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Cisco 2921 with IOS 15.2(3)T1

My ISP has provided a DSL connection (Dialer0 via PPPoATM on EHWIC) and a 3G SIM (Dialer2 via Sierra Wireless box over PPPoE). The 3G is for failover. The ISP supplies me with a /29 IP range and this is routed over DSL directly or the 3G connection using L2TP (Virtual-PPP 1) during failover.

The failover works using and IP SLA and tracking. Using ICMP requests to ISP gateway (which is statically routed from the DSL interface) it tracks reachability and successfully fails over all routing and NAT to the L2TP via 3G when the DSL fails.

The question: I'd like to be able to ping and connect to the router at both its IP addresses (DSL and L2TP) at all times, however when the routing switches, replies and connections only work at the active address because there is no route for the other interface.

It seems like I need a way of creating a route for each interface to allow replies to connection attempts while the other interface has the default route attached. I've investigated policy based routing but my experience in this area is limited and so any help greatly appreciated.

Overview:

Dialer0 via PPPoATM (Primary) (IP A/32)
Dialer2 via PPPoE (3G Backup) (IP B/32)
Virtual-PPP 1 via L2TP via Dialer2 (3G LT2P) (IP C/32)

ISP Supplies IP D/29 which is NATted by the router. They route IP D via DSL or L2TP depending on their reachability tests.

ip route 0.0.0.0 0.0.0.0 D0 10 track 10
ip route 0.0.0.0 0.0.0.0 Virtual-PPP 20
ip route (DSL gateway) 255.255.255.255 D0
ip route (L2TP gateway) 255.255.255.255 D2

Thank you all in advance!

  • If an interface goes down, you simply can't get to it, even through a backdoor. – Ron Maupin Sep 1 '16 at 21:49
  • @RonMaupin I'm not trying to access the down interface, only the standby interface when both are connected. When both interfaces are active and connected, I am only able to connect to the one that holds the default route. If D0 and Virtual-PPP1 are both live, I can ping IP A (D0) but not IP C until D0 goes down and the route transfers to Virtual-PPP1 and then I can ping IP C. I'd like to be able to ping both when both are live to allow external reachability tests. – Basic Monkey Sep 1 '16 at 21:55
  • If both interfaces are up, you should be able to ping both. The return traffic will only flow through a single interface based on the default route. – Ron Maupin Sep 1 '16 at 22:14
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Indeed this can be done using policy based routing, e.g.

access-list 1 permit <ip A>
access-list 2 permit <ip C>

route-map PBRmap permit 10 
    match ip address 1 
    set interface dialer0

route-map PBRmap permit 20 
    match ip address 2
    set interface virtual-ppp1

ip local policy route-map PBRmap

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