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New Member

Path to WAN help

I have a 4503 as the Core on the LAN. It is connected via Gi2/1 to a 3845 Router (Gi0/0 I believe). This Router will be its way out to the internet.

I'm trying to tell the switch to push all non-local traffic out through that one connected Router.

Am I correct with this:

Create a Vlan99 with 100.100.100.127 255.255.255.224

Switchport access vlan 69 to the Gi2/1 port on the 4503.

Put in a 'ip default-gateway 100.100.100.128'

Give the Gi0/0 interface on the Router an IP of 100.100.100.228 255.255.255.224

Assuming that the Router has connectivity to the outside world already via another Interface, would this be the correct path to take?

I know I'm probably missing a lot of variables... but any direction would be appreciated.

  • Other Network Infrastructure Subjects
1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: Path to WAN help

If the 4503 will have EIGRP network statements then certainly you are planning to use the 4503 as a layer 3 router rather than just as a layer 2 switch. As a layer 3 router Martin's comments about routing updates and how to arrange default route are quite appropriate.

As I said the ip default-gateway is appropriate for a layer 2 switch. If you are going to use the 4503 as a layer 3 I would not necessarily remove the ip default-gateway config, but you should understand its limited functionality. The ip default-gateway works for IP "hosts" not for IP routers. Its function is to identify the default gateway for the IP host. That is certainly the familiar concept in configuring PCs. The host needs a default gateway to reach destinations in "remote" networks. A layer 2 switch has an IP address for management purposes and the layer 2 switch functions as an IP host and needs a default gateway.

But the ip default-gateway does not define a default route. A "router" needs a default route. I frequently configure routers to include the ip default-gateway command because there are a few (but only a very few) situations where the default-gateway command is useful on a router: if you disable routing and enable bridging the router will need a default-gateway. If the router does an rx-boot the rxboot image operates as an IP host not as a router.

So I would leave the ip default-gateway configured. But do not be confused and assume that it will supply the default route that you need.

HTH

Rick

7 REPLIES
Purple

Re: Path to WAN help

It wouldn't be vlan 69 , probably a typo .

Create vlan 99 as you did . Create the layer 2 vlan ,99 . Apply the vlan to the interface you want to use G2/1 (switchport access vlan 99) . Do not use the default-gateway command , not what its used for . Point a default static route to 100.100.100.128 .

Hall of Fame Super Silver

Re: Path to WAN help

I am not clear whether you intend to use the 4503 as just a layer 2 switch or whether you intend to use its capabilities as a layer 3 switch. the use of ip default-gateway implies its use as a layer 2 switch. If the 4503 is operating as just a layer 2 switch then the thing that is important is where is the default gateway that is configured on the end stations? As a layer 2 switch the only traffic that is influenced by the ip default-gateway is management traffic from the switch.

If the 4503 is to operate as a layer 3 device then there are several things that we should look at differently. Perhaps you can clarify how you are planning to use it.

Also in your example is you use the 224 mask then the .127 address would be a broadcast address and the .128 address would be a subnet address. Neither of these is appropriate as an interface address.

HTH

Rick

New Member

Re: Path to WAN help

Hmm,

I apologize, it appears I made a few typos in my post. I think the other persons comment was helpful in that it gave me a very simple scenerio in how to set it up, but now I'm wondering if there is more I need to do.

This 4503 is going to be a Core switch that will be routing many different Vlans. It will have EIGRP statements also.

I guess I don't quite understand the differences in "ip default-gateway" and the default route line. My thinking was that any type of traffic that wasn't destined for LAN (routed via Vlan, the 4503), would be passed on to the default-gateway (or now, the default route). Am I completely wrong in this? Or is this statement unclear?

Thanx so much for both responses... much appreciated.

Re: Path to WAN help

Hi,

as you are using the 4503 as a router it needs a default route in the IP routing table pointing to the internet gateway. You could achieve this by configuring a static default route pointing to the internet gateway (3845) as already stated above.

Example:

ip route 0.0.0.0 0.0.0.0 100.100.100.200

(assuming 100.100.100.200 is the interface IP of 3845).

A second option would be to let the 3845 announce a default route through EIGRP to your 4503.

Hope this helps

Martin

Hall of Fame Super Silver

Re: Path to WAN help

If the 4503 will have EIGRP network statements then certainly you are planning to use the 4503 as a layer 3 router rather than just as a layer 2 switch. As a layer 3 router Martin's comments about routing updates and how to arrange default route are quite appropriate.

As I said the ip default-gateway is appropriate for a layer 2 switch. If you are going to use the 4503 as a layer 3 I would not necessarily remove the ip default-gateway config, but you should understand its limited functionality. The ip default-gateway works for IP "hosts" not for IP routers. Its function is to identify the default gateway for the IP host. That is certainly the familiar concept in configuring PCs. The host needs a default gateway to reach destinations in "remote" networks. A layer 2 switch has an IP address for management purposes and the layer 2 switch functions as an IP host and needs a default gateway.

But the ip default-gateway does not define a default route. A "router" needs a default route. I frequently configure routers to include the ip default-gateway command because there are a few (but only a very few) situations where the default-gateway command is useful on a router: if you disable routing and enable bridging the router will need a default-gateway. If the router does an rx-boot the rxboot image operates as an IP host not as a router.

So I would leave the ip default-gateway configured. But do not be confused and assume that it will supply the default route that you need.

HTH

Rick

New Member

Re: Path to WAN help

Wow,

Much thanks to all the posts. They all have been extremely helpful. I tried rating them all but it appears only one went through :(

Thank you for the explanation of default-gateway, Rick. My only question is that in my situation, why would I even leave the default-gateway line if the Core is doing Layer3? Any host that connects to the network is going to have an IP address that belongs to a set Vlan, so wouldn't all the traffic be directed to the default route? (0.0.0.0 line)

I guess I'm not fully understanding why a packet would use default-gateway over the default route.

And as far as setting the default route, how would I do this via EIGRP? And is there an advantage to this?

Thanks again all

Hall of Fame Super Silver

Re: Path to WAN help

Thanks for trying to rate the posts. It is helpful to have feedback about posts that were helpful.

I basically look at ip default-gateway as insurance. A router will not need ip default-gateway over 99% of the time. But what if one of those rare situations occurs and the device (router or switch doing layer 3 routing) is behaving as an IP host instead of a router? I frequently leave the ip default-gateway in the config just to cover that possibility.

And let me try to address another part of your question about why a packet would use default-gateway rather than using default route. The difference is whether the device is operating as a layer 3 "router" (needs default route not default gateway) or is operating as an IP "host" (needs default gateway not default route).

As far as setting the default route there are some advantages in setting it and advertising it through EIGRP. One of the advantages is that the EIGRP routing is dynamic and can react to changes that take place in the network. Another advantage is the amount of work required: if you advertise it in EIGRP you have to set the default route once and then EIGRP advertises it. If you do not advertise the default route with EIGRP then you will need to set the default route on EVERY router in the network.

HTH

Rick

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