Change 1841 Router with 3750G Switches.

Unanswered Question
Nov 17th, 2009

Hi,


We have 1841 router used as our Data Link router. It has 2 interfaces, one with connection from ISP and other for our local network.


We have 2 nos 3750G stackable switches in our stock.

Is it possible that we can replace the router with these switches ?


Please guide. Thanks in advance.

K

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Overall Rating: 3.5 (2 ratings)
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Mohamed Sobair Tue, 11/17/2009 - 01:18


Hi,


what is the primary usage for 1841? what do you require?


Does the 2 3750 Stackable Switches required by you?


Mohamed

ciscokalpesh Tue, 11/17/2009 - 01:24

Hi,

Thanks for reply.


The primary usage of 1841 router is that - it is our Data Link router. All our sites are connected with Data Links (through ISP).


The 2 switches can provide us redundancy if we can replace it. Also we need to configure VLANs in our network for which we need to configure the switches. So if we can replace these switches also as the Data Link router, then we can eliminate the 1841 router from the network.


Thanks again,

K

Mohamed Sobair Tue, 11/17/2009 - 02:12

Hi,


depending on the type of Interfaces installed on the 1814. for your WAN, and keep in mind that some QoS and Nat features are not supported on the switch. Other than that you can replace and get the benefit of the stacking switches for multiple connections.



HTH

Mohamed

ciscokalpesh Tue, 11/17/2009 - 02:27

Hi,

Thanks for reply.


We do not have any Qos and NAT features enabled on this router. I would like to know about the interface configuration.

The router has 2 interface configured - FA 0/0 and FA 0/1. I will need to configure 2 interface on this switch as such. How do i do that ? When i go to the interface config mode, i do not get any option to assign IP address to that interface.


Please guide. Thanks,

K

LordFlasheart Tue, 11/17/2009 - 03:11

Hi,


Your 3750 should support Layer 3 addresses on the switchports but you must issue the "no switchport" command in order to turn the port into a Layer 3 port. Only then can you add an IP address to that port.


Please rate if helpful.


Hope it all works well :-)

ciscokalpesh Tue, 11/17/2009 - 03:36

Hi,


Thanks for reply.


Yes it works ! Thanks.


I would configure these switches as stacks.

1)

One of the gig interface (int-1) as L3 for ISP and other gig interface (int-2) as L3 for internal gateway.

In stacks, i will have to configure same 2 ports (int-1 & int-2) on the other switch too ? So that if one switch goes down the traffic will be moving through other switch ports (int-1 & int-2) physically connected on it.


2)

Can i configure the internal default gateway as vlan1 ip, and then route the traffic from that vlan1 gw to the ISP port configured as L3 port ?


3)

Is there a need to configure HSRP with stackable switches ?


Please guide. Thanks,


K


Joseph W. Doherty Tue, 11/17/2009 - 03:41

As the other posters have described, you can configure a port to be a routed port (as they also described). As an alternative, you can assign the port to a VLAN and assign it an IP address. Assuming there's only one WAN (Ethernet) connection, one advantage of using a VLAN in a stack configuration, you can configure two different stack member ports for the same WAN VLAN. If the connected stack member fails, just reconnect the patch cable to the other stack member port. (I.e. saves time in getting your WAN back on-line.)


BTW, don't forget to route on the 3750 you'll need to enable routing (off by default, I believe).


[edit]

Just noticed one of your follow up replies.


#1 Yes you can config a internal L3 routed port too, but unless you're connecting to another internal router (or L3 switch), internal subnets usually managed by VLANs.


The 2nd part of your question is addressed by my original response (above).


#2 Yes, although often recommended to avoid using VLAN1 for a user VLAN.


#3 Strictly speaking, no because if the active stack master fails the new master will reset or take over the gateway address. However, by default when the new master assumes the IP address the MAC changes. This can confuse hosts that don't repond to the gratious ARP done at the same time. Using a HSRP virtual IP gateway avoids this issue. It also allows you to easily add a backup gateway on another L3 device in the future.

ciscokalpesh Tue, 11/17/2009 - 04:15

Hi,


Thanks for the brief reply.


You wrote - "If the connected stack member fails, just reconnect the patch cable to the other stack member port." Why not i configure 2 different stack member ports in the same VLAN and connect both to the network such that if one goes off the other is active ? Is that possible ?


Thanks,

K.

Joseph W. Doherty Tue, 11/17/2009 - 04:36

It is if you have two such cables (usually not for WAN Ethernet) and you run spanning tree correctly to block the L2 loop.


PS:

Within a LAN, say to another switch, instead of using STP to block one link, with a 3750 stack you can Etherchannel across stack members.

Mohamed Sobair Tue, 11/17/2009 - 03:29


Hi,


Login to the Master Switch of the Stack, turn on the switchport to layer-3 port by issuing: (no switchport) Interface level command, and then assign it an IP address.


HTH

Mohamed

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