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Default route going out through interface

I currently have my default route set to be ip route 0.0.0.0 0.0.0.0 FastEthernet0/0 on my 2611XM.  The problem with this is that it causes the ARP table to be huge as it sends requests out for every single connection through the router.  This results in the first packet being dropped for new IP addresses and causes a noticeable delay when browsing web pages upon the first connection.  Needless to say, this is very annoying.  I know one way to fix this is to assign the next hop an IP address as opposed to an interface.  The problem here is that my IP address through my ISP is DHCP assigned.  Is there a way to resolve this issue without having the next hop as an IP address or asking the ISP to provide me a static address?

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Accepted Solutions
Hall of Fame Super Gold

Re: Default route going out through interface

Andy

You have correctly identified several of the issues when you point a static route (and most especially a static default route) at an Ethernet interface. In addition to the size of the ARP table and the dropped packets while the router sends the ARP request there are several other potential problems:

- this depends on the next hop supporting proxy arp. If the next hop device does not support proxy arp then your static route will not work - and in my experience increasingly network administrators are disabling proxy arp due to security concerns.

- in addition to the memory used by the very large ARP table it will increase CPU utilization - remember that every entry in the ARP table is flushed and re-learned every 4 hours. So the router CPU is removing entries, sending an ARP request, receiving an ARP response, and building a new entry in the ARP table.

Depending on the version of software that you are running, there should be an option in configuring yout static route to specify DHCP where you would normally put the next hop address.

Reza

It is not usually true that you get a different IP address every time that the lease expires. Especially when the interface remains up, when the lease expires the client will usually negotiate to obtain the same IP address, and usually does get the same address assigned.

It is certainly true that sometimes you will get a different IP address when using DHCP and that could impact the routing logic. But for using it on a router it would normally not be every time that the lease expires.

HTH

Rick

4 REPLIES
VIP Super Bronze

Re: Default route going out through interface

asilva203 wrote:

I currently have my default route set to be ip route 0.0.0.0 0.0.0.0 FastEthernet0/0 on my 2611XM.  The problem with this is that it causes the ARP table to be huge as it sends requests out for every single connection through the router.  This results in the first packet being dropped for new IP addresses and causes a noticeable delay when browsing web pages upon the first connection.  Needless to say, this is very annoying.  I know one way to fix this is to assign the next hop an IP address as opposed to an interface.  The problem here is that my IP address through my ISP is DHCP assigned.  Is there a way to resolve this issue without having the next hop as an IP address or asking the ISP to provide me a static address?

Hi Andy,

That is common issue and what I would suggest is to get in touch with your service provider to get a static IP and use that.  With DHCP every time the lease expires you will have a different IP address and that can cause down times.  In addition when you have a set IP address troubleshooting is easier then when you have dynamic IP address.

HTH

Reza

Re: Default route going out through interface

I currently have my default route set to be ip route 0.0.0.0 0.0.0.0
FastEthernet0/0 on my 2611XM.  The problem with this is that it causes
the ARP table to be huge as it sends requests out for every single
connection through the router.  This results in the first packet being
dropped for new IP addresses and causes a noticeable delay when
browsing web pages upon the first connection.  Needless to say, this is
very annoying.  I know one way to fix this is to assign the next hop an
IP address as opposed to an interface.  The problem here is that my IP
address through my ISP is DHCP assigned.  Is there a way to resolve
this issue without having the next hop as an IP address or asking the
ISP to provide me a static address?

Hi,

ARP Input uses up huge amounts of memory when a static route points to a broadcast interface,Every packet for the default route is sent to the FastEthernet0/0. However, there is no next hop IP address specified, and so, the router sends an ARP request for the destination IP address. The next hop router for that destination replies with its own MAC address, unless Proxy ARP is disabled. The reply from the router creates an additional entry in the ARP table where the destination IP address of the packet is mapped to the next-hop MAC address.


Do not create a static default route to an interface, especially if the interface is broadcast (Ethernet/Fast Ethernet/GE/SMDS) or multipoint (Frame Relay/ATM). Any static default route must point to the IP address of the next hop router. After you change the default route to point to the next hop IP address, use the clear arp-cache command to clear all the ARP entries. which can fixes the memory utilization problem aslo in routers.

Hope to Help !!

Ganesh.H

Hall of Fame Super Gold

Re: Default route going out through interface

Andy

You have correctly identified several of the issues when you point a static route (and most especially a static default route) at an Ethernet interface. In addition to the size of the ARP table and the dropped packets while the router sends the ARP request there are several other potential problems:

- this depends on the next hop supporting proxy arp. If the next hop device does not support proxy arp then your static route will not work - and in my experience increasingly network administrators are disabling proxy arp due to security concerns.

- in addition to the memory used by the very large ARP table it will increase CPU utilization - remember that every entry in the ARP table is flushed and re-learned every 4 hours. So the router CPU is removing entries, sending an ARP request, receiving an ARP response, and building a new entry in the ARP table.

Depending on the version of software that you are running, there should be an option in configuring yout static route to specify DHCP where you would normally put the next hop address.

Reza

It is not usually true that you get a different IP address every time that the lease expires. Especially when the interface remains up, when the lease expires the client will usually negotiate to obtain the same IP address, and usually does get the same address assigned.

It is certainly true that sometimes you will get a different IP address when using DHCP and that could impact the routing logic. But for using it on a router it would normally not be every time that the lease expires.

HTH

Rick

Community Member

Re: Default route going out through interface

Rick, great answer.  I've always been so accustomed to putting in a static IP or Ethernet interface for a default route, that I never thought check the other options available.  Thanks for the help everybody.

Andy

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