Multilayer Campus Design Layer 2 Access with Layer 3 Distribution

[ishai]

Multilayer Campus Design

Layer 2 Access with Layer 3 Distribution

Hi

im trying to set up a network with a l3 in the distribution and a l2 in the access

L3 L3 (3845)

| |

|-----L3-------L3---| (3750)

\ /

\ /

L2

But im getting in to troubles when i deploy it

The core is 3845 routers the distribution is 3750 switches talking in between them EIGRP the cross link is a L3 ether channel,

The L2 switch is also a 3750 switch with a MGT VLAN

The vlans are configured on the two distribution switches as (SVI's) and are know in the whole network, no issues there

The problems staring when i test the failover (HSRP) , i disconnect a trunk on the l2 switch and all the network is falling,

EIGRP neighbors go down

HSRP takes a long time to fail over

All Vlan disconnect

but when i make the crosslink a L2 Trunk... then all is good

Who can advise?

Thank you

[ishai]

Thanks to you all guys,

I'm glad I'm not alone with this issue and all are really trying to help, thank you

Ok

Let's summarize (see drawing),

I've a two 3845 routers (WAN routers) and two 3750 as the core/distribution switches

All there 4 network components I would like to have speaking EIGERP

I have also a L3 cross link (ether channel) in between the 3750's,

So far all is good and the network is working, But When I disconnect one of the trunks to the access switches (L2) the problems starts

The SVI's (my Vlans) that I configured on the 3750's go down, I see that HSRP takes very long to switch, And all my EIGRP neighbors are going down as well

Is this a Spanning tree issue, Convergence (PVST)?

Now when I configure the cross link from a L3 to a L2 trunk all seems to work fine

So the questions I have now are as follow

Is this the right logic? Is this the way it should be (EIGRP with a L2 trunk ?)

And Spanning tree ive chosen for Rapid spanning tree protocol (802.1W, was PVST before) any tips?

Thank you all

Ill do some new test with a L3 cross link and Rapidspanning tree

So the questions I have now are as follow

Is this the right logic? Is this the way it should be (EIGRP with a L2 trunk)?

And Spanning tree I've chosen for Rapid spanning tree protocol (802.1W, was PVST before) any tips?

Thank you all

Ps.

I know that Cisco is promoting to implement L3 all the way to the Access layer, but it is not possible for me with this design because I have more then one vlan per access switch and this VLAN is being used on other access switches too,

[Jon Marshall]

Hi

I'm assuming that on each 3750 distro switch you have the same vlans running HSRP eg

3750_1

int vlan 10

ip address 192.168.5.2 255.255.255.0

standby 10 ip 192.168.5.1

etc...

3750_2

int vlan 10

ip address 192.168.5.3 255.255.255.0

standby 10 ip 192.168.5.1

etc...

With a L3 etherchannel between your 2 distro switches the HSRP packets have to traverse the access-layer switches ie. they cannot go directly across the L3 etherchannel.

So, and again it is difficult to be precise without configs etc, when you disconnect a trunk from the access-layer to the distro layer an STP calculation will in all probability start. While STP reconverges no traffic is passed over the access-layer so that means no HSRP traffic can go between the 2 distro switches.

So bearing in mind you cannot do L3 at the access-layer as you say i would recommend

1) Migrate to RSTP - this should significantly speed up your convergence times.

2) Optionally you can change the L3 etherchannel between your distro switches to a layer 2 trunk and then ensure that any blocked ports are to the access-layer switches and not between the 2 distro switches.

Oh yes and

3) Stop disconnecting trunks :-)

Let us know how you get on with your tests

Jon

[royalblues]

The convergence should be faster even with PVST if you make sure the core switches are the root for the vlans and you have uplinkfast enabled on the access switches.

Narayan

Never thought that there would be so much discussion on this topic :-)

[ishai]

Hi Narayan,,

It's a tough topic but confusing im glad a lot of people are looking at it,

Thank you Narayan for getting me in the right direction

Ill do some test today and let you know

[ishai]

Jon

Thank you for your Feed back

I think the RSTP will change the speed /convergence big time but I also think that it is not possible without the L2 Trunk,

Ill do the to test

Test 1 L3 ether channel with RSTP

Test 2 L2 trunk with RSTP

Ill let you know later what I found out

Thanks for now

[Jon Marshall]

No problem, although having just reread the post i've made a bit of a basic error. So before someone a lot smarter than me ie. all the others on this post, jumps in

It isn't an STP issue as you do not have any loops in your network. Because you are running a L3 etherchannel between your distro switches and L3 uplinks to your 3845 routers all your trunks from the access-layer should be forwarding.

Apologies for misleading you.

Jon

[Joseph W. Doherty]

re: STP loops

From one of OP's later posts:

"Ps.

I know that Cisco is promoting to implement L3 all the way to the Access layer, but it is not possible for me with this design because I have more then one vlan per access switch and this VLAN is being used on other access switches too,"?

[edit]

re: uplink fast

Automatic feature of RSTP?

[ishai]

Hi to all,

The good news is that's it working

The bad news is that im back to L2 between the Distribution switches.

This is the short version of the tests

I have tested with two access switches and two distribution/core switches

I had 2 laptops on two different Vlans, pinging each other from two different switches,

During the many trunks disconnects and I've tried many variations (with and with out cross connect, one with the two primary trunks disconnected, and so on …

It was working but not fast and some pings were failing (timeouts)

I connected a third access switch during these test and I've notice that all was working good suddenly, (the third switch became a transit switch all traffic was going through it),

Is tough to explain / show with out the config. Files

But the moment I reconfigured to L2 trunk (etherchannel) all test were good with out any ping timeouts

So now you know what I chose for,

Thank you all for the great tips and different views