few questions to confirm my concept

Answered Question
Sep 6th, 2009

Hi every body.

I have few questions to think about during my flight toorrow.

We have one multilayer switch sw1, and layer 2 switch sw2 and host1. sw2 has vlan2 and host, h1 exists in vlan 2.

The goal is to explore different ways to configure sw1 as default gateway for h1.

The topology is:

Sw1 f0/1--------(f0/2)SW2F0/3------h1

Assume sw2 f0/3 is already placed in vlan 2. vlan2 is using 198.198.198.0/24

With the above assumption im mind, I explore different ways i can configure sw1 as default gateway for h1.

Option 1

Sw1:

int f0/1

no switchport

ip address 198.198.198.1/24

I also put the sw2 f0/2 in vlan 2

Can i do this ?

Option 2:

sw1:

int vlan 2

ip address 198.198.198.1/24

no shutdown.

Then i configure the f0/1 on sw1 and f0/2 on sw2 as trunk.

Is it correct?

Option 3

Int vlan 2'

ip address 198.198.198.1/24

I configure the f0/2 as an access port and put it in vlan 2 on sw1 . Next i configure the f0/2 on sw2 as an access port and put it in vlan 2.

Is it correct?

==========================

Thanks and i assure these are my last questions for next 6 months.

Thanks and have a good day.

I have this problem too.
0 votes
Correct Answer by Jon Marshall about 7 years 2 months ago

Sarah

"Why? as the trunk also carries vlan2, so sw1 can send hsrp hello to sw3 vua that trunk."

Lets look at it from the perspective of SW1.

SW1 connects to SW2 and on SW2 that port is in vlan 2.

SW2 connects to SW3 and on SW2 that port is in vlan 2.

So if SW1 sends a packet out of the routed port it will arrive on SW2 in vlan 2. SW2 will then send that packet out to SW3 because that port is also in vlan 2. So you can see there is a direct L2 path between the SW1 routed port and the SW3 routed port.

Now lets looks at it again from SW1 the other way.

SW1 port is a routed port, it is not part of a vlan. So for it to send a packet in vlan 2 and go via the L2 trunk between SW1 & SW3 the packet would have to be routed onto vlan 2 within SW1 ie. there is no direct L2 path between the routed port on SW1 and the routed port on SW3 via the trunk link.

And if the packet has to be routed then HSRP packets wouldn't work that way. Note that in the first example going via SW2 the packet is not routed, it is simply switched through vlan 2 fron SW1 -> SW2 -> SW3.

Key thing to understand is that routed ports on a switch cannot be part of a vlan. So to get to vlan 2 on the same switch the packet would have to be routed.

It can be a tricky concept to understand.

Jon

Correct Answer by Jon Marshall about 7 years 2 months ago

Sarah

SW1 is connected to SW3 via a L2 trunk.

SW1 is connected to SW2 via a routed port

SW3 is connected to SW2 via a routed port

So HSRP packets would go via SW2 to get from SW1 to SW3. HSRP would indeed work as Peter said.

The reason HSRP packets cannot just go go across the trunk link between SW1 & SW3 is because HSRP packets only work within the same vlan. The routed port on SW1 and the routed port on SW3 are not in a vlan. But the ports on SW2 that SW1 & SW3 connect to are in a vlan.

So there is a L2 path between SW1 & SW3 for that vlan but that path is via SW2. It is not via the direct link between SW1 & SW3.

If instead of routed ports on SW1/SW3 you had configured "int vlan 2" and given them IP addresses then the L2 path would indeed be across the direct link between SW1 & SW3.

That was what i was trying to point out. The difference between a routed port and switchport on a L3 switch. Unfortunately i made a mistake regarding which ports STP would block, thankfully Peter was on hand to correct it.

Have a good flight :-)

Jon

Correct Answer by Jon Marshall about 7 years 2 months ago

Peter

"Maybe I misunderstood you - can you please help me here?"

No, you understood better than me. Your'e correct in what you say. Because the ports are routed then both uplinks from SW2 would indeed be forwarding. Apologies for the misleading information.

Deserves a rating i think.

Jon

Correct Answer by Peter Paluch about 7 years 2 months ago

Hello Jon,

You confused me here:

However if you used option 1 then it would behave quite differently. Because the 2 ports are not members of any vlan the HSRP packets cannot flow between the 2 routed ports across the L2 trunk. HSRP would not work [cut]

It is true that a routed port is not a member of any VLAN. But why should the HSRP Hellos flow through the L2 trunk between SW1 and SW3? Assuming that the SW2 is connected to both SW1 and SW3 and both ports on SW2 are in VLAN2 then the hellos would be sent from SW1 routed port via SW2 to SW3 routed port and vice versa. I think that the HSRP should be working just fine here. And also, no uplink from SW2 would be STP-blocked as the routed ports do not send or receive BPDUs.

