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Does Ping Packet go through Trunk port

Posted on 2016-10-19
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Last Modified: 2016-10-29
If you have L2 Switch with different VLANs, then you Trunk the switch port  to a router trunk port.
Now if PC1 on Vlan 10 pings VLAN 20 (both are on the same switch), then the Ping packet will go through the trunk port of the switch to the Trunk port of the router then back the same path.

in this case can we say Ping works at Layer 2 or at both Layer 2 and Layer 3 ?


I also believe(correct me if I am wrong) The IP routing at the router level works without any extra configuration , not like at the L3 switch level where you have to add the command "IP Routing". correct ?



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Question by:jskfan
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Expert Comment

by:skullnobrains
ID: 41849884
pings are IPs packets. IP is layer 3.

layer 3 packets don't "know" about your vlans. you can think of a vlan as a fake physical switch. IP packets will see it that way. they do not care about what actually happens on level 2

routing is activated by default in all routers as far as i know.
having IPs in each vlans will make the router natively route from one vlan to the next.
you don't need to create routes unless you have multiple routers in which case you need to tell to each router which of it's siblings has an ip in the destination lan
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Author Comment

by:jskfan
ID: 41855635
But Ping Packet will still have to go from the PC plugged to L2 Switch then  through Uplink Port (The Trunk port) and come back to the PC
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skullnobrains earned 2000 total points
ID: 41855671
yes

- your host sees the destination is in a different network -> it looks the path up in the routing table
- it selects the router's ip
- it broadcasts an arp query demanding for the mac corresponding to the router's ip
- the router answers
- it sends the ping
- the switch tags it in VLAN10 and forwards it on the trunk port
- the router receives it tagged, and routes it tagged on vlan20 on the same port
- the packet is traverses the switch, and is sent untagged to the destination host
- pretty much the same happens the other way round for the pong
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Author Closing Comment

by:jskfan
ID: 41865170
Thank you
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