Core layer device selection problem for student, help!

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Dec 8th, 2009
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Hi guys,

I have a problem with a college assignment and would massively appreciate some advice


The assignment focuses on the hierarchical model, vlans, trunking.  It states:

3 sites each with three departments (the departments are different in each site... no site has any matching departments so 9 deps all together)


I have partially done the assignmnet on packet tracer as so:


2960's for access layer each with two fast ethernet aggregated links to a 3560 (each site has one 3560 except for one that has 2 due to limitations on link aggregation and forwarding rates)


I thought this was good so far (and still do) but it came to the core devices and I wanted to use switches ( due to the theory on latency... layer 2 (OSI) traffic is a lot less burden than layer 3 etc).

BUT

I have read in the assignment brief that each site has an existing 2800 series (with 1 fast ethernet port and 1 serial port).

THE QUESTION IS......

What do I do??  Because at all sites that have 1 distribution device (3560)  all the internal traffic in the site will not need to go to the core layer, the routing (within the site) will be handled at distribution..... so does this mean I could use this 2800 at the core.  If this will work, can I, at the site where there are two 3560 distribution L3 switches can I just have a trunk link straight from one to the other???


Otherwise is my only choice (if I have to use this god damn 2800) to put a L2 at distribution then use the 2800 as a router on a stick (lolipop router)???



Some of the stuff I have said might be a bit lacking in knowledge, but please forgive me, Im still learning!!


Many Thanks

Dan

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Jon Marshall Tue, 12/08/2009 - 12:45
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danabersoch wrote:


Hi guys,

I have a problem with a college assignment and would massively appreciate some advice


The assignment focuses on the hierarchical model, vlans, trunking.  It states:

3 sites each with three departments (the departments are different in each site... no site has any matching departments so 9 deps all together)


I have partially done the assignmnet on packet tracer as so:


2960's for access layer each with two fast ethernet aggregated links to a 3560 (each site has one 3560 except for one that has 2 due to limitations on link aggregation and forwarding rates)


I thought this was good so far (and still do) but it came to the core devices and I wanted to use switches ( due to the theory on latency... layer 2 (OSI) traffic is a lot less burden than layer 3 etc).

BUT

I have read in the assignment brief that each site has an existing 2800 series (with 1 fast ethernet port and 1 serial port).

THE QUESTION IS......

What do I do??  Because at all sites that have 1 distribution device (3560)  all the internal traffic in the site will not need to go to the core layer, the routing (within the site) will be handled at distribution..... so does this mean I could use this 2800 at the core.  If this will work, can I, at the site where there are two 3560 distribution L3 switches can I just have a trunk link straight from one to the other???


Otherwise is my only choice (if I have to use this god **** 2800) to put a L2 at distribution then use the 2800 as a router on a stick (lolipop router)???



Some of the stuff I have said might be a bit lacking in knowledge, but please forgive me, Im still learning!!


Many Thanks

Dan


Dan


Firstly the idea that L2 is a lot less burden than L3 is not strictly true these days because of L3 switches. Certainly a router will not perform at L3 anywhere near as well as a switch at L2 in terms of packet throughput but a L3 switch does L3 forwarding in hardware not software.


Having said that how are the 3 sites connected to each other ? the logical use of the 2800 routers would be to connect the serial to the WAN and the ethernet interface to the 3560. The WAN is in effect the network that connects your 3 sites together.


So how were you proposing to link the sites together ?


Jon

danabersoch Tue, 12/08/2009 - 13:26
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Thanks for your reply

yeah like you say the sites will be connected with serial wan links.

I can add wan and lan interfaces to the 2800's (didnt mean I was restricted with interfaces). By doing it like that (access switch----to-----L3 switch----to------2800-----WAN) am I basically covering distribution and core layers with the L3 switch.  and the 2800 is just for access to the other sites.

Jon Marshall Tue, 12/08/2009 - 13:43
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danabersoch wrote:


Thanks for your reply

yeah like you say the sites will be connected with serial wan links.

I can add wan and lan interfaces to the 2800's (didnt mean I was restricted with interfaces). By doing it like that (access switch----to-----L3 switch----to------2800-----WAN) am I basically covering distribution and core layers with the L3 switch.  and the 2800 is just for access to the other sites.


Dan


Access / distribution / core layers can mean different things in different designs ie. a traditional campus design where you have multiple buildings on the same site would have core which would link the buildings/ distrbution which aggregates the access-layer switches within each buliding and connects to to the core and then the access-layer switches.


Now compare that to your scenario and you could argue that the WAN is actually your core, connecting your distribution switches together, although i think most people would not call this the core. Or you could argue that the 3560 is a collapsed core/distribution layer switch per building.


The point is the 3 tier architecture cannot strictly be applied to all designs, it is a guideline not a must do.


Jon

danabersoch Tue, 12/08/2009 - 14:39
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. Or you could argue that the 3560 is a collapsed core/distribution layer switch per building.


This is true I think the sites are meant to be at different locations (stupid given the nature of the assignment it is based on the coursework from ccna 3 LAN switching and Wireless) but I will have to chack tommorrow.


If they are not and the sites are next door I will find out, work on it a bit more and get back to the post.

So if they are in different locations there shouldn't be a problem? (access switch----to-----L3 switch----to------2800-----WAN) because I will need a router to connect the WANs anyway, and all the traffic within the site will be sorted between access and the 3560?


