UPD: Cisco and 2 providers

The problem is as old as today and we will solve it :)
Given: 2 providers (ISP1 and ISP2), internal network and between them a router.
Task: configure one main (ISP1) and one backup channel (ISP2), if the first one drops, it switches to the second one, when the first channel is raised, it switches back.

UPD: Updated, added ip sla, so far without explanation.

Let us proceed from the fact that the switches are also from Cisco :) and for the beginning we will configure 3 Vlans:
Vlan 10 - internal network
Vlan 12 - ISP1
Vlan 14 - ISP2

C2970#vlan data
C2970(vlan)#vlan 10 name OurNet
VLAN 10 added:
Name: OurNet
C2970(vlan)#vlan 12 name ISP1
VLAN 12 added:
Name: ISP1
C2970(vlan)#vlan 14 name ISP2
VLAN 13 added:
Name: ISP2
C2970-Servers(vlan)#exit
APPLY completed.
Exiting....
C2970#

Port number 1 belongs to Vlan 12 -ISP1, port 2 belongs to Vlan 14 (ISP2), the third trunk port is for the router, the other ports belong to Vlan 10, our internal network.

interface FastEthernet0/1
description -=I=- ISP1 -=I=-
switchport access vlan 12
switchport mode access
no cdp enable

interface FastEthernet0/2
description -=I=- ISP2 -=I=-
switchport access vlan 14
switchport mode access
no cdp enable

interface FastEthernet0/3
description -=I=- RTR -=I=-
switchport mode trunk
no mdix auto

interface FastEthernet0/4
switchport access vlan 10
switchport mode access
no cdp enable
...
interface FastEthernet0/24
switchport access vlan 10
switchport mode access
no cdp enable

We’ve dealt with switching, we are now going to our root router, Cisco 2811 with Jos c2800nm-adventerprisek9_ivs-mz.124-24.T1.bin is taken as an example (although it will work on a different root router with another IOS)
')
Configure the interfaces:

interface FastEthernet0/0
no ip address
duplex auto
speed auto
no cdp enable
!
interface FastEthernet0/0.12
description -=I=- Internet over ISP1 -=I=-
encapsulation dot1Q 12
ip address 164.122.12.9 255.255.255.248
ip access-group ACL_INET_OUT_ISP1 out
ip nat outside
ip virtual-reassembly
no cdp enable
!
interface FastEthernet0/0.14
description -=I=- Internet over ISP2 -=I=-
encapsulation dot1Q 14
ip address 122.164.8.17 255.255.255.248
ip access-group ACL_INET_OUT_ISP2 out
ip nat outside
ip virtual-reassembly
no cdp enable
!
interface FastEthernet0/1
no ip address
duplex auto
speed auto
!
interface FastEthernet0/1.10
description to -=I=- LAN -=I=-
encapsulation dot1Q 10
ip address 10.10.10.200 255.255.255.0
ip nat inside
ip virtual-reassembly
no cdp enable

Set up pools for NAT:
ip nat pool ISP1-PUBLIC-IP 164.122.12.9 164.122.12.9 netmask 255.255.255.248
ip nat pool ISP2-PUBLIC-IP 122.164.8.17 122.164.8.17 netmask 255.255.255.248

Access lists (ACL):
ACL'i for interfaces, so that no one breaks :)
ip access-list extended ACL_INET_OUT_ISP1
deny ip any 10.0.0.0 0.255.255.255 log-input
deny ip any 172.16.0.0 0.15.255.255 log-input
deny ip any 192.168.0.0 0.0.255.255 log-input
permit ip host 164.122.12.9 any
deny ip any any log-input

ip access-list extended ACL_INET_OUT_ISP2
deny ip any 10.0.0.0 0.255.255.255 log-input
deny ip any 172.16.0.0 0.15.255.255 log-input
deny ip any 192.168.0.0 0.0.255.255 log-input
permit ip host 122.164.8.17 any
deny ip any any log-input