Maybe I misunderstood you - can you please help me here?

Best regards,

Peter

Correct Answer by Jon Marshall about 7 years 2 months ago

Actually this follows on from a question i was dealing with a while back.

As Giuseppe and Peter say all 3 will work and the most flexible is option 2.

However there is a big difference between option 1 and option 3.

Option 1 makes the switchport a routed port ie. it is not a member of any vlan.

In your example it makes little difference but imagine if you have another L3 switch (SW3) connected to SW1 via a L2 trunk. And sw2 is connected to both SW1 & SW3. You want to run HSRP between SW1 and SW3 for this vlan. This is a very common setup.

With option 3 no problem because you can configure HSRP under the L3 SVI for vlan 2 and the HSRP packets will flow across the L2 trunk link between the 2 L3 SVI's for vlan 2.

However if you used option 1 then it would behave quite differently. Because the 2 ports are not members of any vlan the HSRP packets cannot flow between the 2 routed ports across the L2 trunk. HSRP would not work, assuming the one of the uplinks from sw2 was blocked as there is now no L2 path between the 2 routed ports.

Note if the L2 trunk between SW1 & SW3 ended up being blocked then the 2 uplinks from SW2 would both be forwarding and so HSRP would work.

Finally, looking forward to hearing from you again in 6 months time with all those tricky 6500 questions :-)

Jon

Correct Answer by Peter Paluch about 7 years 2 months ago

Hello Sarah,

Option 1: Absolutely correct. It can be done this way. The downside of this approach is that the link between Sw1 and Sw2 is an access link, not a trunk, and cannot presently serve to carry multiple VLANs and perhaps provide routing between them.

Option 2: Absolutely correct. This is the typical configuration.

Option 3: Absolutely correct. The downside is similar to the Option 1.

Regarding your "last question": I am pretty sure that everybody here enjoys having these conversations with you and would be much happier if this wasn't the last question in the next 6 months. But hey, what do you know? Maybe after spotting your talent you'll get to work with those 6500 right after the first month :) (Never been in army so please forgive my naive jokes here.)

Best regards,

Peter

Correct Answer by Giuseppe Larosa about 7 years 2 months ago

Hello Sarah,

your understanding is correct:

a L2 end-to-end path has to exist between host h1 and its default gateway.

Default gateway can be an SVI or a routed port.

In real world 99.99% of times we use option2 to avoid to dedicate an inter-switch link to vlan2 broadcast domain.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sun, 09/06/2009 - 11:20

Hello Sarah,

your understanding is correct:

a L2 end-to-end path has to exist between host h1 and its default gateway.

Default gateway can be an SVI or a routed port.

In real world 99.99% of times we use option2 to avoid to dedicate an inter-switch link to vlan2 broadcast domain.

Hope to help

Giuseppe

Correct Answer
Peter Paluch Sun, 09/06/2009 - 11:25

Hello Sarah,

Option 1: Absolutely correct. It can be done this way. The downside of this approach is that the link between Sw1 and Sw2 is an access link, not a trunk, and cannot presently serve to carry multiple VLANs and perhaps provide routing between them.

Option 2: Absolutely correct. This is the typical configuration.

Option 3: Absolutely correct. The downside is similar to the Option 1.

Regarding your "last question": I am pretty sure that everybody here enjoys having these conversations with you and would be much happier if this wasn't the last question in the next 6 months. But hey, what do you know? Maybe after spotting your talent you'll get to work with those 6500 right after the first month :) (Never been in army so please forgive my naive jokes here.)

Best regards,

Peter

sarahr202 Sun, 09/06/2009 - 14:16

Thanks Peter, Giuseppe, and Jon

For Peter.

I have never been in army before so don't worry about the naive jokes. Army won't let me work with routers rightaway. During this three months training, they will teach me every thing from assembling the gun to throwing grenades. Actually I was looking at your profile and really amazed to read about the country your are from. It Is very beautiful country.

For Giuseppe.

Let me congratuale you for being a top net pro.

You guys have a wonderful day.

Peter Paluch Sun, 09/06/2009 - 16:13

Sarah,

You are heartily welcome.

Thank you for your compliments about Slovakia - yes, absolutely, it is a wonderful country.

About Giuseppe: agree 100%. Giuseppe, how come that there is no interview with you here on NetPro?

Best regards,

Peter

Joseph W. Doherty Tue, 09/08/2009 - 03:57

"Giuseppe, how come that there is no interview with you here on NetPro? "

Peter, not 100% positive, but strongly suggest it's because Giuseppe, much like you, is a relatively new contributor, who also provides many, many posts of high quality. I'm relatively new contributor myself, and I recall Giuseppe becoming highly active.