Another problem (that I totallty understand if you don't want to indulge) IS:

In one site I cannot accomadate all the access switches with 1 L3 and because of Packet tracer should stick with 3560 (rubbish but .PKT file is worth 25%)

because of this I have two 3560's. my question:    can these two 3560 both hold vlan ip addresses (subnets's gateway) in other words can both have links to devices in the same vlan?

Ellaberating....

Two 3560 = L3a + L3b - there is a sales vlan 10. I have sales connected to L3a and some to L3b ( the only way these devices would be able to communicate is up to the 2800 which I am trying to avoid) so can I have botth L3's as a gateway for the same subnet and then can I link them with a trunk or do I have one with the subnet gateway and for the packets recieved at the other l3 they go over a link to the one with the gateway configured to be routed.

Or do I keep the vlans on seperate L3's and route between the L3's.  Im sorry for the confusing way this has gone!!

Jon Marshall Tue, 12/08/2009 - 15:23
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danabersoch wrote:



. Or you could argue that the 3560 is a collapsed core/distribution layer switch per building.


This is true I think the sites are meant to be at different locations (stupid given the nature of the assignment it is based on the coursework from ccna 3 LAN switching and Wireless) but I will have to chack tommorrow.


If they are not and the sites are next door I will find out, work on it a bit more and get back to the post.

So if they are in different locations there shouldn't be a problem? (access switch----to-----L3 switch----to------2800-----WAN) because I will need a router to connect the WANs anyway, and all the traffic within the site will be sorted between access and the 3560?


Another problem (that I totallty understand if you don't want to indulge) IS:

In one site I cannot accomadate all the access switches with 1 L3 and because of Packet tracer should stick with 3560 (rubbish but .PKT file is worth 25%)

because of this I have two 3560's. my question:    can these two 3560 both hold vlan ip addresses (subnets's gateway) in other words can both have links to devices in the same vlan?

Ellaberating....

Two 3560 = L3a + L3b - there is a sales vlan 10. I have sales connected to L3a and some to L3b ( the only way these devices would be able to communicate is up to the 2800 which I am trying to avoid) so can I have botth L3's as a gateway for the same subnet and then can I link them with a trunk or do I have one with the subnet gateway and for the packets recieved at the other l3 they go over a link to the one with the gateway configured to be routed.

Or do I keep the vlans on seperate L3's and route between the L3's.  Im sorry for the confusing way this has gone!!


Dan


Ask as mich as you like although be aware i'm not familiar with packet tracer so may not be able to help on anything specific to that


Your 2 3560's - yes you can connect them together with a trunk link and have a L3 vlan interface on both switches for the same vlan. They will need a different IP each out of the same subnet.


Then you can run HSRP between the switches. With HSRP  you have one Virtual IP (VIP) and 2 physical IPs, one for each vlan interface. You configure the default-gateway on the client to be the VIP. Only one switch at a time is active so all traffic that is routed goes to the active switch.


You can run MHSRP where you have multiple HSRP groups for the same vlan if you want to use both L3 interfaces for routing but this can be messy and in your scenario is probably not worth the trouble.


GLBP automatically load shares for you but unfortunately GLBP is not supported on 3560 switches.


Jon

danabersoch Tue, 12/08/2009 - 16:09
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Checked it out through google and this sounds perfect for my design, but typically packet tracer does not support HSRP    is there another way to link these l3 switches and use 1 ip add for the clients (I think I know the answer). If not I could have two different gateways,1 forr the sales clients connected to l3a and another for the sales clients connected to l3b (would this work with a standard trunk link connecting the l3's) because if so I could configure my simulation (packet tracer) like that and then in my documentation refer to HSRP as a better solution.


On second thoughts this seems to be getting a bit complex (well sounds interesting but a bit out of the realm of this assignment) and as it is my assignment I think I can change it about to be simpler... to having L3a accomadating 2 vlans and L3b accomodating another.  If I were to do this then I can either route between the L3's or trunk between them.... cant I???

danabersoch Tue, 12/08/2009 - 16:27
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Just to let you know I wont be replying again tonight (its 12-30 here and my head hurts lol) but I will be able to read any replies tommorrow morning.


Thanks a lot for the help today cleared a lot of things up for me!!  

Jon Marshall Tue, 12/08/2009 - 16:30
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danabersoch wrote:


Checked it out through google and this sounds perfect for my design, but typically packet tracer does not support HSRP    is there another way to link these l3 switches and use 1 ip add for the clients (I think I know the answer). If not I could have two different gateways,1 forr the sales clients connected to l3a and another for the sales clients connected to l3b (would this work with a standard trunk link connecting the l3's) because if so I could configure my simulation (packet tracer) like that and then in my documentation refer to HSRP as a better solution.


On second thoughts this seems to be getting a bit complex (well sounds interesting but a bit out of the realm of this assignment) and as it is my assignment I think I can change it about to be simpler... to having L3a accomadating 2 vlans and L3b accomodating another.  If I were to do this then I can either route between the L3's or trunk between them.... cant I???


The simplest thing to do is just configure L3 vlan interfaces on one of the 3560 switches and then connect the other via a L2 trunk.


You could do what you suggest about having 2 vlans on one and 1 on the other but it just complicates things. If you did you could run a trunk between the switches and have all the vlans on both switches. If you dual connected your access-layer switches one to each 3560 then this would give you some level of resilience in that if one switch died at least not all vlans would be unuseable but because you cannot run HSRP then you would lose the ability to route the vlans whose L3 interfaces were on the switch that died.


Jon

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