Understanding Passive interface in EIGRP

jskfan
jskfan used Ask the Experts™
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I need to give an example in order to understand Passive interface:
Let s say I have a router R2 which is an ABR between OSPF are 0 and area 3 and EIGRP AS 10.
it has S0/1 connecting to area 0 and E0 connecting to area 3 and S0/2 connecting to EIGRP AS 10
now if I configure S0/1 and E0 as passive interfaces , then area 0 and area 3 will not get any routes from EIGRP 10 and the other way around I guess will also be true,EIGRP10 will not get any OSPF routes from area 0 and area 3.

My question is if later, I will try to redistribute OSPF into EIGRP 10 and EIGRP 10 into Area 0 and Area 3,will still be possible even if the afore-mentioned interfaces are configured as passive ?

Thank you
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it depends on your purpose. Passive interface is one way traffic. Usually redistribution is two way traffic. If you hope it has full function between ERGRP and OSPF, you do not configure that interface as passive.

Author

Commented:
by  the way , Passive interface in EIGRP blocks the incoming and outgoing advertisement.
What I have concluded from reading on the Net, is some routers version will advertize EIGRP with the classfull IP adress by default when you redistribute them into other other Routing Protocols such as OSPF. so the work around is to block the advertizement manually by making the interfaces that connect the ABR to the OSPF as Passive, then for instance, create Access-List that encompass the classless subnets of the EIGRP and Tag them, so that later when you redistribute the EIGRP routes you can use the Route Map to mach the Tag.

*** I might be going too far, but I wanted to make sure that Passive interface will come handy in the case where the router version will advertise the classful network ie, 172.16.10.0 as 172.16.0.0 even when you specify the wild card mask of 0.0.0.255.
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Author

Commented:
thpipfh: I have been to that link before I posted the Question. Thank you though.

What I need to know, in case you want to redistribute Classles IP address of a subnet from EIGRP AS to OSPF when the router version(in the EIGRP AS) does allow only the classful subnet.

because in some version if you type for example : Network 10.24.24.0 ?
the help does not show the wild card mask to enter.
what s the work around ?

Commented:
Please give me what model device you are using ??

Commented:
what is the ios vertion ?

Author

Commented:
If you have seen CBT Nuggets CCIE videos Advanced OSPF configuration. the instructor, had to use the passive interfaces to prevent the router from advertizing EIGRP domain routes, because the ASBR allow only classful IP addresses from EIGRP
So he used passive interfaces, then as a work around he used access-list that permits the /24 subnets mixing it up with Tags and Route maps.. it is not really clear to me...but that would be helpful if any Cisco expert out there can elaborate on that with an example
Commented:
Passive interface has nothing to do with redistributing routes between OSPF and EIGRP.

Also, passive interface has nothing to do with traffic (one way traffic).

To make an interface 'passive' in EIGRP (or OSPF) means that the router will not send or receive any EIGRP/OSPF 'hello' packets on that interface, and consequently the router will not form any EIGRP (or OSPF) neighbor relations ships on that interface.

However, the router will advertise the network that is configured on the passive interface to other neighbors - neighbors formed on other interfaces.
Commented:
PS. An alternative to use Passive interface, is to redistribute connected subnets, with a filter for just that interface network. With OSPF that gives an external route, while with passive it is an internal route.
TimotiStDatacenter Technician
Top Expert 2012
Commented:
Agree with pergr: you don't even need to configure S0/1 and E0 to run EIGRP to do redistribution.
Also, what type is area 3?

Author

Commented:
Thank you

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