problems with static default next hop

Answered Question
Aug 4th, 2010

I have an 891 router and I am configuring a static default pointing out one of the WAN links.  The problem is, specifying a next hop of the wan interface (Fa8 in this case) does not work.  However, specifying a next hop of DHCP works perfectly.

ip route 0.0.0.0 0.0.0.0 fastethernet8  (no work)

ip route 0.0.0.0 0.0.0.0 dhcp  (works)

Why would this be?

Thanks,

-J

I have this problem too.
0 votes
Correct Answer by manish arora about 6 years 4 months ago

I think the reason for the default route statement with a physical interface not working is that the upstream router is not sending any proxy-arp for the ip addresses you are requesting arp for.

In simple words, If you set up a default route to physical interface and you are trying to reach 4.2.2.2 , your router will look into its routing table and will not find any route for it other than a default route pointing to Fethernet 8, so it will think that 4.2.2.2 is directly connected to it from that interface and will try ARP for 4.2.2.2 , having not recieve anything than from the upstream router ( which is not sending any proxy-arp ) it will just drop it as it will not be able to make a frame for that address 4.2.2.2.

hope i am making any sense here

Manish

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
millerjw Wed, 08/04/2010 - 14:56

Yep Yep... that's exactly what I did.  It doesn't work if you specify a next hop of a physical interface. It takes the command, shows the command in the config, but won't route.

-J

millerjw Wed, 08/04/2010 - 15:06

sure is... it's one of the two wan ports on the router.  The 891 has two WAN interfaces.  Fa8 and Gig0.

-J

manish arora Wed, 08/04/2010 - 15:11

By next hop of physical interface , you mean the exit interface of the router right ?

manish

millerjw Wed, 08/04/2010 - 15:14

Yes, that is correct, Fa8 is the exit interface on the router.  The 891 has switchports Fa0-7 and wan interfaces fast 8 and gig 0.

-J

Correct Answer
manish arora Wed, 08/04/2010 - 15:30

I think the reason for the default route statement with a physical interface not working is that the upstream router is not sending any proxy-arp for the ip addresses you are requesting arp for.

In simple words, If you set up a default route to physical interface and you are trying to reach 4.2.2.2 , your router will look into its routing table and will not find any route for it other than a default route pointing to Fethernet 8, so it will think that 4.2.2.2 is directly connected to it from that interface and will try ARP for 4.2.2.2 , having not recieve anything than from the upstream router ( which is not sending any proxy-arp ) it will just drop it as it will not be able to make a frame for that address 4.2.2.2.

hope i am making any sense here

Manish

millerjw Wed, 08/04/2010 - 15:49

ahhhh... you may be correct here.  There is a no proxy-arp set on the dhcp gateway address. I will need to test this tomorrow am (as it's a production net)  thanks for the tip, I'll let you know how it works.

-J

millerjw Thu, 08/05/2010 - 07:37

yep, that was the problem.  Removing proxy-arp solved it.

-J

Actions

This Discussion