Routing with ISDN as a backup Link

Unanswered Question
Dec 16th, 2007

Hi Everyone

We are using Leased line as primary link b/w sites and ISDN is used as a backup link ( Static routing used on routers) , but there is one problem that when Primary link goes down , the ISDN link used as a backup link (adminsitrative distance higher than that of leased line in the routing table -- routes put administratively) is Up and when seeting the routing table using sh ip route command , the leased line routes are not flushed out and some one suggested that while putting the static route in the route for leased line use exit interface aswell as next hop ip for example

ip route 172.x.x.x serail 1/0

and the problem got resolved -- i mean routes were flused whenever leased line got down

so my question is what is the logic behind putting both exit interface and next hop ip in the route

and what is this called ( i mean any reference on cisco site to it)

I have this problem too.
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Danilo Dy Sun, 12/16/2007 - 07:36


You mean when the leased line is down (down/down or up/down), the route thru leased line is still in the routing table? i.e.

S 172.x.x.x/16 [1/0] via

Route will be injected in the routing table when its define gateway is available/reachable or the exit interface is up/up. If the define gateway is not available/reachable or the exit interface is down/down or up/down, the route will be flush.



syedsohailsarwar Wed, 12/19/2007 - 10:13

Hi Medan

thats is where the problem is , i mean when exit interface specified in the ip route command say s1/0 is having status as UP/Down -- status UP , protocol Down , the route never gets flushed from the routing table and while using exit interface aswell as next hop ip , this problem doesnot resurface and route gets automatically flushed from routing table .

i have checked this on production routers in our setup since our network is spanned across india using LL as primary link and ISDN as backup link

is there any reference on Cisco website which explains something about specifying both exit interface and next hop ip while putting static route in a router

Danilo Dy Wed, 12/19/2007 - 20:37


Do you enable keepalive in your serial1/0 interface? Can you post the config of your serial1/0 interface and "show interface serial1/0" output?



syedsohailsarwar Thu, 12/20/2007 - 08:28

Hi dandy

No we dont enbale kepalives ( i dont know whether they are enabled by default)

and the configuration of serial interface is

sh run int s1/0

int s1/0

ip address 172.x.x.x.

description *** 64 K Lease Line to XXX ***

bandwidth 64

encapsulation ppp

so its quite straight forward configuration using encapsulation as ppp and subnet of /30

and bandwidth 64 which makes it useless unless we are using dynamic routing since we are using static route at our access locations

smothuku Mon, 12/17/2007 - 02:09

Hi Sohail ,

Can you paste the previous and present config of the router.i.e before adding exit interface for static route and with next-hop ip in the static route.

if possible paste the " show ip route" in both the cases by shutting serial interface.



syedsohailsarwar Wed, 12/19/2007 - 10:16

do u require complete configuration of the router or simple routing table and interface configuration plus sh version

smothuku Wed, 12/19/2007 - 20:41

Hi Sohail ,

It is better if you get us complete config of the router.




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