ACL'i for those who are online:
ip access-list extended ACL_NAT-INET_OUT_ISP1
remark /--------------------------------------
remark Black Hole RFC 1918
remark --------------------------------------/
deny ip any 10.0.0.0 0.255.255.255 log-input
deny ip any 172.16.0.0 0.15.255.255 log-input
deny ip any 192.168.0.0 0.0.255.255 log-input
remark /--------------------------------------
remark SMTP For Exchange
remark --------------------------------------/
permit tcp host 10.10.10.254 any eq smtp
deny tcp any any eq smtp log-input
remark /--------------------------------------
remark WSUS
remark --------------------------------------/
permit ip host 10.10.10.251 any
remark /--------------------------------------
remark Full Aceess for Odmins
remark --------------------------------------/
permit ip host 10.10.10.10 any
remark /--------------------------------------
remark Access for Exchange
remark --------------------------------------/
permit ip host 10.10.10.254 any
remark /--------------------------------------
remark Deny SMTP for TI (gate)
remark --------------------------------------/
deny tcp host 10.10.10.249 eq smtp any log-input
remark /--------------------------------------
remark Access to Internet for TI (gate)
remark --------------------------------------/
permit ip host 10.10.10.249 any
remark /--------------------------------------
remark Self
remark --------------------------------------/
permit ip host 164.122.12.9 any
remark /--------------------------------------
remark Deny All
remark --------------------------------------/
deny ip any any log-input

ip access-list extended ACL_NAT-INET_OUT_ISP2
remark /--------------------------------------
remark Black Hole RFC 1918
remark --------------------------------------/
deny ip any 10.0.0.0 0.255.255.255 log-input
deny ip any 172.16.0.0 0.15.255.255 log-input
deny ip any 192.168.0.0 0.0.255.255 log-input
remark /--------------------------------------
remark SMTP For Exchange
remark --------------------------------------/
permit tcp host 10.10.10.254 any eq smtp
deny tcp any any eq smtp log-input
remark /--------------------------------------
remark WSUS
remark --------------------------------------/
permit ip host 10.10.10.251 any
remark /--------------------------------------
remark Full Aceess for Odmins
remark --------------------------------------/
permit ip host 10.10.10.10 any
remark /--------------------------------------
remark Access for Exchange
remark --------------------------------------/
permit ip host 10.10.10.254 any
remark /--------------------------------------
remark Deny SMTP for TI (gate)
remark --------------------------------------/
deny tcp host 10.10.10.249 eq smtp any log-input
remark /--------------------------------------
remark Access to Internet for TI (gate)
remark --------------------------------------/
permit ip host 10.10.10.249 any
remark /--------------------------------------
remark Self
remark --------------------------------------/
permit ip host 122.164.8.17 any
remark /--------------------------------------
remark Deny All
remark --------------------------------------/
deny ip any any log-input

Route-map:
route-map NAT-TO-ISP1 permit 10
description -= Routing/NAT to ISP1, outgoing traffic =-
match ip address ACL_NAT-INET_OUT_ISP1
match interface FastEthernet0/0.12
!
route-map NAT-TO-ISP2 permit 10
description -= Routing/NAT to ISP2, outgoing traffic =-
match ip address ACL_NAT-INET_OUT_ISP2
match interface FastEthernet0/0.14

And now the most interesting is tracking:

track 100 list boolean and
object 101
object 102
object 103
object 104
delay down 2 up 3
!
track 101 interface FastEthernet0/0.12 line-protocol
delay down 2 up 5
!
track 102 interface FastEthernet0/1 line-protocol
delay down 2 up 5
!
track 103 ip route 164.122.12.9 255.255.255.248 reachability
delay down 2 up 5
!
track 104 ip sla 104
delay down 2 up 5
!
track 200 list boolean and
object 201
object 202
object 203
object 204
delay down 2 up 3
!
track 201 interface FastEthernet0/0.14 line-protocol
delay down 2 up 5
!
track 202 interface FastEthernet0/1 line-protocol
delay down 2 up 5
!
track 203 ip route 122.164.8.17 255.255.255.248 reachability
delay down 2 up 5
!
track 204 ip sla 204
delay down 2 up 5

and routes:

ip route 0.0.0.0 0.0.0.0 164.122.12.9 10 tag 100 name -=[ISP1]=- track 100
ip route 0.0.0.0 0.0.0.0 122.164.8.17 20 tag 200 name -=[ISP2]=- track 200

and now actually what we check the availability of our gateways:

ip sla 104
icmp-echo 164.122.12.9 source-interface FastEthernet0 / 0.12
request-data-size 1
timeout 700
threshold 600
frequency 30
ip sla schedule 104 life forever start-time now
ip sla 204
icmp-echo 122.164.8.17 source-interface FastEthernet0 / 0.14
request-data-size 1
timeout 700
threshold 600
frequency 30
ip sla schedule 204 life forever start-time now