Route Change on Distant Link Failure

[Brandon James]

I have ran into a routing problem that I think should be simple but for some reason I am unable to think my way through it. I have provided a simple depiction of my network in the attached image.

Quick back story on the network that can probably be ignored. When I got to this network, the 6 Mbps link was not utilized. I introduced HSRP to provide for basic redundancy. EIGRP 100 area is the local campus where 255 is connections to a COLO. The local campus was a flat layer 2 network consisting of numerous buildings (RTR X represents multiple campus sites).

Router B is the HSRP active router. IP SLA is used to decrement the priority of RTR B if ICMP fails to reach RTR A (sourced out SVI/VLAN 912), allowing for RTR C to preemptively assume the active router role.  RTR B has a default route to 10.255.0.1, RTR C has a default route to 10.255.0.9.  This setup works well for the flat layer 2 network.

I am working to migrate the network to a Layer 3 topology and must do so in a phased approach as to provide minimal disruption to the network. My goal is to establish independent routing between each building (each RTR represents a building) and my phased plan to accomplish this is to assign a /16 to each building (each building will need about 10 VLANS that will each have a /24). I have created SVI’s at each building and have EIGRP 100 routing between each. This allows me to incrementally migrate host from the flat layer 2 network to a local building subnet that routes as needed. Once I get all host migrated to the routed SVIs I will convert the trunk links between each building to routed interfaces. Because of the complexities of some of our system designs, this could take up to 6 months, hence the reason for a phased approach that allows for the flat layer 2 network to still operate between each building while also introducing the routed VLANs.

This is my problem, because RTR C has a default route to 10.255.0.9, the SVIs that are directly connected route over the 6 Mbps link. I can fix this by pointing RTR Cs default route to RTR B but if the 500 Mbps link fails and RTR C assumes the active router role, there is an obvious problem. How can I get RTR C to use ROUTE A then dynamically change to ROUTE B when the 500 Mbps link fails and/or RTR C becomes the HSRP active router?

Thanks in advance for any assistance that anyone is able to provide.

[Mark Malone]

Hi

one way is you could use ip sla tracking with route -map with next hop verify reachability syntax  , set the ping to the far end of the 500mbps circuit , thenwhen that ip becomes unreachable the ip sla should swap over to the next hop that's set which would be 10.255.0.9

example of what im talking about in terms of config

http://www.firewall.cx/cisco-technical-knowledgebase/cisco-routers/861-cisco-router-pbr-ipsla-auto-redirect.html

[Brandon James]

Mark / Jon, thank you for your input, it is greatly appreciated. I am going to lean towards Marks suggestion.

I believe something like this should work?

RTR C Config:

track 8 ip sla 1 rechability
!
ip sla 1
 icmp-echo 10.255.0.1
 threshold 3000
 timeout 3000
 frequency 3
!
ip sla schedule 1 life forever start-time now
!
ip route 0.0.0.0 0.0.0.0 RTR.B.IP.ADD track 8
ip route 0.0.0.0 0.0.0.0 10.255.0.9 10
!

If I understand correctly, the static route with track 8 will use the default distance metric of 1 and be installed in the routing table as long is ICMP reachability exist to 10.255.0.1. When ICMP fails, the route will be removed and 10.255.0.9 becomes active?

I have a static route installed on Router C saying to use router B to reach host 10.255.0.1.

I should have a maintenance window this weekend and will give it a test.

Thanks,

Brandon

[Mark Malone]

Hi looks ok I would add a distance metric at the end of the secondary route

ip route 0.0.0.0 0.0.0.0 RTR.B.IP.ADD track 8 20

[Jon Marshall]

Mark

That is the primary route so it should have no AD added as far as I can tell.

Jon

[Mark Malone]

yep my mistake back to front apologies

[Jon Marshall]

Are you running EIGRP between routers B and C ?

I ask because if you are and you already have IP SLA tracking setup on router B you could use EIGRP to propagate a default route to C.

Just another alternative to consider.

Jon