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Cisco switch SVI

Posted on 2016-10-30
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Last Modified: 2016-10-31
This question is based on the attached network diagram.  I am trying to understand why sw2 could not ping 10.10.100.254 when I removed SVI 100 from sw2. Couldn't it go through sw0 and sw0 can do the intervlan and reach 10.10.100.254 for sw2. Thanks

net diagram
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Question by:leblanc
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Expert Comment

by:Niten Kumar
ID: 41866164
The vlan needs to be created on the switch so that the switch is able to tag packets for that particular vlan.
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by:leblanc
ID: 41866288
The vlan100 was created on sw2. I could only ping 10.10.100.254 if the SVI is configured on sw2. In other words, it only worked if I assigned an IP address to int vlan 100 on sw2. I'd like to know why. Thx
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Expert Comment

by:Predrag Jovic
ID: 41866419
vlan100 was created on sw2
You need also vlan 100 on sw1 and sw3 and VLAN100 to be enabled on all trunks.
If it is GNS3 lab, it happens sometimes that I need to add SVI to make ping work (although it does not make much sense).
All you need for ping is:
-  VLAN100 on every switch (except fw - IP address assigned to interface)
- VLAN100 allowed on all trunks
- on sw2 Fa0/3 configured as access port VLAN 100
- routing configured on FW with proper routes (without routing default gateway should be configured as 10.10.100.1) for the case that you are pinging from other VLANs. If you are pinging from sw0 SVI VLAN100 you can ignore this line.

But again, if it is GNS3 and everything configured as above - I would go with GNS3 bug.

Question: how did you create VLANs on switches?  should be created through database VLAN:
switch>en
switch# datebase vlan
switch# vlan 100
switch#  exit
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Author Comment

by:leblanc
ID: 41866969
I did not use GNS3. I am using Packet Tracer. Yes I have everything setup as you mentioned  I ping from sw2 to fw. I can ping fine if I have the SVI configured. If I remove the SVI, I cannot ping the fw. The same issue applied to sw1 & sw3.

PS. Also, you can create vlan without using the database command with gns3 v13.1.3. I tried the database vlan command and it did nto work. Thanks
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Expert Comment

by:Predrag Jovic
ID: 41866996
Can you paste configurations?
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by:SIM50
ID: 41867023
Can you test this for me in your config on sw2 please?
1. clear arp table
2. with SVI 100 interface, ping 10.10.100.254
3. check arp by sh ip arp | i 10.10.100.254
4. clear arp table again
5. shutdown or remove SVI 100 and ping 10.10.100.254
6. check arp table by sh ip arp | i 10.10.100.254

Please post results of steps 3 and 6.
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by:leblanc
ID: 41867059
sw2-1#clear arp
sw2-1#ping 10.10.100.254

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.100.254, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 0/0/1 ms
sw2-1#sh ip arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.20.2              -   000A.41DE.74A1  ARPA   Vlan20
Internet  10.10.100.3             -   000A.41DE.74A1  ARPA   Vlan100
Internet  10.10.100.254           1   000C.CF16.1703  ARPA   Vlan100
sw2-1#clear arp

sw2-1(config)#no int vlan 100
sw2-1(config)#
%LINK-5-CHANGED: Interface Vlan100, changed state to administratively down
%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan100, changed state to down

sw2-1#ping 10.10.100.254

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.100.254, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

sw2-1#clear arp
sw2-1#sh ip arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.20.1              0   000A.F3BC.2DB7  ARPA   Vlan20
Internet  10.10.20.2              -   000A.41DE.74A1  ARPA   Vlan20
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Expert Comment

by:SIM50
ID: 41867073
That's your answer then why you need to have SVI 100 up.
Every packet needs to have source and destination. ARP traffic is constrained to a vlan so the switch needs to have one interface in that vlan to use as a source for ARP packets.
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Author Comment

by:leblanc
ID: 41867086
My thought was as I don't have SVI 100 on sw2, sw2 will go to his default gateway sw0 to be intervlan to vlan 100 thus sw2 should be able to ping 10.10.100.254. Doesn't it work that way?
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by:leblanc
ID: 41867094
This is the original question from my post "I am trying to understand why sw2 could not ping 10.10.100.254 when I removed SVI 100 from sw2."
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Expert Comment

by:Predrag Jovic
ID: 41867098
Original answer - paste configurations.
:)
It should work if you have configured VLAN 20 and VLAN 100 on every switch.
You need VLAN 20 and 100 on every switch and enabled on every trunk along the way, otherwise it will not work.
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Assisted Solution

by:SIM50
SIM50 earned 100 total points
ID: 41867107
My thought was as I don't have SVI 100 on sw2, sw2 will go to his default gateway sw0 to be intervlan to vlan 100 thus sw2 should be able to ping 10.10.100.254. Doesn't it work that way?

You still need to resolve IP to MAC.
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Accepted Solution

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Predrag Jovic earned 300 total points
ID: 41867118
# sh vlan
or
# sh vlan brief
on every switch should show VLAN 20 and VLAN 100 present
also interface vlan 20 on sw2 should be up up
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LVL 26

Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 300 total points
ID: 41867123
10.10.100.1 should resolve arp for 10.10.100.254 in this case, since it is L3 device connected to both VLANs.
Sure interface VLAN for 20 and 100 on sw0 should be up up also and ip routing should be enabled on that one.
FW need to have route 10.10.20.0 255.255.255.0 10.10.100.1 (if routing is enabled on this one).
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Assisted Solution

by:Craig Beck
Craig Beck earned 100 total points
ID: 41867530
If you removed SVI 100 from SW2 it has no IP so it needs to use one of its other SVIs.  The problem is that you don't have VLAN 20 trunked through SW1 or SW3 so you can't reach your default gateway and therefore SW0 can't route traffic for SW2.
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Author Comment

by:leblanc
ID: 41867537
Predrag,

I think you got it. I need to have all vlans on all switches. I do not need to have an IP address assigned to int vlan100 on sw2. However, from the FW, I need a static route pointed to the internal network with next hop as sw0 because that is where all intervlan is done. I did that and I was able to pin 10.10.100.254 from all switches.
No I understand why. Thx
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Expert Comment

by:Predrag Jovic
ID: 41867543
You are welcome.
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