11-15-2009 09:11 PM - edited 03-06-2019 08:37 AM
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?
11-15-2009 09:37 PM
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?
11-15-2009 11:00 PM
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).
11-15-2009 10:48 PM
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
11-16-2009 12:41 AM
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.
11-16-2009 09:44 AM
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
11-16-2009 04:25 AM
"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.
Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: