Redundant WAN question

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May 17th, 2010
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Hi

I have a  remote office with to WS-C4510R-E as core switches witch is connected via an etherchannel (trunk). All access switches have one connection to each core. And here we are running GLBP.

Today we have one layer 2 connection to our main site and we are running OSPF over that connection. What do we need to do if we want to connect an additional layer 2 connection. One to each core switch?

Regards

Mikkel

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Jon Marshall Mon, 05/17/2010 - 03:59
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Mikkel


Can you post the visio as .jpg.


Jon

Jon Marshall Mon, 05/17/2010 - 04:07
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Siemens_SWP wrote:


Hi Jon


Here you go.


Mikkel


Mikkel


Thanks for that.


Are the 2 routers in your diagram connecting to the WAN ?


If so i would simply configure the connections from the routers to the core switches as L3 P2P links using /30 addresses eg.


core switch 1


int gi0/1

no switchport

ip address 192.168.5.1 255.255.255.252


router 1


int gi0/1

ip address 192.168.5.2 255.255.255.252



core switch 2


int gi0/1

no switchport

ip address 192.168.5.5 255.255.255.252


router 2


int gi0/1

ip address 192.168.5.6 255.255.255.252


Because you are using GLBP then traffic from the access-layer switches could end up going to either core switch. Each core switch will then see it's shortest path via it's direct connection to the WAN router. If one of the WAN routers fail then the interconnect linnk between your core switches can be used to get to the other WAN router.


Jon

Siemens_SWP Mon, 05/17/2010 - 04:18
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Hi Jon

Thanks for the answer.

I have done that! But I am not sure it is working as expected.

If I do a "show ip route" one the core switches I can only see routes from the connected router! Like:

O IA    10.150.2.6/32
           [110/211] via 149.212.15.177, 00:00:37, GigabitEthernet9/44
O IA    10.150.1.5/32
           [110/207] via 149.212.15.177, 00:00:37, GigabitEthernet9/44

So what will happen if I shut down Gi9/44?

No default route is configured on the switch!

Mikkel  

Jon Marshall Mon, 05/17/2010 - 04:29
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Siemens_SWP wrote:


Hi Jon

Thanks for the answer.

I have done that! But I am not sure it is working as expected.

If I do a "show ip route" one the core switches I can only see routes from the connected router! Like:

O IA    10.150.2.6/32
           [110/211] via 149.212.15.177, 00:00:37, GigabitEthernet9/44
O IA    10.150.1.5/32
           [110/207] via 149.212.15.177, 00:00:37, GigabitEthernet9/44

So what will happen if I shut down Gi9/44?

No default route is configured on the switch!

Mikkel  


Mikkel


You only see routes from the connected router because the core switches are not seeing equal cost paths. The reason being that there is an extra hop between the core switches ie.


core 1 gets routes from router 1. Core 1 also gets routes from core 2 which got it's routes from router 2. But that path will be longer than going direct to router 1 from core 1. I'm assuming you are running OSPF between the 2 core switches ?


If you shut down gi9/44 then you should see the routes pointing to the other core switch being installed in the routing table.


If you want each core switch to see 2 equal cost paths then you could make the connections between the core switches and routers all be on the same vlan and this vlan would need to be allowed on the interconnect as well. So instead of using L3 P2P links which creates additional hops the router LAN interfaces and the core switch interfaces connecting to the routers are all allocated to the same vlan.


Jon

Jon Marshall Mon, 05/17/2010 - 04:35
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Mikkel


Quick follow up to previous post.


If you did use a single vlan to get 2 equal cost paths then be aware that the core interconnect could become more utilised. Also because you using GLBP then they are kind of "fighting" each other ie. GLBP distibutes between the core switches but then the traffic might then be sent back across the interconnect depending on which of the 2 equal cost routes was chosen. So you could end up with suboptimal paths.


The ideal solution is to use L3 P2P and connect each WAN router to both core switches, that way you benefit from GLBP and you still get 2 equal cost paths to each remote destination.


Jon

Siemens_SWP Mon, 05/17/2010 - 05:19
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Hi Jon


The ideal solution is to use L3 P2P and connect each WAN router to both core switches, that way you benefit from GLBP and you still get 2 equal cost paths to each remote destination.




Sounds nice! But how will I do this! Will I then need like 4 small subnet? Anything else I should thik about? Do you have a link?


Se drawing

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Jon Marshall Mon, 05/17/2010 - 05:29
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Siemens_SWP wrote:


Hi Jon


The ideal solution is to use L3 P2P and connect each WAN router to both core switches, that way you benefit from GLBP and you still get 2 equal cost paths to each remote destination.




Sounds nice! But how will I do this! Will I then need like 4 small subnet? Anything else I should thik about? Do you have a link?


Se drawing


Yes you would need 4 /30s. I notice on your diagram you have used /31s. You can do but if you are using private addressing can't see any huge benefit, up to you though.


Obviously you also need spare ethernet interfaces on each of your WAN router.


This also assumes that each WAN router is receiving routes for the same destinations.


Jon

Siemens_SWP Mon, 05/17/2010 - 06:04
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Hi Jon


Im using public IP´s and I have to use at least 80% of all the IP´s I have. So I think I will go for the /31.


Today the setup is like this:


I have 2 sites 200 km apart.


See drawing


In each end I have 2 4500 series switches. Between the sites there is 2 layer 2 connections.


So if I want to spare interfaces on the switches, how will I do that?

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Jon Marshall Mon, 05/17/2010 - 06:12
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Siemens_SWP wrote:


Hi Jon


Im using public IP´s and I have to use at least 80% of all the IP´s I have. So I think I will go for the /31.


Today the setup is like this:


I have 2 sites 200 km apart.


See drawing


In each end I have 2 4500 series switches. Between the sites there is 2 layer 2 connections.


So if I want to spare interfaces on the switches, how will I do that?


Sorry this is getting a bit confusing. I thought we were just talking about the main site. If all the devices are L3 switches then you just need spare ports to interconnect everything. Obviously between sites you are limited by your connections but i though we were talking about connectivity between your main site core switches and WAN routers (which may or may not be L3 switches) in the same site ?


Jon

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