Core layer switches IP address for routing

Unanswered Question
Jul 17th, 2009

For routing process I add a IP address of each Vlans subnet that active on each Access and Distribution switches (Have a port with that Vlan on the switch) to the corresponding Vlan Interface of them.

Which IP address should I add to the Core switch for routing?

Should I add a IP of each vlan that in the LAN to each vlan interface of Core layer switch?

I want run OSPF routing protocol on the LAN.

I have this problem too.
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Giuseppe Larosa Sat, 07/18/2009 - 04:00

Hello Reza,

>> Which IP address should I add to the Core switch for routing?

if you want to implement a L3 routed core every link betweeen core device and a distribution device is a L3 link with its own IP subnet.

For example if you have 16 distribution pairs and two core switches:

10.10.10.0/30 dis11 to core1

10.10.10.4/30 di12 to core2

10.10.10.8/30 dis21 to core1

10.10.10.12/30 di22 to core2

...

10.10.10.128/30 disF1 to core1

10.10.10.132/30 disF2 to core2

this under the idea to have not a full mesh between core routers and distribution devices

then you need also a L3 link between the two cores (at least one)

Each L3 device should also have a loopback interface to be used as OSPF router-id and for management purposes (telnet and so on)

you can use /32 loopbacks taken from same block for example

10.255.254.1/32 core1

10.255.254.2/32 core2

10.255.254.3/32 dis11

10.255.254.4/32 dis12

to make the routing function the core switches have to talk OSPF on all links to distribution nodes

router ospf 10

router-id 10.255.254.1

network 10.10.10.0 0.0.0.255 area 0

network 10.255.254.1 0.0.0.0 area 0

network area commands work like ACL statements and first statement starts OSPF on each interface whose ip address belongs to 10.10.10/24 space

Second command is used to advertise its own loopback.

router-id command allows to define the OSPF router-id.

Distribution nodes have to advertise client Vlans and to take part in OSPF communication on point to point link.

if you use a L2 access layer design client vlans are served by distribution nodes.

if you use a L3 access layer design the access layer switches take part in OSPF and have to advertise their own client vlans.

Hope to help

Giuseppe

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