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Home > Archive > CCNP > January 2001 > ISDN backup
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| the priority connection is a T1 dedicated and the ISDN serves as the backup. When the ISDN is active I can only ping the routers but not the hosts connected to each router. This is the configuration:
username Test_two password 7 XXXXXXX
!
isdn switch-type XXXXX
!
interface Ethernet0
ip address 192.10.218.1 255.255.255.0
!
interface Serial0
ip address 100.50.218.1 255.255.255.252
backup interface BRI0
!
interface BRI0
ip address 100.50.218.5 255.255.255.252
encapsulation ppp
dialer idle-timeout 300
dialer map ip 100.50.218.6 name Test_two broadcast 1XXXXXXXX
dialer map ip 100.50.218.6 name Test_two broadcast 1XXXXXXXX
dialer load-threshold 2 either
dialer-group 1
isdn switch-type XXXXXX
isdn spid1 XXXXXXXXXXXX
isdn spid2 XXXXXXXXXXXX
ppp authentication chap
ppp multilink
!
ip classless
ip route 10.10.218.0 255.255.255.0 100.50.218.2
ip route 10.10.218.0 255.255.255.0 100.50.218.6
dialer-list 1 protocol ip permit
-----peer router configuration--------
username Test_one password 7 XXXXXXX
!
isdn switch-type XXXXX
!
interface Ethernet0
ip address 10.10.218.1 255.255.255.0
!
interface Serial0
ip address 100.50.218.2 255.255.255.252
backup interface BRI0
!
interface BRI0
ip address 100.50.218.6 255.255.255.252
encapsulation ppp
dialer idle-timeout 300
dialer map ip 100.50.218.5 name Test_one broadcast 1XXXXXXXX
dialer map ip 100.50.218.5 name Test_one broadcast 1XXXXXXXX
dialer load-threshold 2 either
dialer-group 1
isdn switch-type basic-ni
isdn spid1 XXXXXXXXXXXX
isdn spid2 XXXXXXXXXXXX
ppp authentication chap
ppp multilink
!
ip classless
ip route 192.10.218.0 255.255.255.0 100.50.218.1
ip route 192.10.218.0 255.255.255.0 100.50.218.5
dialer-list 1 protocol ip permit
----------------------------------
THANKS!
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| Turn off route caching on your serial and bri interfaces.
no ip route-cache
MadChef
[This message has been edited by MadChef (edited 01-25-2001).] | |
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| quote:
Originally posted by MadChef:
Turn off route caching on your serial and bri interfaces.
Care to explain in a few words? Like for dummies?
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| quote:
Originally posted by dmaftei:
Care to explain in a few words? Like for dummies?
This could go on for a while....
Symptoms: he can ping the new networks that just came up (the isdn link) but can't ping the ethernet interface of the remote router when ISDN is up. I assumed he could ping the ethernet interface when the T1 was up and that the ISDN link probably wasn't up very long while he was trying to test it. Indeed it would have been kinda tough to keep it up very long because the idle timeout was 5 minutes and since he could route between the two networks it would be tough to keep interesting traffic flowing. Anyway....
Fast switching is enabled by default so the router will cache all routing entries. That means that the knows that traffic to the remote networks go out it's serial interface and it doesn't bother to check the routing table each time it gets a new packet destined for that network. If it doesn't check the routing table, it never realizes that the next hop is no longer available, so it keeps blindly sending packets out that interface and they immediately fall off the world and die. So you disable fast switching (no ip route-cache) on the serial interface so you can quickly switch traffic over to the bri interface if the serial link fails. Eventually the cached entry would have timed out (I don't know how long it takes. Anyone?) but this makes it happen in a more timely fashion.
If he was using a floating static route to bring up the ISDN interface by building that second static route with an AD of like 200 instead of using the backup command, the above situation would apply again. If the ISDN link was up and the T1 was down, the router would cache entries for the ISDN link and would continually send interesting traffic down the link and it would constantly stay up. In his case, the backup command will shut the isdn interface down after the T1 has come up for a minute or so (again, I can't remember the default values and usually end up hard coding it anyway). So you turn off route caching on the BRI interface as well.
It IS a performance hit, but it's hardly worth bothering about over slow speeds such as this.
As an aside, I THINK that cef switching, because of its design, wouldn't cause the same problem. Could anyone comment on that?
MadChef
By the way, CCnotyet, I take it this worked? | |
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| Madchef,
Thanks! It worked. I used this configuration to backup a dedicated line but I used the floating static route on ISDN backing up the frame-relay line. | |
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| quote:
Originally posted by CCnotyet:
Madchef,
Thanks! It worked. I used this configuration to backup a dedicated line but I used the floating static route on ISDN backing up the frame-relay line.
Excellent! That's my preferred way of backing up interfaces.
MadChef
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