Routed port priority

[Laurent Soffray]

Hello,

I have a 4503 switch, two 3560 and a couple of 2960.

I've created a L3 route between the 4503 and the first 3560. I've linked the first 3560 and a 2960 with a L2 link and then link this 2960 to the second 3560 with a L2 link.

Now, I want to add another L3 routed link between the second 3560 and my 4503.

I know that I'm going to create a loop, and that's why I want to know how to set the priority between two routed ports on two differents switches.

I know that there's no STP for the routed port.

Can you help me?

[Laurent Soffray]

I've found the answer:

HSRP.

[Jon Marshall]

lsoffray1 wrote:
Hello,
I have a 4503 switch, two 3560 and a couple of 2960.
I've created a L3 route between the 4503 and the first 3560. I've linked the first 3560 and a 2960 with a L2 link and then link this 2960 to the second 3560 with a L2 link.
Now, I want to add another L3 routed link between the second 3560 and my 4503.
I know that I'm going to create a loop, and that's why I want to know how to set the priority between two routed ports on two differents switches.
I know that there's no STP for the routed port.
Can you help me?

Laurent

You won't create a loop that way.

A L2 loop is concerned with the physical connections within your network and that is what STP does ie. it blocks links so that there can be no loop.

A L3 loop has very little to do with physical connections, it is more to do with a router having a route to a network that points back to the router that sent it the information. This can be caused by a number of things redistribution being one of the more common issues.

What you describe ie. each 3560 connected to the 4500 with a L3 routed port is a common setup and works very well. The 4500 will see 2 equal cost paths to each destination on the 3560's and so will automatically use both links for forwarding traffic. This will not create a loop.

Jon

[Giuseppe Larosa]

Hello Laurent,

the scenario that you describe is quite common in campus networks:

the use of L3 point to point links provide you the capability to handle link redundancy at OSI layer 3: no problems of loops and no need for STP

forget for a moment that these are switches and look at them as routers:

all you need are two static routes on C4503 one with next-hop = C3560_1 ip address on first L3 p2p link, second static route with next-hop = C3560_2 ip address on second L3 p2p link

each C3560 will probably have a default static route using the appropriate C4503 IP address (that on the L3 link between the two devices) as next-hop.

both C3560 need to have an SVI interface on the common Vlan/IP subnet (with a different IP address on each)  and HSRP could be used there not towards the C4503.

The fact that the Vlan/broadcast domain associated to the IP subnet can have a redundant L2 topology is handled by the two C3560 using STP but from the point of view of the C4503 this is not of direct interest.

Hope to help

Giuseppe