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Passive Interfaces and Static Routes

Can you have a passive interface if all your routing is done via Static routes i.e. there is no routing protocol present

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Passive Interfaces and Static Routes

If you are just using static routes, then you could use a "floating static". That is, you underpin the static route to Serial2/0 with another one to Null0, but with a higher administrative distance. When the Serial goes down the Null0 takes over. For example:

ip route 192.168.42.0 255.255.255.0 Serial2/0

ip route 192.168.42.0 255.255.255.0 Null0 250

The route to Serial2/0 normally has an adminstrative distance of 1, and so it masks the Null0 route. When Serial2/0 goes down, the Null0 route takes its place at administrative distance 250.

Kevin Dorrell

Luxembourg

10 REPLIES
Purple

Re: Passive Interfaces and Static Routes

The concept of a passive interface only makes sense with a routing protocol enabled. When an interface is configured as passive, different actions are taken depending on the routing protocol in use:

- RIP will not send updates out the passive interface but will process updates received over it

- EIGRP/OSPF will not send/receive Hellos received over the interface. They will, however, advertise the IP subnet associated with the interface

Hope that helps - pls do rate the post if it does.

Paresh

New Member

Re: Passive Interfaces and Static Routes

Hi,

I shall elaborate:

A link went down to a site recently. It appeared that the traffic destined for this site had no where to go and therefore the ?gateway of last resort? was used which is a firewall. Hence traffic for this downed site was bouncing from firewall to router continuously until the firewall eventually crashed.

Is there a command that can be used to tell the router to simply drop traffic destined for a route that is down, rather than return it to the Gateway of last resort?

The router in question only uses Static routes. No routing protocol is in use.

Thanks

Hall of Fame Super Silver

Re: Passive Interfaces and Static Routes

Patrick

Would I be correct in assuming that the interface that stopped working was some type of Ethernet? It is a recognized issue that static routes over Ethernet interfaces will typically stay in the routing table even when the next hop is not reachable. For the static route to be withdrawn the inteface must go into protocol down state and the Ethernet interfaces typically do not go protocol down.

Cisco has introduced a feature called Reliable Static Routing with Object Tracking which addresses this issue. This link has information that should get you started:

http://www.cisco.com/en/US/products/sw/iosswrel/ps5413/products_feature_guide09186a00801d862d.html

HTH

Rick

New Member

Re: Passive Interfaces and Static Routes

Hello Rick,

It was a serial interface that went down:

interface Serial2/0

Re: Passive Interfaces and Static Routes

If you are just using static routes, then you could use a "floating static". That is, you underpin the static route to Serial2/0 with another one to Null0, but with a higher administrative distance. When the Serial goes down the Null0 takes over. For example:

ip route 192.168.42.0 255.255.255.0 Serial2/0

ip route 192.168.42.0 255.255.255.0 Null0 250

The route to Serial2/0 normally has an adminstrative distance of 1, and so it masks the Null0 route. When Serial2/0 goes down, the Null0 route takes its place at administrative distance 250.

Kevin Dorrell

Luxembourg

New Member

Re: Passive Interfaces and Static Routes

Hello Kevin,

This seems to make sense. I'll give it a go. Thanks for the advice.

Pat

Re: Passive Interfaces and Static Routes

Hi,

I assume that the setup is as follows,

You have a static route in your router to reach the remote network pointing to the other end serial ip.

You also have a default route in your router pointing to the firewall.

In this case, if the serial interface goes down, the traffic to the remote network will flow to your firewall, which dont have any ip connectivity to the remote network in your case.

If so, you can add another static route pointing the next hop as null interface with higher metric.

ip route mask null0 200

SO, whenever, the serial link goes down, the traffic to the remote network N.N.N.N will dropped in the null0 interface. When the link is back, the traffic flow via the serial link..

HTH

-VJ

Re: Passive Interfaces and Static Routes

Hi Patrick,

What you are tying to achieve can be done by using a feature called reliable static routes with object tracking..Same point is discussed several times in this forum.

Have a look at the following URL for more information on this..

http://www.cisco.com/en/US/products/sw/iosswrel/ps5413/products_feature_guide09186a00801d862d.html#wp1071672

-VJ

New Member

Re: Passive Interfaces and Static Routes

can any body explain me the sequence in the link given by Vijay:

http://www.cisco.com/en/US/products/sw/iosswrel/ps5413/products_feature_guide09186a00801d862d.html#wp1071672

how one by one configuration will work? means what will be the working sequence for the configuration? it has lots of configuration.

plz help me

dporter

New Member

Re: Passive Interfaces and Static Routes

please any one can explain me the sequence of working on my previous post?

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