Default Static Route Failover

jsmcniel88 · ‎12-20-2011

We have a dedicated T3 (se 1/0) and two back up T1s (se 0/0/0 and se 0/1/0). The previous engineer added static routes like so:

Cisco IOS Software, 3800 Software (C3845-IPBASE-M), Version 12.4(3h), RELEASE SOFTWARE (fc2)

#show ip int brief

Interface IP-Address OK? Method Status Protocol

GigabitEthernet0/0 207.1X.162.129 YES NVRAM up up

GigabitEthernet0/1 10.143.199.1 YES NVRAM administratively down down

Serial0/0/0 1X0.81.61.46 YES NVRAM up up

Serial0/1/0 160.81.X7.170 YES NVRAM up up

Serial1/0 1X0.81.76.26 YES NVRAM up up

ip route 0.0.0.0 0.0.0.0 Serial1/0

ip route 0.0.0.0 0.0.0.0 1X0.81.X6.25 5

ip route 6X.41.1X8.253 255.255.255.255 Null0

Our issue is that when the t3 link fails the Internet fails over to the two t1s, when the t3 link comes back up I have to manually shut the T1s interfaces to fail the route back over to the t3. My question is what is the best way to solve this problem so the routes will fail over automatically back to the t3 when the link comes back up.

I have a read a little about IP SLAs but not sure if that is the way to go. I also do not see that command in this IOS so what IOS do i need to go to? Or can this be done another way using IPBASE?

Thanks

mccmhtac1 · ‎12-20-2011

Please post the results of : sh ip route

jsmcniel88 · ‎12-20-2011

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

64.0.0.0/32 is subnetted, 1 subnets

S 64.41.168.253 is directly connected, Null0

160.81.0.0/16 is variably subnetted, 3 subnets, 2 masks

C 160.81.X7.128/25 is directly connected, Serial0/1/0

C 1X0.81.76.24/30 is directly connected, Serial1/0

C 1X0.81.61.0/25 is directly connected, Serial0/0/0

207.1X.162.0/25 is subnetted, 1 subnets

C 207.1X.162.128 is directly connected, GigabitEthernet0/0

S* 0.0.0.0/0 is directly connected, Serial1/0

mccmhtac1 · ‎12-20-2011

I am not an expert at this stuff, but after looking at your config I am not sure how both Ts are used in the failover. Are they bonded?

I would setup two static default routes. The first to your T3 via: 1X0.81.76.24/30. Then the second to the other T1s IP with a higher Administrative Distance.

Example for the two routes:

ip route 0.0.0.0 0.0.0.0 1X0.81.76.24/30 240 nameofconnection(T3)

ip route 0.0.0.0 0.0.0.0 160.81.X7.128/25 250 nameofconnection(T1s)

The router will use the smaller AD when all circuits are up..

my 2 cents

jsmcniel88 · ‎12-20-2011

They are bonded.

mccmhtac1 · ‎12-20-2011

ok. What is the next hop for the bonded IPs? Use that for the second static route.

jsmcniel88 · ‎12-20-2011

Use the same higher AD for the second static (250)? Also should I leave the loop prevention (null0) on the route? Or is that even needed in this case?

mccmhtac1 · ‎12-20-2011

To be honest I am not sure why you have the route to null0. I just read that is to put packets in the bucket of no routes are found. Seems to be common practice, I don't use it and have a failry complex network.

The default Administrative Distance is 1 for static routes. If you are using routing protocols they each have thier own AD much higher. Since I only see static routing, you could do this and it shoud work fine:

ip route 0.0.0.0 0.0.0.0 1X0.81.76.24/30 nameofconnection(T3)

ip route 0.0.0.0 0.0.0.0 160.81.X7.128/25 5 nameofconnection(T1s)

Again, my solution only routes to one T1, I am unclear how yours are setup. Typically there are only two IPs on bonded Ts, one at each end.

mccmhtac1 · ‎12-20-2011

Please don't remove the null route till you investigate what that subnet is it is tossing

ajay chauhan · ‎12-20-2011

Backup route will only work when primary interface goes down . In case link is up and outage next in ISP it wont detect failure.

Yes IP SLA can be used config will look like-

interface ethernet 0
description primary-link
ip address 10.1.1.1 255.0.0.0

interface ethernet 1
description backup-link
ip address 10.2.2.2 255.0.0.0

ip sla monitor 1
type echo protocol ipIcmpEcho 172.16.23.7
timeout 1000
frequency 3
threshold 2

ip sla monitor schedule 1 life forever start-time now
track 123 rtr 1 reachability
access list 101 permit icmp any host 172.16.23.7 echo

route map MY-LOCAL-POLICY permit 10
match ip address 101
set ip next-hop 10.1.1.242
set interface null 0
!
ip local policy route-map MY-LOCAL-POLICY
ip route 0.0.0.0 0.0.0.0 10.1.1.242 track 123
ip route 0.0.0.0 0.0.0.0 10.2.2.125 254

Richard Burts · ‎12-20-2011

There are a couple of things about this that I do not understand and hope that you can provide clarification.

Your original post says that there are 2 backup T1s. So I would have expected 2 floating static backup default routes. Your post shows only 1. Is there really only 1 floating static default route or did you not post all the static routes?

In general I would expect a static route over a T3 to work while the T3 is up and to be withdrawn from the routing table when the T3 goes down. This would allow the floating static to be used in the routing table. And I would expect that when the T3 came back up that the floating static would go out of the routing table. Your original post says that this did not happen and this surprises me. Perhaps there is something about these interfaces that is different from what I expect. Perhaps you could post the configuration of the 3 interfaces and perhaps also post the output of show interface for these interfaces.

HTH

Rick

[edit] Using IP SLA to monitor the routes would make sense if the primary static default route were not being withdrawn from the routing table. As I read this thread it does not seem that this is the problem. The problem seems to be about going back to the original default route. I do not see how IP SLA would solve that problem.

HTH

Rick

mccmhtac1 · ‎12-20-2011

I totally agre with Mr. Burts, something is up with the backup T1s.

Richard Burts · ‎12-20-2011

Thanks for the additional information.

The original poster described the 2 T1s as bonded. Perhaps he is using slightly different terminology than I do. When I think about bonded serial interfaces I think about things like Multilink PPP to bond several seperate physical interfaces and to provide a single logical interface for them. What we have in the configuration are 2 T1, using HDLC protocol, and each with its own separate and distinct IP subnet.

I do not see anything in the output given so far that explains why the static default route using the T3 did not come back into the routing table when the T3 came back up. I note that there is a service policy input configured on the interface and wonder if it might have some impact.

I also note that the code on this 3845 is relatively old. I wonder if the 3845 were running more recent code if the behavior would be different?

HTH

Rick

HTH

Rick

mccmhtac1 · ‎12-20-2011

I do not understand how those T1s are IP'd. This route:

ip route 0.0.0.0 0.0.0.0 1X0.81.X6.25 5 does not seem to be in the same subnet as either T1:

ip address 1X0.81.61.46 255.255.255.128

!

interface Serial0/1/0

ip address 160.81.X7.170 255.255.255.128

Could 1X0.81.X6.25 be the Multilink IP? Though no Multilink config is showing up.

Again, I am not an expert, still learning, but makes no sense, neither do the x's in the IP addresses.

Is there more to this config?

jsmcniel88 · ‎12-20-2011

I work in VOIP so I am not very experienced with this routing. I called the previous engineer and he said that the T1s are bonded by the ISP. I asked him about the different IPs on each T1 and he didn't have a good answer.

If I add the ADs to the end of the route would this fix the issue or should I upgrade the IOS first and see if it solves the problem?