Implementation with InterVLAN routing and Dynamic Routing in a switch

[Tang-Suan Tan]

Hi all :

I am implementing three tiers hierarchical network.

There are different VLANs routing in the network and also VLAN access control for routing and blocking diffrent VLANs between the access network. The links between the access switches and distribution switches are in the trunk links which carries a lot of different VLANs.

As for the links bwtween distribution switches and core swiitches, they are in routing mode which has only one subnet.Correct me if I am wrong.

My questions are as below :

1. Can I configure the SVI interface at the Root Switch which is manually set at the distribution layer? This distribution switch will have also the routed port which are uplink to the core switch. The core switch is set with only OSPF routing without any VLAN.

Same apply to this same distribution switch, can I configure the Uplink port as layer 3 routing port and configure it with OSPF routing?

We know that links between the distribution switch and access switch are in trunk link come with different VLANs, is the OSPF (if we can set in this distribution switch) can recognize all these different subnets in different VLAN and so it can do routing out from the Uplink port?

2. Can anybody has any sample configuration file for this type of configuration in the distribution switch? If OK, please share with me.

Many thanks.

Warmest regards,

Tangsuan Tan

[Giuseppe Larosa]

Hello TangSuan,

1)   you will configure an SVI for each client facing Vlans on both distribution switches ( if you have two).

If you have two distribution switches you can provide First Hop Redundancy using either HSRP, VRRP or GLBP to provide an always-on default gateway to clients in each client Vlan.

When using HSRP or VRRP for doing this is good practice to make the HSRP active/VRRP master to be on the distribution switch that is root bridge for the vlan.

The uplink port to the core switch can be a routed link and it is used to build an IGP adjacency with the core switch.

A typical configuration for OSPF on the distribution tree can be the following

2)

router ospf 10

!  link to core switch in area 0

router-id

network 10.10.10.0 0.0.0.255 area 0

! all client vlans in a non zero area to allow for inter area filtering when necessary     

network 10.0.0.0 0.255.255.255 area 2

! avoid to send OSPF hellos on client vlans

passive-interface default

no passive-interface 

no passive-interface

!

OSPF works on the interfaces that have IP addresses that match network ... area commands regardless if the interfaces are SVIs or routed ports.

The L3 interfaces facing client IP subnets can be made passive because there is no need to exchange OSPF messages on all these Vlans.

Just one vlan between distribution switches is needed to run OSPF

Or two when running OSPF in multi area to have one in area 0 and one in non zero area.

The use of non-zero areas allows for inter-area filtering of internal routes.

This is something that may be useful in the future

Ir also allows for route summarization.

On each device use a loopback interface with a /32 subnet mask IP address to provide a consistent router-id reachable in the OSPF domain,

Enforce router-id with manual configuration.

Hope to help

Giuseppe

[Tang-Suan Tan]

Hi Giuseppe :

Thanks to your reply!

I have questions to your answer and would you please don't mind spend some of your previous time to answer for me :

1. Since this distribution switch is come with InterVLAN routing and layer 3 dynamic OSPF, if we configure it as router OSPF 10, will the switch become an OSPF router and will this affects the SVI or InterVLAN routing?

2.  Your reply mainly in the dynamic OSPF routing. Since this OSPF will link the non-zero area to area 0 (backbone or core layer), from my understanding the non-zero area will be all the different VLANs or subnet from access swithces as well as distribution switches, how to link all these different VLANs or subnet to the area 0 within this distribution switch? Do you have any configuration example to show on this requirement?

3. Do you have full configuration example from real world distribution switch which has one portion of Dynamic Routing to core layer and another portion with InterVLAN routing with the access layer? Or you can provide any link or related document on this type of configuration?

thanks and warmest regards,

Tangsuan Tan

[Giuseppe Larosa]

Hello Tangsuan,

1) intervlan routing is provided once the global

ip routing

command is configured on the distribution switch,

OSPF as a routing protocol is used only on core facing links to advertise access IP subnet to core and to receive routes from core (usually including a default route).

2)   In my proposed template the distribution switch is configured to act as ABR ( Area Border Router) acting as interconnect between area 0 and non zero area. All this is done automatically without the need of additional commands.

However, additional commands can be added to perform the following actions:

- creating less specific routes representing the access layer IP subnet ( route summarization)

- filtering some routes from reaching core if and when necessary

The use of a single area OSPF domain would not allow these optional operations.

3)

As I wrote before for inter vlan routing all is needed is:

ip routing   

+

configuration of SVI interfaces ( L3 interfaces associated to Vlan)

interface vlan X

ip address A.B.C.D 255.255.255.0

no shut

+

configuration of HSRP or other first hop redundancy protocol

In each L3 Vlan you define an HSRP group to provide a fault tolerant default gateway to end user devices

! active distribution root bridge for vlan and HSRP active role

spanning-tree priority vlan x 0

interface vlan X

standby 1 ip A.B:C.E

standby 1 priority 110

standby 1 preempt

standby 1 authentication chosenpwd

! stadnby distribution

spanning-tree priority vlan x 1

interface vlan X

standby 1 ip A.B:C.E

standby 1 priority 100

standby 1 preempt

standby 1 authentication chosenpwd

To have some load sharing among the two distribution switches you can choice:

to make dis1  root bridge and HSRP active for even vlans like 2,4,6, and so on

to make dis2 root bridge and HSRP active for odd vlans like 1,3,5 and so on.

The use of spanning-tree priority 0 for root bridge ensures the root bridge role,  priority 1 for the secondary root bridge.

Hope to help

Giuseppe