L2 / L3 circuit??

[shinakuma123]

Hi 

I've noticed some strange things, which i want to clarify is feasibly possible. Situation is as follows:

- 1 Router at site A, 1 Router at site B

- Dedicated 1 gig link connecting the 2 together provided via a carrier

- Link at Router A side connects to port Gi1/1 and link at Router B side connects to Gi2/2

Now link is up and passing traffic and all is well, but what I noticed is, one end (on router A port gi1/1) is set as Switchport and access a particular vlan, and the other end (Router B, port gi2/2) is set with a IP address.

So one end is L2 and the other end is L3.... is such situation possible? i thought both ends of a link have to match, ethier switchport so L2 or with ip address so L3....

Can anyone clarify this? their is no other switches or routers in between so am wanting to know how this is working .

thanks! (P.s - Jon Marshall :D I am relying on your expertise buddy :P)

[Jon Marshall · Accepted solution]

P.s - Jon Marshall :D I am relying on your expertise buddy :P)

That could be an unwise thing to do :-)

Yes it's perfectly possible although not the norm.

On the end with the port as a L2 switchport you would obviously need a L3 vlan interface for that vlan and then the routing happens between the SVI on one device and the L3 routed port on the other.

Don't see it much nowadays for a pure routed link but back in the day if one end was a 6500 with CatOS this was the only way you could do it on the 6500 end because you couldn't make a port L3.

Obviously now with IOS on the 6500s that has changed.

Jon

[shinakuma123]

Hi Jon

Thanks alot for your prompt reply!! you really are the backbone of Cisco support :P.

Just to clarify things, yes the router A end is a 6500 and the router B end is Cisco 7204 vxr, The L3 for the IP's assigned to both ends (because they are BGP peers) is on router A... 

But the port gi1/1 on router A is set as switchport and as access on a different vlan...

could you explain this to me a bit more, am still confused how such a setup could work,

1) is that link essentially L2 or L3 or both? 

2) Is possible to trunk VLANs over that link, if we set the router B side to switchport also? If we do this do we lose the L3 bgp adjacency? 

3) What type of traffic flows on this link since its a mixture? layer 2 traffic or layer 3?

If you could explain a bit more on how this is working i would appreciate that, I have never seen such a setup and just stumbled on it at work, need to sort this mess asap

thanks

[Jon Marshall]

The L3 for the IP's assigned to both ends (because they are BGP peers) is on router A... 

Happy to explain but not sure what you mean by the above ie. how can both ends be on router A.

Just so I understand you have a 6500 with a L2 switchport in a vlan and then presumably you also have a L3 vlan interface (SVI) for the same vlan with an IP address from the same IP subnet as the IP address on the L3 port on the 7200 router ?

Is that the case ?

Jon

[shinakuma123]

Ok let me explain:

Router A - Gi1/1 ----> Router B Gi2/2

Router A config:

ip address xxx.xxx.xx.218 255.255.255.252
 ip access-group xxx-filter in
 no ip mroute-cache
 load-interval 30
 duplex auto
 speed 1000
 media-type gbic

Router B config:

switchport
 switchport access vlan 30
 switchport mode access
 load-interval 30
 speed nonegotiate
 spanning-tree bpdufilter enable

Now as you saw, router A has a /30 configured with itself being .218.

Router B has .217 but configured on a SVI on a different VLAN:

!
interface Vlan35
 ip address xxx.xxx.xx.217 255.255.255.252 secondary

This VLAN is not in the config on router B, as thats set to vlan 30 access,

Vlan 35 goes to our filtering device, which then passes it back to whereever its going..

Can you explain this, as its odd!

[Jon Marshall]

That is odd to say the least.

I can't see how the packet gets delivered to the SVI as it is for a different vlan than the access port.

If you ping the x.x.x.218 address using the source interface as vlan 35 does it work ?

Jon

[shinakuma123]

I can ping xx.xx.xx.218 from Router B (.217) using vlan 35, it wont let me use vlan 30 , because its L2 on the device, the l3 is not on here.. any suggestion please, i need to figure how this is working

[Jon Marshall]

Can you post the configuration of router B ?

In your explanation you said router B was the 7200, is that the case ?

Jon

[shinakuma123]

I cant post config sorry, am bound by NDA policies as this is LIVE core equipment. 

Router A is the 7204 vxr, Router B is 6509 

Check private message please.. :)

[Jon Marshall]

Just responded.

Jon