Solved: eigrp "network" command - adding interfaces/network into eigrp routing

SJ K · ‎05-27-2015

Hi all,

As eigrp is a classless routing protocol but in default is doing the auto summarizaton at classful boundaries. I am abit confuse on the "network" command when we issue router eigrp xx.

q1) does the network command in anyway affect / influence the subnet mask that is being send over in route advertisement to another router ?(when no auto summary is use)

q2) is the network command used only to enable interfaces for EIGRP if their IP fall within the network range. By range - should it be classful or classless ? or it can be either of them on what are the network/interfaces that we want to enable eigrp ?

what will be the difference if we put

"network 10.10.0.0" -- what netmask will the router assume to search for interfaces under this network range or

"network 10.10.0.0 0.0.255.255"

q2.5) I have only try the following options

10.10.0.1/16 - IP address of interface.
R2(config-router)#network 10.10.0.1 0.0.0.0 -- interface not included in eigrp
R2(config-router)#network 10.10.0.1 -- interface included in eigrp

This means that the network command is not to be use to specify a particular interface to turn on, but the interface which are connected to the network that is being specified in the network command.

So if the 2nd command works "network 10.10.0.1" -- what network is it actually referring to ? 10.0.0.0/8 or 10.10.0.0/16 ?

q3) I have asked the above because I see that in EIGRP "network" command, it is optional to include the netmask or wildcard mask and they are recorded as it is inside the running config.

Assuming if I have 2 interfaces fa0/0 and fa0/1 running 10.10.0.1/16 and 10.20.0.1/16 respectively; if i only want to enable fa0/0 for EIGRP. What are the options that I have ?

a) issue network 10.10.0.0 255.255.0.0

b) issue network 10.0.0.0 but set fa0/1 to passive interface

c) but i cannot issue just network 10.0.0.0 - right ? (it will trigger both interface).

Please advise.

Regards,
Noob

Peter Paluch · ‎05-27-2015

Hi Koh,

The network command as used in EIGRP (and RIP and OSPF as well) only adds directly connected networks to the routing protocol process but has absolutely no influence on what specific netmasks are going to be advertised with the network addresses, and also has absolutely no relation to any summarization whatsoever.

Basically, consider the network command as defining a range of addresses. Any interface whose own IP address falls under this range will be added to EIGRP and its real subnet/netmask advertised. That's about it.

q1) does the network command in anyway affect / influence the subnet mask that is being send over in route advertisement to another router ?

No, it does not.

q2) is the network command used only to enable interfaces for EIGRP if their IP fall within the network range. By range - should it be classful or classless ?

The classless format using the wildcard mask is considered better because it gives you a more precise control over the networks you will truly advertise. Purists advocate for using the network command as follows:

interface FastEthernet0/0
 ip address 192.0.2.1 255.255.255.128
 no shutdown
!
router eigrp 1
 network 192.0.2.1 0.0.0.0

Note that here, the network command including the wildcard mask basically defines a one-element-long range of addresses, exactly the 192.0.2.1. Because the Fa0/0 has this IP address and thus falls into this range, it will be added to EIGRP and its true network 192.0.2.0/25 will be advertised.

what will be the difference if we put

"network 10.10.0.0" -- what netmask will the router assume to search for interfaces under this network range or

"network 10.10.0.0 0.0.255.255"

Writing network 10.10.0.0 without a wildcard argument will cause the router to replace the address with its classful counterpart 10.0.0.0. This is true for all addresses - if no wildcard mask is given, the configured address will be masked by its classful mask, and the resulting major network (that's the name for this classful address) will be configured instead. As a result, all your directly connected interfaces whose address is 10.x.y.z would be added to EIGRP.

