OSPF between sites

Unanswered Question
Nov 15th, 2009

Hello

I have 3 sites and i ant to interconnect them with OSPF. IOS feature set allows it. Pls look at the attachement.

There are Cisco 3750 switches stacks on each site. The problem is site 1 has two stacks connected via trunk line. I need to keep this trunk working and implement optimal routing.

What is the correct way to start OSPF routing between the stacks on Site 1?

Will it be correct if I create new vlan on Stack 1 and 2 only for ospf connectivity, and declare all other vlans passive-interfaces?

Or is it better to make routed link between stacks (and keep existing trunk too) and run OSPF over this link?

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CriscoSystems Sun, 11/15/2009 - 21:37

To give you the best answer, it would help to know how Layer 3 routing is currently set up. Do you have a router-on-a-stick setup on either one of the switches?

Is there currently Layer 3 routing between the sites (static, or another non-OSPF protocol)? If so, what is the topology?

DPodtikhov Sun, 11/15/2009 - 23:00

Inter vlan routing is done by the 3750 switches themselves, there is no router-on-a-stick setup here.

Site 2 and 3 are new ones, in fact, there is only one link at the moment - the link between site 1 and 2. Other links will be ready soon.

Routing between this sites is static. Switches at sites 1 and 2 are connected by L3 routed port (not an SVI).

Giuseppe Larosa Sun, 11/15/2009 - 22:48

Hello Dmitry,

>> The problem is site 1 has two stacks connected via trunk line. I need to keep this trunk working and implement optimal routing.

it is just enough to deploy OSPF using an SVI interface:

let's suppose new vlan 333 is for routing between the two stacks.

you create the vlan at layer 2

vlan 333

name ospf-backbone

you add the vlan to the list of permitted vlan on trunk between the two stacks

int type x/y

switchport trunk allowed vlan add 333

you create the SVI for vlan 333

int vlan 333

ip address 10.33.33.1 255.255.255.252

! very important

no shut

in ospf you add a network statement for IP subnet of vlan 333

router ospf 10

network 10.33.33.0 0.0.0.3 area 0

this allows you to keep your L2 trunk and to have a path where OSPF is running on vlan 333.

We do so in several campuses

note:

if you have a port-channel between the two stacks make changes to the list of vlans only on the logical interface port-channel they will be automatically reported to member links.

trying to change the vlan list on a member link can cause serious troubles like bridging loops even on powerful standalone C6500 devices.

Hope to help

Giuseppe

DPodtikhov Mon, 11/16/2009 - 00:41

Thank you for your reply.

I suppose I will additionally have to declare all the vlans, except OSPF backbone vlan, passive-interfaces. Otherwise OSPF will form neighbour relationships on each vlan across the stack.

So the setup will look like this:

Stacks on site 1 use new SVI interface to form an OSPF adjacency.

Sites 2 and 3 are connected to Site 1 switches via the routed ports.

Giuseppe Larosa Mon, 11/16/2009 - 09:44

Hello Dmitry,

>> I suppose I will additionally have to declare all the vlans, except OSPF backbone vlan, passive-interfaces

yes that is correct

>> Sites 2 and 3 are connected to Site 1 switches via the routed ports.

recommended or you will join the campuses

Hope to help

Giuseppe

Joseph W. Doherty Mon, 11/16/2009 - 04:25

"What is the correct way to start OSPF routing between the stacks on Site 1? "

Normally, insure the one or more subnets that exist on the trunk are covered by an OSPF network statement. (NB: in your 1st follow up post you mention "Routing between this sites is static." but your using OSPF?)

"Will it be correct if I create new vlan on Stack 1 and 2 only for ospf connectivity, and declare all other vlans passive-interfaces?

Or is it better to make routed link between stacks (and keep existing trunk too) and run OSPF over this link? "

Either would be better for faster OSPF convergence if the connection is lost between the two Site 1 stacks. Both also avoid possible issues with multiple paths when max exceeds max supported by OSPF. Both also help preclude rogue OSPF neighbors. Of the two, I would lean toward using a dedicated VLAN on the trunk. Another option would be to use one or a couple of existing VLANs configured active and others configured passive.

PS:

BTW, ". . . implement optimal routing." also would need to consider bandwidths between L3 devices and typical traffic patterns between them.

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