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Switch does not have arp entry but does have a mac-address entry

Answered Question

All,



I have two switches connected to each other.  Switch-A is the core switch which connects to mutliple switches and Switch-B is connected to hosts.



On Switch-B, I see an arp entry and mac-address entry for a particular host.  But on Switch-A, I am only seeing a mac-address entry for the host but no arp entry.


The issue is, we use DHCP reservations for hosts, and therefore the host does not get its DHCP reserved address.


I can ping the DHCP server from Switch-B and Switch-A.


I doubled\tripled check the correct MAC address of the host's network interface on the DHCP server's reservation entry, and it has the correct MAC address of the host's interface.



Is there anything else I can do to make this works?  I am sure I am missing something.


Thank you all and have a great New Year!

Correct Answer by Jon Marshall about 3 years 7 months ago

I do not see an ARP entry for the host on Switch-B,  but I do see an entry in the Mac-Address table, and the same holds for  Switch-A.

If switch B is not routing then you should only see the mac in the mac address table. Not sure why you have a default route on switch B if it is not routing though.


What about switch A. Is that the switch where all the inter vlan routing is done ?


The DHCP server is in the 10.10.70.x network, what is the vlan for that and where is the routing done for that vlan ?


To understand how/why things work you need to understand what switches are doing what.


Jon

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Jon Marshall Mon, 12/30/2013 - 11:55
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On Switch-B, I see an arp entry and mac-address entry for a particular host.  But on Switch-A, I am only seeing a mac-address entry for the host but no arp entry.

Which switch is responsible for routing the vlan that the host is in ?


It sounds like it is switch B and not switch A.


Jon

Thanks for the reply.


On Switch-B, there is an interface VLAN ip address that is being routed and a single default static route to Switch-A interface VLAN.


For example:



Switch-B:


VLAN3: Ports 1-48

Interface VLAN3: ip address 10.10.10.1 /24

Static Route: ip route 0.0.0.0 0.0.0.0 10.2.2.1


Switch A:


VLAN3: Ports 1-48

Interface VLAN3: ip address 10.2.2.1 /24

Static Route: ip route 0.0.0.0 0.0.0.0 10.1.1.1



Does this help?

Jon Marshall Mon, 12/30/2013 - 12:41
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The switches are using a completely different subnet for vlan 3 which does not make sense.


Which subnet is the host in ?


How is switch B connected to switch A ?


What is the subnet for vlan 3 meant to be ie. which switch is right.


If the host is in subnet 10.10.10.0/24 then you will not see an arp entry on switch A for this host.


Jon

Thanks again.  See inline for response:




Which subnet is the host in ?

>>> 10.5.5.32


How is switch B connected to switch A ?

>>> Uplink from Switch-B Gi1/0/1 to Switch-A Gi1/0/5


What is the subnet for vlan 3 meant to be ie. which switch is right.


>>> I gave the incorrect IPs for the interface vlans:


>>> Switch-B: Interface VLAN3 - 10.7.2.24 /23


>>> Switch-A: Interface VLAN3 - 10.7.2.1 /23



If the host is in subnet 10.10.10.0/24 then you will not see an arp entry on switch A for this host.


>>> OK if that is the case then how come I cannot get the DHCP issue the host its reserved IP address I assigned?



Thanks for helping.

Jon Marshall Mon, 12/30/2013 - 13:14
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If the host is in subnet 10.5.5.x then where is the L3 interface for that subnet ie. vlan 3 is 10.7.2.x/23  ?


Is the uplink a trunk port ?


I need to understand the topology to understand where the arp entry should be.


Where/what is the DHCP server ?


Jon

Ok here you go, hope this helps better. I need the host on Switch-B to get an ip address from the DHCP server.




                                                                                    Gi1/0/5             Gi1/0/1

DHCP Server    --- 10.10.0.0 /23 Network ----  Switch-A  ---------------  Switch B

(10.10.70.10/23)                                             - IntVLAN3 IP:                  - IntVLAN3 IP:

                                                                          10.10.7.1/23                     10.10.7.24/23

                                                                       - OSPF 1                          - VLAN3: P1-48

                                                                          network 10.10.0.21        - Def route: 0.0.0.0

                                                                          network 10.10.7.0            0.0.0.0 10.10.7.1

                                                                       - VLAN3: P1-48                        


                                                                                                                                     |

                                                                                                                                     |

                                                                                                                                     |

                                                                                                                           Host on

                                                                                                                        Fas1/0/48

Jon Marshall Mon, 12/30/2013 - 15:42
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I'm trying to help, honestly, but i need you to answer the questions i'm asking.


Your diagram shows switch B with a default route and your initial post said you see the host in the arp table which suggests it is routing.


I asked which subnet the host was in and you said 10.5.5.x so i asked where the L3 interface is for that subnet and you have just ignored the questions.


So can you answer these questions -


1)  Why do you have a default route on switch B pointing to the same IP subnet on switch A ie. you don't route for the same subnet ?


2) what is the link between the switches ie. a L2 access link in vlan 3, a trunk link ?


3) what vlan is the host meant to be in ?


4) what subnet you do want the host to get an IP from ?


5) if the subnet in 4) is not vlan 3 then which vlan is it and where is the vlan routed ?


6) the DHCP sever is on the 10.10.70.x/23 subnet. Where is the routing for that vlan done ?


