Migration from Cisco 2911 combo rtr/switch to 8200 and 9300 separate

[cisco-newbie2022]

Hello,

This is my first attempt at migrating from a combination RTR/Switch to two separate units. The router only has two ethernet ports that are already going to be connected to other equipment. The older unit's internal connection wasn't considered when this equipment was bought, and now I'm left with: how does this router(8200-1N-4T) connect to the switch(9300)? Should I get an SFP with ethernet port and connect it to the switch? Should I get a switch module installed in the router for a layer 2 connection(more expensive)? Should this connection be routed, and was it a layer 3 connection in the 2911? Am I overthinking this? Any advice would be appreciated. 

interface GigabitEthernet1/1
description Internal switch interface connected to EtherSwitch Service Module
no ip address

[Giuseppe Larosa]

Hello @cisco-newbie2022 ,

you haven't provided details on how the configuration was with the Cisco C2911 with Etherswitch module

>> how does this router(8200-1N-4T) connect to the switch(9300)?

if you only have two physical interfaces on the 8200 you need to connect one of them to the Catalyst 9300 and you can use subinterfaces on the C8200 side and an 802.1Q L2 trunk on the Cat 9300 side.

Other options are possible if you enable ip routing on the catalyst 9300 you could move one link to the Cat9300 and use a single routed link with appropriate static routes.

Hope  to help

Giuseppe

[cisco-newbie2022]

I have the SFP's but etherchannel would be overkill for what this system needs. Can I trunk all the vlans on the switch side (SFP) and then set up the router to accept the trunk? I have tried that and cant ping back and fourth. The switch setup seems pretty simple, switchport mode trunk, switchport trunk vlan all, but the router does seem to accept it? I enabled CDP on each device and they can see each other.

[Richard Burts]

You have not provided details about the configuration of these devices. On the switch side it should be fairly simple: on the interface used to connect to the router configure it as trunk with all vlans allowed. On the router it is somewhat more complex - on the router interface connecting to the switch you configure multiple subinterfaces, one subinterface for each of the vlans being sent over the trunk. On each subinterface you identify the vlan tag and configure an appropriate IP address and mask.

[liviu.gheorghe]

Hello @cisco-newbie2022 ,

the Cisco 8200-1N-4T has 4 GigabitEthernet ports, 2 of them are with RJ45 connectors and 2 with SFP.

You can connect the Cisco 8200 to the 9300 switch with 1 or 2, eventually buying additional SFP's. I recommend connecting two links between the devices and configuring an Etherchannel - this way you have the advantage of 2 Gbps bandwidth between the two devices and port redundancy. The etherchannel link can be configured as a 802.1Q L2 trunk between the devices as mentioned by @Giuseppe Larosa .

Hope this helps.

[Joseph W. Doherty]

There are a couple of ways to approach this combo.

Firstly though, since the 9300 is a L3 switch, why do you need the router?

If you need the router, for features unique to the router, you could make it both a router on a stick to the 9300, for L2, and/or make it a peer router, for L3, i.e. all external device copper connections are via the 9300.  If there's some feature the router must perform, traffic can be directed to the router on switch trunk interface connected to router subinterface interface.  Optionally, you could bind the switch and router using Etherchannel to provide both port redundancy and possibly about, on average, 50% more bandwidth.

Possibly, what I'm describing isn't clear without diagrams and/or example configuration.  Doing such might also appear complex because often network engineers are still thinking the constraints using routers and L2 switches, not all the potential of a L3 switch.

For example, given host <> (access port V#) L3 switch (access port V#) <>  (not subinterface) router

the host and router share a L2 connection.  But, on a L3 switch, you can add a SVI for V#.  Then host has two L3 interfaces available to it.  Perhaps you use the L3 switch SVI as the subnet gateway, so external to L3 switch network outbound traffic is routed or redirected to the router, but the router can return traffic to the host bypassing the SVI.

Or host <> (access port V#) L3 switch (non switch port - aka routed port) <>  (not subinterface) router

Now all external to the L3 traffic must be routed to and from the router.

Or host <> (access port V#) L3 switch (trunk) <>  (subinterfaces) router

This latter, depending on what SVIs have been defined, can be any of the above.

The important take away, much like a ROAS, the router doesn't need any connections beyond what the switch can provide, unless the switch runs out of ports or you don't have compatible media (e.g. copper vs. fiber) ports.

If your Ethernet module in the 2900 was just a L2 model, you had VLANs defined for the L2 ports, and SVIs to do L3 with those VLANs and the router's ports.  When you interconnect a router and L3 switch, you don't need SVIs as you can use router subinterface ports, as you would with a ROAS, but with a L3 switch, you can still use L3 on the switch, which provides two ways to interact with the router's L3.