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Highlighted
Beginner

2 Gateways - Same Subnet

Hello All:

Can I do this. 2 T1's terminated in 2 seperte routers but servicing clients on same subnet internally.

So I would split clients ...some pointing to one gateway, some to the other.

Let's say Router#1

FA0 - 10.0.1.1 <gateway #1>

Router#2

FA0 - 10.0.1.2 <gateway #2>

Would this cause any issues?

Thanks

5 REPLIES 5
Highlighted
Advisor

Re: 2 Gateways - Same Subnet

You can do this. You can run HSRP between the two routers, on the lan side, create two standby groups on each interface. standby group1 will be active router for half the clients, and standby group 2 will be active for half the clients.

r1

int fa0/0

ip add 10.0.1.1

standby 1 ip 10.0.1.3

standby 1 preempt

standby 1 priority 110

standby 2 ip 10.0.1.4

stanbdy 2 preempt

r2

ip add 10.0.1.2

standby 1 ip 10.0.1.3

standby 1 preempt

standby 2 ip 10.0.1.4

stanbdy 2 preempt

standby 2 priority 110

For half the clients in your network, use 10.0.1.3 as default gateway and use 10.0.1.4 as default gateway for the other half. This will ensure that even if one router fails, one half clients that were serviced by the failed router, will still be able to get out through the other router.

HTH

Highlighted
Beginner

Re: 2 Gateways - Same Subnet

I wondered how this was was configured, I have designed solutions with multiple instance's of HSRP for several clients so they have failover either way.

In the situaion it is also important to make sure there is some kind of failover on traffic comming back into the network i.e Static routes if connecting to the net(ISP would need to configure) or HSRP again on the other side if a point 2 point

Highlighted
Beginner

Re: 2 Gateways - Same Subnet

This sounds like a job for Gateway Load Balancing

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a00801541c8.html

Similar to HSRP, but both routers actively accept traffic.

Highlighted
Beginner

Re: 2 Gateways - Same Subnet

Thanks....have you ever implemented this? I read through the article and the concept is clear but some of the config is not.

I gather from the article that the secondary IP's configured are the "virtual ip" and that all members participating in this group must have the same "virtual ip" which is in fact the gateway ip to be used for all clients?

How does one specify which router is the AVG? Is this it in step 5?

Step 5

glbp group ip [ip-address [secondary]]

Example:

Router(config-if)# glbp 10 ip 10.21.8.10

Enables GLBP on an interface and identifies the primary IP address of the virtual gateway.

•After you identify a primary IP address, you can use the glbp group ip command again with the secondary keyword to indicate additional IP addresses supported by this group.

Anyone have a sample config that I can take a look at.

Thanks again...very helpful

Highlighted
Beginner

Re: 2 Gateways - Same Subnet

I have never used it however, this is how I understand it to be configured:

1. Decide which router will be the AVG and assign the highest priority to this router interface.

router(config-if)#glbp 1 priority 200

router2(config-if)#glbp 1 priority 150

router3(config-if)#glbp 1 priority 100

2. Decide if you want to use the preempt option.

router(config-if)#glbp 1 preempt

3. Now for the AVF's. GLBP uses a weighting function to decide which router becomes an AVF. The router begins with a router and this weight is reduced each time an interface goes down. The amount by which its reduced is configurable. The default maximum weight is 100. If you want to make a dynamic weighting adjustment you must specify an interface to track and how to adjust the weight.

router(config)#track 1 interface eth0/0 line-protocol

* you must choose between line-protocol and ip routing

4. Next you must define the weighting thresholds. Maximum weight can range from 1-254 with 100 being default. The upper (default 100) and lower (default 1) thresholds define when the router can and cannot be the AVF.

router(config-if)#glbp 1 weighting 100 lower 1 upper 100

5. Lastly, you must configure glbp to know which objects to track so that the weighting can be adjusted.

router(config-if)#glbp 1 weighting track 1 decrement 10

6. On the AVG router you need to specify the load balancing method. Round-Robin, weighted, or host-dependent

router(config-if)#glbp 1 load-balancing round-robin

7. Assign the virtual IP to the group. This is performed on the AVG:

router(config-if)#glbp 1 ip 10.10.10.10

Issue the following command on the AVF's. These guys will learn the virtual IP from the AVG.

router2(config-if)#glbp 1 ip

HTH,

Brian