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Load-balancing across four T1's on 2 routers

 
 
Sean-Usenet
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Posts: n/a
 
      08-31-2006
I am setting up the following:

- 2 Cisco 2800 series routers, each has two T-1 internet connections.
- Those 2 routers are also connected to a 100mb layer-3 switch.
- Our firewall will also connected to that layer-3 switch.
- The firewall's' default gateway will be that layer-3 switch.
- The workstations are behind the firewall, and will use the firewall
as their default gateway

- OSPF will be running on the 2 routers and the layer-3 switch, and
also on 2 routers on the ISP's site.
- The OSPF area will be Totally Stubby, thus the ISP's routers will be
advertising default routes into our network.

As long as all four T-1's are up, everything should work fine:

- The workstations will route outbound packets to the firewall
- The firewall will route the packets to the layer-3 switch
- The layer-3 switch is running OSPF and will see two equal cost
default routes, and will load-balance traffic between our two routers
- The routers will in turn also have two defaults routes (1 route
through each T-1), and load-balance traffic across each T-1


My problem is what happens when one T-1 goes down? Our layer-3 switch
will still see equal cost routes and split the traffic across the two
routers, even though one router has 1/2 the bandwidth.

Can someone help me with this problem? Please let me know if you have
any questions on what I explained above! Here is a diagram of the
setup, i hope it looks ok:

R1 R2 (ISP Routers)
|| ||
|| || (4 total T-1s)
|| ||
R1 R2 (Our Routers)
| |
\ /
\ /
\ /
Layer-3
Switch
|
|
|
Firewall
|
|
|
Layer-2
Switch
|
|
|
|
Workstations


Thanks!
Sean

 
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Merv
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      08-31-2006
How many FastEthernet ports on your 2800 routers ?

 
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Igor Mamuzic
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      08-31-2006
Maybe you could find solution on one of these links:
- if you have 12.3 IOS see:
http://www.cisco.com/en/US/products/...0801d1e95.html
or
- if you have 12.4 IOS see:
http://www.cisco.com/en/US/products/...0801d1e95.html


B.R.
Igor



"Sean-Usenet" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>I am setting up the following:
>
> - 2 Cisco 2800 series routers, each has two T-1 internet connections.
> - Those 2 routers are also connected to a 100mb layer-3 switch.
> - Our firewall will also connected to that layer-3 switch.
> - The firewall's' default gateway will be that layer-3 switch.
> - The workstations are behind the firewall, and will use the firewall
> as their default gateway
>
> - OSPF will be running on the 2 routers and the layer-3 switch, and
> also on 2 routers on the ISP's site.
> - The OSPF area will be Totally Stubby, thus the ISP's routers will be
> advertising default routes into our network.
>
> As long as all four T-1's are up, everything should work fine:
>
> - The workstations will route outbound packets to the firewall
> - The firewall will route the packets to the layer-3 switch
> - The layer-3 switch is running OSPF and will see two equal cost
> default routes, and will load-balance traffic between our two routers
> - The routers will in turn also have two defaults routes (1 route
> through each T-1), and load-balance traffic across each T-1
>
>
> My problem is what happens when one T-1 goes down? Our layer-3 switch
> will still see equal cost routes and split the traffic across the two
> routers, even though one router has 1/2 the bandwidth.
>
> Can someone help me with this problem? Please let me know if you have
> any questions on what I explained above! Here is a diagram of the
> setup, i hope it looks ok:
>
> R1 R2 (ISP Routers)
> || ||
> || || (4 total T-1s)
> || ||
> R1 R2 (Our Routers)
> | |
> \ /
> \ /
> \ /
> Layer-3
> Switch
> |
> |
> |
> Firewall
> |
> |
> |
> Layer-2
> Switch
> |
> |
> |
> |
> Workstations
>
>
> Thanks!
> Sean
>



 
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James
Guest
Posts: n/a
 
      08-31-2006
I am 99% sure that your layer three switch will see four equal cost
routes not two, when one T1 goes down it will then see three routes.
Your layer three switch will take this into consideration when making
its balancing decision.

