How Layer 2 switch works without default gateway

[mahesh18]

Hi Everyone,

4500 switch is connected to 2960 switch.

4500  config

Vlan 10

name Data

It has ip helper configured that points to DHCP.

From 4500  switch port  - port  x connects to 2960 port.

Port x is configured as trunk between 4500 and 2960.

***********************************************************************************

2960 config

vlan 10

name data

All user ports are configured under vlan 10 and as access ports.

Port x is trunk port connected frpm 2960  to 4500 switch allowing vlan 1 and 10 only.

This switch has no default gateway configured.

We connected user PC on 2960 switchports and they were able to get the IP from DHCP server and were able to access the network

My question is how users on 2960 switch are able to access the network without ip default-gateway configured  on 2960 switch?

[pjmonline]

The switch only needs a default gateway if it will be doing routing. In your case the 4500 will do the routing and should require a default route. The dhcp server hands the client its ip address along with subnet mask and default route. The rest is handled by the 4500.

Sent from Cisco Technical Support iPhone App

[darren.g · Accepted solution]

mahesh18 wrote:
Hi Everyone,
4500 switch is connected to 2960 switch.
4500  config
Vlan 10 
name Data
It has ip helper configured that points to DHCP.
From 4500  switch port  - port  x connects to 2960 port.
Port x is configured as trunk between 4500 and 2960.
***********************************************************************************
2960 config
vlan 10 
name data
All user ports are configured under vlan 10 and as access ports.
Port x is trunk port connected frpm 2960  to 4500 switch allowing vlan 1 and 10 only.
This switch has no default gateway configured.
We connected user PC on 2960 switchports and they were able to get the IP from DHCP server and were able to access the network
My question is how users on 2960 switch are able to access the network without ip default-gateway configured  on 2960 switch?

The 2960 passes the requests to the 4500 switch over the trunk, which in turn passes them to the default gateway. As long as the ports are in the same access VLAN, and that VLAN is trunked to a device where there *IS* access to the default gateay, then the traffic will be passed.

Layer 2 does not need a default gateway at layer 3 to allow devices to communicate. You can have a bunch of devices in a single layer 2 VLAN and provided they're all configured with the same IP subnet at layer 3, they will *all* be able to communicate with each other - they just won't be able to talk to anything outside that VLAN. You only need layer 3 when you need to move your traffic outside the single subnet environment (which is almost always these days, I admit).

If your DHCP server is connected to an access port in VLAN 10, then *any* device in VLAN 10 will be able to get an IP address from it, and access other devices. Only when you need to get OUT of VLAN 10 do you need a default gateway for higher level protocols to work.

Cheers

[mahesh18]

Hi Darren,

Thanks for reply.

So layer 2 switch 2960 has no interface vlan 10

It is has layer 2 vlan 10

Also there is no management IP on layer 2 .

So on layer 2 switch we just create interface vlan   for management purposes like telnet?

[darren.g]

mahesh18 wrote:
Hi Darren,
Thanks for reply.
So layer 2 switch 2960 has no interface vlan 10
It is has layer 2 vlan 10
Also there is no management IP on layer 2 .
So on layer 2 switch we just create interface vlan   for management purposes like telnet?

Mahesh.

On the layter 2 switch, I would create an IP address on VLAN1's (the untagged VLAN) SVI - and use the same VLAN on the other switch, in the same IP subnet range, for its management address.

There are, of course, exceptions - but I usually use VLAN 1 for management because I know it exists by default on even non-layer3 switches like the 2950 and 2960.

Cheers

[mahesh18]

Many thanks Darren and Joseph.

Have a great weekend.

[Joseph W. Doherty · Accepted solution]

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Posting

My question is how users on 2960 switch are able to access the network without ip default-gateway configured  on 2960 switch?

The default-gateway doesn't support connected hosts to the switch, its purpose to notify the switch (as a host) what the gateway IP is.

If you connected your 2960 to a 4500 access port in VLAN10, clients connecting to the 2960 would work the same provided the port they connected to on the 2960 and the port connecting to the 4500 were in the same VLAN.  I.e. VLAN 10 not required on the 2960 nor is trunk link.  (NB: an unmanaged non-VLAN capable switch would also work in this configuration.)