Jon Marshall Tue, 09/08/2009 - 09:09

Peter

As Joseph says it really depends on how active people are ie. Giuseppe is very active and has moved up the tables very quickly so there is just a lag between being on the all time table and getting an interview.

I'm assuming that you have both now seen Giuseppe's interview though as it has just been posted to the site.

Giuseppe - good to put a face to your name.

Joseph, can't be long before you pop up as well ?

Jon

Giuseppe Larosa Tue, 09/08/2009 - 10:35

Hello Peter, Jon and Joseph

Peter: I'm a newcomer in comparison to Jon, Richard Burts, Paolo and others. As you can see your desire to see my interview has been satisfied. :-)

Jon: I hope we will have a chance to meet in some event in the future.

I can say I had the same thought about Jon because his interview appeared one year ago if I'm not wrong.

And yes I expect to see also Joseph's interview and later Peter's one.

Hope to help

Giuseppe

Correct Answer
Jon Marshall Sun, 09/06/2009 - 12:22

Actually this follows on from a question i was dealing with a while back.

As Giuseppe and Peter say all 3 will work and the most flexible is option 2.

However there is a big difference between option 1 and option 3.

Option 1 makes the switchport a routed port ie. it is not a member of any vlan.

In your example it makes little difference but imagine if you have another L3 switch (SW3) connected to SW1 via a L2 trunk. And sw2 is connected to both SW1 & SW3. You want to run HSRP between SW1 and SW3 for this vlan. This is a very common setup.

With option 3 no problem because you can configure HSRP under the L3 SVI for vlan 2 and the HSRP packets will flow across the L2 trunk link between the 2 L3 SVI's for vlan 2.

However if you used option 1 then it would behave quite differently. Because the 2 ports are not members of any vlan the HSRP packets cannot flow between the 2 routed ports across the L2 trunk. HSRP would not work, assuming the one of the uplinks from sw2 was blocked as there is now no L2 path between the 2 routed ports.

Note if the L2 trunk between SW1 & SW3 ended up being blocked then the 2 uplinks from SW2 would both be forwarding and so HSRP would work.

Finally, looking forward to hearing from you again in 6 months time with all those tricky 6500 questions :-)

Jon

Correct Answer
Peter Paluch Sun, 09/06/2009 - 12:31

Hello Jon,

You confused me here:

However if you used option 1 then it would behave quite differently. Because the 2 ports are not members of any vlan the HSRP packets cannot flow between the 2 routed ports across the L2 trunk. HSRP would not work [cut]

It is true that a routed port is not a member of any VLAN. But why should the HSRP Hellos flow through the L2 trunk between SW1 and SW3? Assuming that the SW2 is connected to both SW1 and SW3 and both ports on SW2 are in VLAN2 then the hellos would be sent from SW1 routed port via SW2 to SW3 routed port and vice versa. I think that the HSRP should be working just fine here. And also, no uplink from SW2 would be STP-blocked as the routed ports do not send or receive BPDUs.

Maybe I misunderstood you - can you please help me here?

Best regards,

Peter

Correct Answer
Jon Marshall Sun, 09/06/2009 - 12:37

Peter

"Maybe I misunderstood you - can you please help me here?"

No, you understood better than me. Your'e correct in what you say. Because the ports are routed then both uplinks from SW2 would indeed be forwarding. Apologies for the misleading information.

Deserves a rating i think.

Jon

sarahr202 Mon, 09/07/2009 - 18:39

Hi Jon.

I might not have a chance to check your reply as i am staying at a hotel,waiting to be flown tomorrow morning by Air force 1 (just kidding).

I will quote you for easy reference.

Option 1 makes the switchport a routed port ie. it is not a member of any vlan.

"In your example it makes little difference but imagine if you have another L3 switch (SW3) connected to SW1 via a L2 trunk. And sw2 is connected to both SW1 & SW3. You want to run HSRP between SW1 and SW3 for this vlan. This is a very common setup."

If i understood you correctly, then sw3 is connected to sw1 by a trunk. That means port on sw1, connected to sw3 is not access port but a trunk port. If the port on sw1 ,connected to sw3 is routed port then sw3 is not connected to sw1 by trunk.

If you have chance ,please elaborate on it i.e if the sw1 is connected to sw3 by truink or not. If sw1' port connected to sw3 is routed port.

Based on the Peter response to your post, port on sw1 connected to sw3 is routed port. That means we have a topology something like this:

Sw1 is connected to Sw2 by f0/1 which is routed port, and to sw3 by f0/3 which is also routed port. Sw2 and sw3 is also connected ( though i did not know if Jon means whether they are connected by trunk or access port).

According to Peter response, Hsrp hello flow from sw1 to sw2 and from sw2 to sw3. Why does hsrp hello not flow from sw1 to sw3 directly i.e they are connected to each other directly?

Thanks and have a good day.

