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ping with source specified

Posted on 2014-03-27
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Last Modified: 2014-04-08
When I ping the remote site from my core switch, it does not work. But when I specify "source vlan 10", it works. Why is that?
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Question by:leblanc
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rfportilla earned 167 total points
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Because whichever vlan the switch is using by default does not have access to the Internet, but VLAN 10 does.  Are you familiar with VLAN's?  They are used to segment traffic like a router.
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by:leblanc
ID: 39960540
My layer 3 core switch has a default route pointed to the distribution WAN layer 3 switch, then to the FW to access the internet. I have SVIs configured on my core switch. I believe all of my VLAN can access the Internet.
I did not have to specify the source vlan with the ping before I moved from layer 2 to layer 3 between the core switch and the distribution WAN layer 3 switch.
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by:giltjr
giltjr earned 333 total points
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What IP address does your L3 core use by default?

Maybe your WAN L3 switch or your firewall does not have a route back to that address/subnet?
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by:leblanc
ID: 39960679
On my L3 core, my default route is pointed to the next hop /30 of the WAN distribution L3 switch interface. All subnets are /24.
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by:rfportilla
ID: 39961338
I don't know your switch.  Is there a command to determine the default network interface and/or vlan?  I think there might be a default vlan that is used internally that might be default in the management if.  I've seen many switches with extra interfaces and vlans that don't seem like they are needed, but they exist (and not always easily visible).  

In either case, is this just a curiosity or is there an issue stemming from this?
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by:giltjr
giltjr earned 333 total points
ID: 39961362
Assuming your setup is like:

CORE-L3 < -- > WAN-L3 <----> Firewall

Typically a L3 switch (your CORE-L3) will use the IP address of the SVI based on your routing table.  So if you are ping'ing 1.1.1.1, what is the IP address of the router it will use as the next hop.

I would assume that it would be the IP address in CORE-L3 that is on the subnet between CORE-L3 and WAN-L3.

So first thing to check is does the firewall have a route back to that subnet.
Second thing to check is does the firewall have a policy that would allow ICMP to/from that subnet.
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