Jon

Jon Marshall Mon, 12/30/2013 - 15:49
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If you want the host to get an IP from the DHCP server then whichever vlan the host is in you need to go to the switch that routes for that vlan and on the L3 SVI you need to add an ip helper-address ie.


int vlan

ip helper-address 10.10.70.10


Jon

My apologies for the delay and for not answering your questions earlier.  I was trying to wrap up from work.


On Switch-A, there is an "ip-helper address" under the IntVLAN3 interface.



1)  Why do you have a default route on switch B pointing to the same  IP subnet on switch A ie. you don't route for the same subnet ?


>>> The configs on both switches we done by another engineer.  But the ip for the host is on a different subnet as the subnet for the IntVLAN3.  Host reserved address is 10.10.73.12/23.


2) what is the link between the switches ie. a L2 access link in vlan 3, a trunk link ?


>>> No they are both access ports.  Switch-A--Gi1/0/5 and Switch-B--Gi1/0/1 are on the same VLAN3.


3) what vlan is the host meant to be in ?


>>> VLAN3 which is what it is in now.


4) what subnet you do want the host to get an IP from ?


>>> 10.10.70.10 /23


5) if the subnet in 4) is not vlan 3 then which vlan is it and where is the vlan routed ?


>>> I believe it is being routed via OSPF.



6) the DHCP sever is on the 10.10.70.x/23 subnet. Where is the routing for that vlan done ?


>>> Via OSPF, I think.



Thanks for your help.

Jon Marshall Tue, 12/31/2013 - 03:47
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edited      

Jon Marshall Tue, 12/31/2013 - 03:48
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Okay, i think there may be some confusion here.


Vlan 3 is asociated with the 10.7.2.2/23 network ie. 10.7.2.1 -> 10.7.3.255


The host on switch B is in vlan 3. But you say you want the host to get an IP from the 10.10.70.x/23 network. So having an ip helper-address under vlan 3 will not work for this. The host cannot be in vlan 3 and get an IP from 10.10.70.x because that is not the subnet associated with vlan 3.


The ip helper-address will only return an IP from the 10.7.2.x subnet not the 10.10.70.x network.


So what vlan is associated with the 10.10.70.x/23 subnet ?


I am not criticising the engineer but the configs on the switch A & B don't look right. You do not route to the same vlan. It may be that vlan 3 on switch B is for management but then switch B should be using a default gateway and not a default route.


If the link between switch A and B is an access link in vlan 3 then any device connected to switch B can only be in vlan 3 which means, due to the ip helper-address under vlan 3, that it will only get an IP from the 10.7.2.x subnet.


So if you want to give it an address from 10.10.70.x you need to -


1) work out which vlan is associated with the 10.10.70.x subnet


2) work out where the L3 interface for that vlan is ie. which device routes for the 10.10.70.x vlan


3) add an ip helper-address under that vlan interface


4) create a trunk between switch A and B and allow that vlan across


5) allocate the port on switch B that the host is connected to into that vlan


However i'm not suggesting you should go ahead and do this because just from the info you have provided it is not at all clear what your setup is. Like i say the config on switches A & B look wrong to me and it may be that i am not understanding exactly how your network is setup.


Jon

Jon Marshall Tue, 12/31/2013 - 04:01
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I noticed from the original post that you say switch A is the core switch so it may be that all the L3 vlan interfaces are on there. Can you post the output of this command from both switch A & B -


switch# sh ip int brief | include Vlan


As i said before, if you see an arp entry on switch B it suggests switch B is routing for vlan 3 which appears odd as switch A also has a vlan L3 interface and it is meant to be the core switch. In addition to the above can you also post from both switches -


switch# sh ip route


Generally speaking the core switch (switch A) would be responsible for routing between vlans and the other switches (switch B for example) would simply be L2 with vlans on it and use switch A to route between those vlans. But that does not appear to be what is happening in your network.


Edit - forgot to ask. What is the default gateway on the host set to. Is it the vlan 3 IP on switch B or the vlan 3 IP on switch A ?


Jon

Thanks Jon.


I reviewed what you wrote earlier.  I  tried not giving a DHCP reservation and allow the DHCP server to issue  any IP, but that doesn't seem to work either.  It is strange because it  did work yesterday but for some reason today, it did not get a random IP  address from the DHCp server.


From Switch-B, I can  ping the DHCP server and Switch-A's IntVLAN3 IP directly with success.   From Switch-A, I can ping the DHCP server and Switch-B's IntVLAN3 IP.


From the DHCP server, I can ping Switch-B's IntVALN3 IP, and I can piing Switch-A's IntVLAN3 IP.


When I perform a 'sho ip route", there is no result, because there is no default gateway configured on Switch-B.


I do not see an ARP entry for the host on Switch-B,  but I do see an entry in the Mac-Address table, and the same holds for  Switch-A.


The host has been configured for DHCP, so no physical IP/SM/DG is configured.

Correct Answer
Jon Marshall Wed, 01/01/2014 - 04:17
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I do not see an ARP entry for the host on Switch-B,  but I do see an entry in the Mac-Address table, and the same holds for  Switch-A.

If switch B is not routing then you should only see the mac in the mac address table. Not sure why you have a default route on switch B if it is not routing though.


What about switch A. Is that the switch where all the inter vlan routing is done ?


The DHCP server is in the 10.10.70.x network, what is the vlan for that and where is the routing done for that vlan ?


To understand how/why things work you need to understand what switches are doing what.


Jon

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