Correct Answer
Jon Marshall Mon, 09/07/2009 - 20:27

Sarah

SW1 is connected to SW3 via a L2 trunk.

SW1 is connected to SW2 via a routed port

SW3 is connected to SW2 via a routed port

So HSRP packets would go via SW2 to get from SW1 to SW3. HSRP would indeed work as Peter said.

The reason HSRP packets cannot just go go across the trunk link between SW1 & SW3 is because HSRP packets only work within the same vlan. The routed port on SW1 and the routed port on SW3 are not in a vlan. But the ports on SW2 that SW1 & SW3 connect to are in a vlan.

So there is a L2 path between SW1 & SW3 for that vlan but that path is via SW2. It is not via the direct link between SW1 & SW3.

If instead of routed ports on SW1/SW3 you had configured "int vlan 2" and given them IP addresses then the L2 path would indeed be across the direct link between SW1 & SW3.

That was what i was trying to point out. The difference between a routed port and switchport on a L3 switch. Unfortunately i made a mistake regarding which ports STP would block, thankfully Peter was on hand to correct it.

Have a good flight :-)

Jon

sarahr202 Mon, 09/07/2009 - 20:39

Thanks Jon ,I just got your response just in time ( 25 minutes to 10 o clock curfew).

You have a good day

sarahr202 Mon, 09/07/2009 - 20:59

Hi Jon

Just want to understand the following( excerpt from your post)

If instead of routed ports on SW1/SW3 you had configured "int vlan 2" and given them IP addresses then the L2 path would indeed be across the direct link between SW1 & SW3.

You mean trunk between sw1 and sw3 will be used, because the direct link between two is a trunk. But then you said hsrp hello flow only in same vlan. The trunk between sw1 and sw3 does not belong to any vlan.

Can you please explain the above ?

Thanks

Jon Marshall Mon, 09/07/2009 - 21:03

Sarah

"You mean trunk between sw1 and sw3 will be used, because the direct link between two is a trunk. But then you said hsrp hello flow only in same vlan. The trunk between sw1 and sw3 does not belong to any vlan."

You are correct in that a trunk doesn't belong to any one vlan but as long as vlan 2 is allowed on that trunk then there is a direct L2 path between the vlan 2 SVI's on SW1 & SW3.

Edit - what i meant about HSRP is that the HSRP packets are not routed so they remain within the vlan they were generated in. Each vlan would have it's own HSRP packets, assuming you are running HSRP for that vlan.

Jon

sarahr202 Mon, 09/07/2009 - 21:15

Thanks Jon.

Just convinced the person-in charge to let me use internet for 30 more minutes.

In the scenario you suggested, i.e

Sw1 connected to sw3 by trunk

sw3 is connected to sw2 by routed port.

sw1 connected to sw2 by routed port.

Hsrp hello will flow from sw1-sw2-sw3. The direct trunk between sw3 and sw1 is not used.

Why? as the trunk also carries vlan2, so sw1 can send hsrp hello to sw3 vua that trunk.

What i understand is this if svi is used for hsrp, then hsrp hello will travel down the link between sw1 and sw3.

But the physical port is being used as routed port and hsrp is configured then sw1 will send hsrp hello to sw2, sw2 then to sw3.

But the quesion is why is it so?

Thanks Jon for your patience. I will pick up this conversation after 6 months.

You have a goood day Jon.

Correct Answer
Jon Marshall Mon, 09/07/2009 - 21:26

Sarah

"Why? as the trunk also carries vlan2, so sw1 can send hsrp hello to sw3 vua that trunk."

Lets look at it from the perspective of SW1.

SW1 connects to SW2 and on SW2 that port is in vlan 2.

SW2 connects to SW3 and on SW2 that port is in vlan 2.

So if SW1 sends a packet out of the routed port it will arrive on SW2 in vlan 2. SW2 will then send that packet out to SW3 because that port is also in vlan 2. So you can see there is a direct L2 path between the SW1 routed port and the SW3 routed port.

Now lets looks at it again from SW1 the other way.

SW1 port is a routed port, it is not part of a vlan. So for it to send a packet in vlan 2 and go via the L2 trunk between SW1 & SW3 the packet would have to be routed onto vlan 2 within SW1 ie. there is no direct L2 path between the routed port on SW1 and the routed port on SW3 via the trunk link.

And if the packet has to be routed then HSRP packets wouldn't work that way. Note that in the first example going via SW2 the packet is not routed, it is simply switched through vlan 2 fron SW1 -> SW2 -> SW3.

Key thing to understand is that routed ports on a switch cannot be part of a vlan. So to get to vlan 2 on the same switch the packet would have to be routed.

It can be a tricky concept to understand.

Jon

sarahr202 Tue, 09/08/2009 - 03:03

Thanks Jon For bearing with me.

See you guys in 6 months

Have a nice day!

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