: 9721556
Just adding to what lrmoore said:

The only time when a interface can be down and will appear to be up, is when using  DDR (dial on demand routing)
If you look at the interface you'll see :
router#sh int dialer 1
Dialer1 is up (spoofing), line protocol is up (spoofing)

The spoofing of the interface will allow the route to be kept in the routing table.



Local interface i.s.o. next hop address also have a different administrative distance.
Every route in a routing table have an administrative distance.  They are as follows:

0     for a static route pointing to a local interface
1     for a static route pointing to a next hop address
90   for EIGRP
100 for IGRP
110 for OSPF
120 for RIP

etc etc

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Author Comment

by:praflik
ID: 9724097
I mispoke earlier.  The default route does not go to an Enternet Interface; it does go to a serial interface.

Actual router config.

ip route 0.0.0.0 0.0.0.0 Serial0/0.1
ip route 0.0.0.0 0.0.0.0 Serial1/0.1 10

Will I have the same "arp" problem mentioned above?  Does ARP happen on a Serial Interface?
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Expert Comment

by:lrmoore
ID: 9724727
Yes, same issues.
Did you read the link in my post? It specifically addresses serial interfaces.

With both of these statements, the second is totally irrelevant because the first one will always be used. If you intended to have a higher admin cost, you must remove the first one.

>ip route 0.0.0.0 0.0.0.0 Serial0/0.1
>ip route 0.0.0.0 0.0.0.0 Serial1/0.1 10

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Author Comment

by:praflik
ID: 9725168
Yes I read the link in your post.  By my reading there are two issues addressed.

One issue was the ARP issue when using a broadcast address routed to an interface.  In their explination they used an Ethernet Interface as the example route (ip route 0.0.0.0 0.0.0.0 Ethernet0.)

The other issue is the "the floating static route never gets installed in the routing table when the primary link is shut down" problem in which they used a separate example involving serial interfaces.  Although they specify "ip route 0.0.0.0 0.0.0.0 Serial3/0" as a route in the second example they never indicate that there is any problem with it; the example refrences only the other routes.  Furthermore, when they apply the commands to fix R1's configuration they don't make a change to "ip route 0.0.0.0 0.0.0.0"  Maybe I'm just being obtuse.

The intention of routing as above was to use Serial0/0.1 as the primary route and Serial1/0.1 as a backup route.  From what you've said and what I've read...if Serial0/0.1 ever goes down the backup route S1/0.1 won't work.

Apparently we've been lucky and the primary had never gone down because we've never had downtime.
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Accepted Solution

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lrmoore earned 50 total points
ID: 9725285
I misread the config you posted and did not see the S0 and S1..

As standard practice, I always use an explicit IP address in a static route.

As shown in the link for R1:
S*   0.0.0.0/0 is directly connected, Serial3/0

For R2
S*   0.0.0.0/0 [1/0] via 10.10.10.1

Notice that a "directly connected" interface will not always go down just because L3 connectivity is lost (i.e. PVC goes down, but interface stays up)

Also notice, that the "solution"
R1(config)#ip route 172.31.10.0 255.255.255.0 Serial3/2 10.10.10.2
R1(config)#ip route 172.31.10.0 255.255.255.0 Serial3/3 192.168.20.2 250

Adds both the interface, and the remote IP address..

This statement is made irrespective of the interface type (Ethernet vs Serial)
"Specifying a numerical next hop on a directly connected interface will prevent 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."


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