VLANs & Switching/Service performance

[StarTAC]

hi all..

i've been doing some reading and i found that one of the ways of slowing down your core network is if you route between VLANs....

i just wanted to understand whether by this, they mean that actual routing between one VLAN to the other, because i have a setup where i've disected a Catalyst 2900XL switch into 3 segments.. and i am using two routers, each with 2 fast ethernet interface modules, to connect all three segments together...

with this kind of setup, is there any special routed protocols being used, or only the data link and physical layer are involved here...?.. can a setup like this be bad for my core network..?..

all help appreciated..

 

[Packetmaster]

This is a very subjective situation. Here we go...

1st- Layer III is involved. You are sending Ethernet Frames to the router which get converted into packets then sent to the destination network or VLAN (layer III) then converted back into Ethernet frames.

2nd- Performance: The rate at which your router can forward packets is the measure performance between locally direct attached Ethernet segments(or VLANS) measured in Packets per second (pps).

3rd-Platform used:
-A 2621 can forward 25 Kpps. This is about 20Mbps/second depending on the byte size of the packet.
-A 3524 (Layer III Switch) can forward 6500(64 byte)-8000(1500 byte) Kpps. This is about 5.4 Gbps (Wire Speed).

4th-The byte size and processing method matter. Smaller byte packets (64byte) send less data and create more processing overhead than larger (1500byte) packets. Cisco Express Switching performance is better than fast switching.

You do not indicate what kind of router you are using. I would recommend a layer III switch if high performance is required. The 3524 offers great performance for the price. If you are using a 2621 form an 802.1q trunk on the fast Ethernet 0/0 and do inter vlan routing with on the same interface with CEF enabled. It will not be faster unless you are not using CEF, but will let you save some hardware.
I would say your setup is probably weak for a core. This is ultimately determined by the amount of data and number of host you have. The real measure, are your users complaining about network performance?

 

[StarTAC]

well, i am using a 2621 router for my core connection.. that is, it will be responsible for routing connections to the WAN [Internet]... it has 2 fast ethernet modules and 2 serial modules....

my connection to the WAN currently is via ethernet.. my satellite modem has an ethernet interface.. so i am not using my serials.. but i will be changing satellite service soon, and my satellite modem for that connection will require a serial connection... so i will end up using on one of my fast ethernet ports....

for my distribution network.. i will be using a 3640 router... it also has 2 fast ethernet modules and will connection the second VLAN segment to the third, on the same switch...

i just wanna know something... isn't segmenting a switch into VLANs the same as having that many switches in your racks.. can i expect a difference in performance if i use two switches as opposed to using one segmented switch..?... does the implementation of a VLAN create performance issues..?..

what would u then recommend my setup be... basically, i have one switch VLAN'ed into 3 segments.. one segment has my satellite equipment and the 2621 router one fast ethernet interface... the second segment has my cache server, bandwidth manager and the 2621 router on the second fast ethernet interface... and the third segment has my servers, services and the 3640 router, which has one fast ethernet connection to the bandwidth manager, and the second one on its segment...

all advice and help appreciated..

 

[Packetmaster]

Yes, to anwser the number of switch question. Vlans are individual Layer II subnets or networks. It makes little difference if you have 1 or 20 vlans on a switch. The only need for a second switch would be more ports or a dual attached server arrangement (high availability).

VLAN trunking EXAMPLE:

http://www.cisco.com/en/US/tech/tk389/tk390/technologies_configuration_example09186a00800949fd.shtml

I would use the 3640 (50-70 kpps) as the Active VLAN router. Use one interface as an 802.1q trunk on the 3640 to the 2900XL. Use sub interfaces to perform the Layer III routing. You can create another trunk on the 2621 (25Kpps) and create HSRP standby interfaces for redundancy. Enable CEF on both routers. Then I would place a WIC module in the 3640 for any serial communications or of course the 2621 if no wic slot is present on the 3640.

 

[StarTAC]

this sounds good...

i have learned that CEF cannot be used on interfaces that run some other features, such as policy-based routing...

so what i will do is enable CEF on both fast ethernet interfaces of my 2620 router [the 2621 has been depolyed at another branch] and on only one interface of my 3640 router, the one that connects to the bandwidth manager, as the other fast ethernet interface that is connected to the switch, will be running policy-based routing for redirection of

http://WWW traffic

to my cache server....

does this sound okay to you.. ?...

all help appreciated...