A solution without using dynamic routing on the central router is as follows. As the requirement is to route traffic to particular ISP links based on source subnet, policy-based routing is required. Note that no other routing is configured other than the policy-based routing, which takes precendence over the standard routing table. IPSLA is required for failover.
! Configure and schedule IP SLA operations
ip sla 1
icmp-echo 1.1.1.2
ip sla schedule 1 life forever start-time now
!
ip sla 2
icmp-echo 2.1.1.2
ip sla schedule 2 life forever start-time now
!
! Configure Object Tracking to track the operations
!
track 1 ip sla 1 reachability
track 2 ip sla 2 reachability
!
! Configure ACL
ip access-list standard FROM_LAN1_ACL
permit 192.168.1.0 0.0.0.255
!
ip access-list standard FROM_LAN2_ACL
permit 192.168.2.0 0.0.0.255
!
! Configure PBR policing on the router
route-map PBR_RM 10
match ip address FROM_LAN1_ACL
set ip next-hop verify-availability 1.1.1.2 1 track 1
set ip next-hop verify-availability 2.1.1.2 2 track 2
!
route-map PBR_RM 20
match ip address FROM_LAN2_ACL
set ip next-hop verify-availability 2.1.1.2 1 track 2
set ip next-hop verify-availability 1.1.1.2 2 track 1
!
! Apply PBR policy on both incoming interfaces of the router i.e. 192.168.1.x/24 & 192.168.1.x/24
interface ethernet 0/0
ip address 192.168.1.x 255.255.255.0
ip policy route-map PBR_RM
!
interface ethernet 0/1
ip address 192.168.2.x 255.255.255.0
ip policy route-map PBR_RM
!
! Display PBR related information
show route-map PBR_RM
show track brief
show ip sla stat
config adapted from source:
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/nx-os/IPSLA/configuration/guide/b_Cisco_Nexus_7000_Series_NX-OS_IP_SLAs_Configuration_Guide/b_Cisco_Nexus_7000_Series_NX-OS_IP_SLAs_Configuration_Guide_rel_6-x_chapter_01000.pdf
Alternatives to this really depend on the WAN service provided by the ISPs. Any of the following would be more elegant solutions if you have access to them.
1. You could consider using a dynamic routing protocol such as OSPF between the central router and the two ISP routers, which you could use to engineer traffic paths without recourse to policy-based routing.
2. Another alternative would be to run BGP down to the central router if this is used on the ISP WANs.
3. A FHRP like HSRP using MHSRP (more than one HSRP group on the ISP routers) would be a simple way to engineer outgoing traffic as per the requirements.
For 2. and 3. if this is an public internet based WAN service rather than private such as MPLS VPN, you may need to source provider independent IP address space.
Hope this helps. Please rate if it does.