James


Igor Mamuzic wrote:
> Maybe you could find solution on one of these links:
> - if you have 12.3 IOS see:
> http://www.cisco.com/en/US/products/...0801d1e95.html
> or
> - if you have 12.4 IOS see:
> http://www.cisco.com/en/US/products/...0801d1e95.html
>
>
> B.R.
> Igor
>
>
>
> "Sean-Usenet" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) oups.com...
> >I am setting up the following:
> >
> > - 2 Cisco 2800 series routers, each has two T-1 internet connections.
> > - Those 2 routers are also connected to a 100mb layer-3 switch.
> > - Our firewall will also connected to that layer-3 switch.
> > - The firewall's' default gateway will be that layer-3 switch.
> > - The workstations are behind the firewall, and will use the firewall
> > as their default gateway
> >
> > - OSPF will be running on the 2 routers and the layer-3 switch, and
> > also on 2 routers on the ISP's site.
> > - The OSPF area will be Totally Stubby, thus the ISP's routers will be
> > advertising default routes into our network.
> >
> > As long as all four T-1's are up, everything should work fine:
> >
> > - The workstations will route outbound packets to the firewall
> > - The firewall will route the packets to the layer-3 switch
> > - The layer-3 switch is running OSPF and will see two equal cost
> > default routes, and will load-balance traffic between our two routers
> > - The routers will in turn also have two defaults routes (1 route
> > through each T-1), and load-balance traffic across each T-1
> >
> >
> > My problem is what happens when one T-1 goes down? Our layer-3 switch
> > will still see equal cost routes and split the traffic across the two
> > routers, even though one router has 1/2 the bandwidth.
> >
> > Can someone help me with this problem? Please let me know if you have
> > any questions on what I explained above! Here is a diagram of the
> > setup, i hope it looks ok:
> >
> > R1 R2 (ISP Routers)
> > || ||
> > || || (4 total T-1s)
> > || ||
> > R1 R2 (Our Routers)
> > | |
> > \ /
> > \ /
> > \ /
> > Layer-3
> > Switch
> > |
> > |
> > |
> > Firewall
> > |
> > |
> > |
> > Layer-2
> > Switch
> > |
> > |
> > |
> > |
> > Workstations
> >
> >
> > Thanks!
> > Sean
> >


 
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Merv
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      08-31-2006

There may be some additional things to consider ...

What happens if an ISP upstream router becomes partitiononed from the
rest of the ISP network - the T1 will stay up but your traffic will be
blackholed - believe it happens.

Also what approach is being planned to load balance the traffic across
each of the pairs of T1s ?

 
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Merv
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      08-31-2006


> I am 99% sure that your layer three switch will see four equal cost routes not two


That depends.

It would be true if the T1s are not bundled and a default route is
configured to point to next hop on each of the two T1's

However if MLPPP we used to bundle the T1's for load balanicng then
there would only be one default route per 2800 and thus only two in
total seen by the layer 3 switch.

 
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Sean-Usenet
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      08-31-2006
Hello

There are 2 FE ports on each router, with only 1 FE port in use.


Merv wrote:
> How many FastEthernet ports on your 2800 routers ?


 
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Sean-Usenet
Guest
Posts: n/a
 
      08-31-2006
We do not plan on using MLPPP. As I understand, our layer-3 switch
will only have 2 default route entires - 1 for each router, not 4
default route entries - 2 from each router. Isn't that not correct?

Merv wrote:
> > I am 99% sure that your layer three switch will see four equal cost routes not two

>
> That depends.
>
> It would be true if the T1s are not bundled and a default route is
> configured to point to next hop on each of the two T1's
>
> However if MLPPP we used to bundle the T1's for load balanicng then
> there would only be one default route per 2800 and thus only two in
> total seen by the layer 3 switch.


 
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Merv
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      08-31-2006
What is the origin of default route on each 2800 ?

Is it provided by the ISP via a dynamic routing protocol ?

Or is it via static routes configured on the 2800 ?

 
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Merv
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      08-31-2006


> We do not plan on using MLPPP.


That being the case and assuming you will be using CEF, be aware that
the two T1 will not be evenly load balanced in real time as CEF does
per destination load balancing.

 
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