arp question

[niro]

I have an access switch plugged into our core with using a trunk port. To manage this switch I've added a vlan to it which also exists on the core (so it's routed throughout our network)...this switch also contains some vlans which are local to this switch and are not routed through the rest of the network. IP Routing is not enabled on the switch, and the trunk port only allows the routed vlan.

My problem is, for some reason when I try to connect to this switch it always fails, I can't even ping it. I'm getting an incomplete arp entry for my PC. I ran an arp debug and noticed that it's sending the arp request from one of the vlans that are not on the core...and the vlan is not allowed on the trunk....which would explain why it's not getting a response. How do I prevent that from happening?

[lgijssel]

It looks like the switch does not have a route to your PC. When the switch doesn't do ip routing, you need to set it's default gateway to an ip adress in the correct subnet on the core switch or other device that should perform the routing.

For protocols like telnet and snmp you can also "fix" the interface that must be used by configuring:

ip telnet source-interface

ip snmp source-interface

regards,

Leo

[niro]

That did not work. The source IP address requesting the mac on the switch is a network that the core does not have routes to...so even if I add the default route to the switch and enable ip routing it won't work...as the core won't know where to send the packets back.

The ip telnet source-int won't work either, as the switch is not even able to resolve my mac address.

Any other ideas?

[Edison Ortiz · Accepted solution]

Can you ping the switch from the Core?

You mentioned the switch does not have "ip routing" enable (which is fine) but have you configured a "ip default-gateway" on the switch so packets destined to other subnets know where to go?

Also, is the source workstation connected physically to this switch?

Can this workstation connect to the core (telnet)?

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Edison.

[niro]

ah ok...using ip default-gateway worked...ip route 0.0.0.0 0 didn't. I'm still a bit confused as to why it sourced the mac request from that vlan rather then one of the ones allowed on the trunk.

[Edison Ortiz]

ip route 0.0.0.0 0.0.0.0 will only work if you have ip routing enabled.

With ip routing disabled, you need ip default-gateway.

Without knowing where to go, it was picking a random Vlan, and the chosen Vlan was the one you saw in your debug.

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Edison.

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