Packet Flow

[Maro.Cisco]

PC--------------------L2switch-------------core----------------------L2switch

172.16.4.2           172.16.11.1           172.16.11.222               172.16.11.2

Vlan 4                     Vlan2                       Vlan2    (SVI)                Vlan2

                                                           172.16.4.222

                                                                Vlan4 (SVI)

PC wants to ping 172.16.11.2

1)ARP request for 172.16.4.222 from PC

2)packet will be sent from PC to Core with destination MAC of 172.16.4.222 and Destination IP of 172.16.11.2

3)ARP request for 172.16.11.2 from Core

4) Packet will reach 172.16.11.2

If L2Switch ( 172.16.11.2) doesnt have default gateway set how will it be able to reply to 172.16.4.2???

[daniel.dib · Accepted solution]

1) PC compares DST IP to local address and the address it not in same subnet.

2) ARP for default gateway at 172.16.4.222

3) Core replies with its MAC address for 172.16.4.222

4) PC sends packet with SRC IP = 172.16.4.2, DST IP = 172.16.11.2

5) L2 switch looks at DST MAC in VLAN 4, VLAN 4 must be configured on the switch

6) If DST MAC is known send it towards core otherwise flood it to all ports except incoming in VLAN 4

7) Frame is sent towards core, tagged with VLAN 4

8) Packet arrives to core, pop VLAN tag. DST MAC = SVI VLAN 4 so process the packet

9) Look up 172.16.11.2 in routing table

10) Send packet to 172.16.11.2 with SRC IP = 172.16.4.2, DST IP = 172.16.11.2, SRC MAC = SVI VLAN 2, DST MAC = L2 switch VLAN 2

11) L2 switch processes packet because DST MAC was for it. If it is L2 only it must have GW configured otherwise it can't forward the packet.

The only exception would be if L2 switch has a network mask that makes it believe that 172.16.4.2 is in the same subnet and if the core device has proxy ARP enabled.

Daniel Dib
CCIE #37149

Please rate helpful posts.

[Maro.Cisco]

Daniel thats what i thought but my customer network has all the L2 switches without any GW and they are replying to the ping , how come ???

[Joseph W. Doherty · Accepted solution]

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Posting

Maro.Cisco wrote:
Daniel thats what i thought but my customer network has all the L2 switches without any GW and they are replying to the ping , how come ???

Do GWs support proxy?

[Maro.Cisco]

Actually im not sure tomorrow ill check this out when im at the customer site , but if proxy configured on the core then in this case how the L2 switch will be able to reply to the ping requests

[Jon Marshall · Accepted solution]

A L2 switch in terms of connecting to it or from it basically acts as an end device. If you do not configure a default gateway on an end device then it believes every destination IP is reachable on the network and simply arps out for it.

So when the L2 switch arps out for the PC mac address the core switch, assuming it is running proxy arp,  simply replies to the arp request with it's own mac address and the switch sends the packet to the core switch.

If your core switch is not running proxy arp then i can't see how it is working.

Jon

[Maro.Cisco]

Actually i just checked and yes all the SVI's created on core switch does have Proxy-ARP enabled by default , so L2switch broadcast ARP request , Core switch recieve it and then reply with VLAN 2 SVI mac address to L2switch.Please correct me if im wrong and thanks for your help