ARP Table with Internet Addresses in it?

[Matthew Martin]

Hello All,

I believe the answer to my question was asked and answered here. But, I would just like to verify if the same thing applies to me.

There are basically 2 interfaces of importance. Serial0/1/0:0 is for our MPLS private link and Gi0/0/0 is connected to a Local Broadband Cable modem for Internet only access. Since the local broadband modem would not allow me to modify the local LAN addressing, I had to configure Gi0/0/0 with DHCP.

The IP Routing table is getting all its routes from BGP, which should send all private subnets back over the MPLS (*i.e. the Serial interface). So I had configured a default Route, which should send everything else through the Local Broadband Modem (*Gi0/0/0), which works just fine.

!
interface GigabitEthernet0/0/0
 description Broadband Internet Gateway
 ip dhcp client client-id GigabitEthernet0/0/0
 ip address dhcp
 ip nat outside
 zone-member security INTERNET
 media-type rj45
 negotiation auto
!
!.....cut.....
!
interface Serial0/1/0:0
 ip address <ip-address> 255.255.255.252
 zone-member security WAN
 encapsulation ppp
 service-policy output AutoQoS-Policy-Trust
!
!.....cut.....
!
ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0/0 dhcp
!

So, am I supposed to remove the "ip route 0.0.0.0 ...." command from the config, is the command not necessary?

Also, if I do remove the default route command, will all Non-BGP routes (*routes to the Internet) still go through the Gi0/0/0 interface?

Thanks in Advance,

Matt

[Richard Burts · Accepted solution]

Matt

I am glad that you found that discussion in which I was a participant. Basically yes you have the same issue.

First let us deal with why your arp table has Internet addresses in it and then we can talk about how to fix it. The issue is caused by the format of your default route. When a route is configured which points at the exit interface but does not specify a next hop (and if the interface is Ethernet) then IOS considers everything reached through that interface as directly connected. And since it considers them directly connected IOS will arp for every address it attempts to forward through that interface. And so every Internet address that you attempt to access will cause an entry in the arp table for that address.

The simple part of how to fix this issue is that since it is cause by a static route specifying exit interface and not specifying a next hop you either remove the reference to the exit interface or you specify a next hop. Since you are using DHCP it gets a bit tricky to use a next hop address. But you ought to be able to figure out what is the address of the ISP device by observing traffic from that device and use that as the next hop. But the simple answer (and I believe the better answer) is to remove the Gig interface from the static route and simply specify that the default route is learned by DHCP.

HTH

Rick

[Matthew Martin]

Hey Rick, thanks for the reply and the detailed explanation, very much appreciated!

So to change the route like you suggested, to have the default route learned by DHCP, would the route just look like this, below?

ip route 0.0.0.0 0.0.0.0 dhcp

Thanks Again,
Matt

[Richard Burts]

Matt

Yes that is what the route statement would look like.

HTH

Rick

[Matthew Martin]

Ok cool, thanks again for the help!

-Matt

[Richard Burts]

Matt

You are welcome. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. I look forward to seeing you active in the community.

HTH

Rick