Load sharing query

[Raja_D]

Hi,

Need advise for the below mentioned requirement.

Cisco router : 18xx

Lan network : 10.54.21.0/24 Primary & 10.54.74.128/26 secondary configured on a SVI interface of the router VLAN2..

Need to ensure that the Primary subnet 10.54.21.0/24 should pass through Primary WAN link 1 & Secondary subnet 10.54.74.128/26 to pass through secondary WAN link 2.

Primary wan link : 10.25.199.184/30

Secondary wan link: 10.24.128.51/30

Incase if the Primary WAN Link ( 10.25.199.184/30) fails the traffic for the Primary subnet ( 10.54.21.0/24 ) should automatically failover through the Secondary WAN link (10.24.128.51/30) and the same applies to secondary subnet (10.54.71.128/26) in vice versa.

BGP attributes that can be used is Local preference for Primary wan link 1 & AS-PATH prepend for secondary wan link 2...

please advise..

[Borgenstrand]

Depending on how everything is setup, you can use a route-map.
Create one access-list per subnet and then create a route-map to match those access-lists and then set the outgoing interface to one or the other.

interface FastEthernet0/0
ip policy route-map RMAP

access-list 1 permit 10.54.21.0 0.0.0.255
access-list 2 permit 10.54.74.128 0.0.0.63
route-map RMAP permit 10
 match ip address 1
 set interface Serial1/0
!
route-map RMAP permit 20
 match ip address 2
 set interface Serial1/1

Please note that in my example, the serial 1/0 is the primary and serial 1/1 is the secondary WAN link. 

[Raja_D]

Thank you Borgenstrand for your answer.

As per my understanding your answer would load share the traffic equally for two different subnets respectively onto the two available wan links , but how would the secondary subnet 10.54.74.128/26 be reacheable in network incase if the secondary wan link goes down ?

I would like to have both the Lan subnets to be always reacheable in network even if any of the either wan links goes down and a switchover to that Lan subnet on which the wan link fails should automatically pass through the other available wan link that is in the network. 

Please advise.. 

[Borgenstrand]

When I tried this in GNS3, the traffic from primary LAN always went to the primary WAN and the secondary LAN always went to the secondary WAN. If I broke the WAN connection it would choose the other one. However, it is not truly load balancing. If 90% of your internet traffic comes from the primary LAN, 90% of the internet traffic would then go to the primary WAN link. 

This however depends on how your network is set-up. Do you have two different ISPs or two links to one ISP? 

[Raja_D]

Yes we have two different ISP's and there are 2 different wan links connected on 2 Seperate interfaces (Fastethernet) ports on router. 

[Borgenstrand]

When I tried this out in GNS3, it worked as it should. The thing is, that is for outgoing traffic. Is outgoing traffic all that you are concerned about? 

[Raja_D]

I am concerned about both forwarding and reverse traffic.. 

[Borgenstrand]

In regards of outgoing traffic, you can do what I wrote before. 

In regards of ingoing traffic, it will not be load-balanced completely. Even if you add as-path prepend, BGP could still choose the other link due to the fact it has less as-paths to you.

I labbed this up in GNS3:

access-list 1 permit 10.54.21.0 0.0.0.255
access-list 2 permit 10.54.74.128 0.0.0.63

route-map PrimaryWANRMAP permit 10
match ip address 2
set as-path prepend 1
route-map PrimaryWANRMAP permit 20
match ip address
exit
route-map SecondaryWANRMAP permit 10
match ip address 1
set as-path prepend 1
route-map SecondaryWANRMAP permit 20
match ip address
exit

router bgp 1
neighbor 1.1.1.2 remote-as 2
neighbor 1.1.1.2 route-map PrimaryWANRMAP out
neighbor 2.2.2.2 remote-as 3
neighbor 2.2.2.2 route-map SecondaryWANRMAP out

In the above example I used BGP AS numbers 1-4. 

So, you should use multiple route-maps to do what you need. 

[Raja_D]

Thank you Borgenstrand for your reply... 

[Borgenstrand]

You are welcome.

 It would be great if you could rate helpful responses.  :)