null interface caused floating static route issue

Answered Question
Jul 10th, 2007

I build two GRE tunnel with keepalive, and use floating static route for backup purpose. But i found it works improperly due to a route point to null0.


ip route 10.20.1.0 255.255.255.0 1.2.4.6

ip route 10.20.1.0 255.255.255.0 1.2.4.10 200

ip route 1.2.0.0 255.255.0.0 null0 --->this route caused floating static route works improperly. Even if the Tunnel500 is down, the route 10.20.1.0 to 1.2.4.6 will keeping active. But if i remove the route 1.2.0.0 to null0, it works fine. I got little confuse, anyone would help will be much appreciated.

!

interface Tunnel500

ip address 1.2.4.5 255.255.255.252

ip mtu 1500

ip tcp adjust-mss 1350

keepalive 5 1

tunnel source 10.86.2.2

tunnel destination 10.90.10.194

!

interface Tunnel501

ip address 1.2.4.9 255.255.255.252

ip mtu 1476

ip tcp adjust-mss 1436

keepalive 5 1

tunnel source 10.86.4.2

tunnel destination 10.90.10.198

Correct Answer by Paolo Bevilacqua about 9 years 7 months ago

Hi,


What is happening, is that in presence of "ip route 1.2.0.0 255.255.0.0 null0", the router is trying recursive routing, because the routing information for 1.2.4.6 is derived from the said static route.


To work around this, either use dynamic routing over tunnel, or point the static route to the interface and not IP address, or both IP address AND interface.


A good discussion is presented at:

http://www.cisco.com/en/US/customer/tech/tk365/technologies_tech_note09186a00800ef7b2.shtml


Hope this helps, please rate post if it does!

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Overall Rating: 5 (2 ratings)
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Paolo Bevilacqua Tue, 07/10/2007 - 16:33

Can you send output of "show ip route 10.20.1.0 255.255.255.0" when tunnel500 is up, and again when it is down ?

And again the above, when "ip route 1.2.0.0 255.255.0.0 null0" is not configured.



flymen331 Wed, 07/11/2007 - 07:05

r1#sh ip route 10.20.1.0 -------When "ip route 1.2.0.0 255.255.0.0 null0" is configured, no matter the tunnel500 is up or down, the "sh ip route 10.20.1.0" output will be the same

Routing entry for 10.20.1.0/24

Known via "static", distance 1, metric 0

Redistributing via ospf 1

Routing Descriptor Blocks:

* 1.2.4.6

Route metric is 0, traffic share count is 1

!

r1#sh ip route 10.20.1.0 ------- When tunnel500 is up and "ip route 1.2.0.0 255.255.0.0 null0" is not configured.

Routing entry for 10.20.1.0/24

Known via "static", distance 1, metric 0

Redistributing via ospf 1

Routing Descriptor Blocks:

* 1.2.4.6

Route metric is 0, traffic share count is 1

!

r1#sh ip route 10.20.1.0 ------- When tunnel500 is down and "ip route 1.2.0.0 255.255.0.0 null0" is not configured.

Routing entry for 10.20.1.0/24

Known via "static", distance 200, metric 0

Redistributing via ospf 1

Routing Descriptor Blocks:

* 1.2.4.10

Route metric is 0, traffic share count is 1

!

Correct Answer
Paolo Bevilacqua Wed, 07/11/2007 - 07:39

Hi,


What is happening, is that in presence of "ip route 1.2.0.0 255.255.0.0 null0", the router is trying recursive routing, because the routing information for 1.2.4.6 is derived from the said static route.


To work around this, either use dynamic routing over tunnel, or point the static route to the interface and not IP address, or both IP address AND interface.


A good discussion is presented at:

http://www.cisco.com/en/US/customer/tech/tk365/technologies_tech_note09186a00800ef7b2.shtml


Hope this helps, please rate post if it does!

flymen331 Wed, 07/11/2007 - 08:26

p.bevilacqua,


It's greatly helpful. Thank you so much!!


I point the static route to IP address AND interface, it works fine.

ip route 10.20.1.0 255.255.255.0 Tunnel500 1.2.4.6

ip route 10.20.1.0 255.255.255.0 Tunnel501 1.2.4.10 200

Paolo Bevilacqua Wed, 07/11/2007 - 08:51

In reality, the IP address is redundant, as your tunnel interfaces are point-to-point. But, it doesn't harm neither.


Thanks for the nice rating and good luck!

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