Writing network 10.10.0.0 0.0.255.255 will configure this particular network for EIGRP, meaning that all interfaces having their address as 10.10.x.y would be added to EIGRP.

q2.5) I have only try the following options

10.10.0.1/16 - IP address of interface.
R2(config-router)#network 10.10.0.1 0.0.0.0 -- interface not included in eigrp
R2(config-router)#network 10.10.0.1 -- interface included in eigrp

This means that the network command is not to be use to specify a particular interface to turn on, but the interface which are connected to the network that is being specified in the network command.

So if the 2nd command works "network 10.10.0.1" -- what network is it actually referring to ? 10.0.0.0/8 or 10.10.0.0/16 ?

If the interface was truly configured as 10.10.0.1/16 then the command network 10.10.0.1 0.0.0.0 should have definitely added that interface to EIGRP. If you are saying it has not then you must have had a typo somewhere, or the interface wasn't up/up.

The network 10.10.0.1 command "collapsed" into network 10.0.0.0. If that one caused that interface to be added to EIGRP then its IP address must have been different from 10.10.0.1.

The 2nd command in fact referred to 10.0.0.0/8.

q3) I have asked the above because I see that in EIGRP "network" command, it is optional to include the netmask or wildcard mask and they are recorded as it is inside the running config.

Assuming if I have 2 interfaces fa0/0 and fa0/1 running 10.10.0.1/16 and 10.20.0.1/16 respectively; if i only want to enable fa0/0 for EIGRP. What are the options that I have ?

a) issue network 10.10.0.0 255.255.0.0

b) issue network 10.0.0.0 but set fa0/1 to passive interface

c) but i cannot issue just network 10.0.0.0 - right ? (it will trigger both interface).

You need to clarify what you mean by "enable for EIGRP". Does that mean you just want to advertise that network in EIGRP, or that you want to advertise it and establish EIGRP adjacencies over it with other routers?

Option a) would add the fa0/0 interface to EIGRP and allow establishing adjacencies over it. The network 10.10.0.0/16 would be advertised, the network 10.20.0.0/16 would not be advertised.

Option b) would add both interface to EIGRP. EIGRP adjacencies will be established over fa0/0 only because fa0/1 is a passive interface, meaning that its network will be advertised but no EIGRP packets will be exchanged over it. Both 10.10.0.0/16 and 10.20.0.0/16 networks will be advertised.

Option c) would do something you don't want to do according to your description: it would advertise both 10.10.0.0/16 and 10.20.0.0/16 and allow EIGRP adjacencies to be established both over fa0/0 and fa0/1.

Best regards,
Peter

Assuming if I have 2 interfaces fa0/0 and fa0/1 running 10.10.0.1/16 and 10.20.0.1/16 respectively; if i only want to enable fa0/0 for EIGRP. What are the options that I have ?

a) issue network 10.10.0.0 255.255.0.0

b) issue network 10.0.0.0 but set fa0/1 to passive interface

c) but i cannot issue just network 10.0.0.0 - right ? (it will trigger both interface).

- See more at: https://supportforums.cisco.com/discussion/12517846/eigrp-network-command-adding-interfacesnetwork-eigrp-routing#sthash.lnSlVVLg.dpuf

View solution in original post

Peter Paluch · ‎05-27-2015

Hi Koh,

Q1) What if I want a network to be advertised, but doesn't want its interfaces to be doing any route advertisement ? -> the solution will be passive interface - right ?

I am not entirely sure what you mean by "doesn't want its interfaces to be doing any route advertisement". Can you perhaps rephrase your question in different words?

Before you ask, though, keep in mind that EIGRP can only advertise networks that are either directly connected or learned by EIGRP (let's ignore redistribution for now). If you want to advertise a directly connected network then you must use a network command that covers the IP address of the interface whose network you want to advertise. If, in addition, you want that interface to disallow any EIGRP communication and thus prevent any remote routes from being sent out and received from that interface, you would use the passive-interface as well.

You do not do anything to advertise a network you have already learned over EIGRP yourself. These are advertised further over all non-passive EIGRP-enabled interfaces by default (again, let's forget about split horizon for now).

Are you guys working as lecturers in university or something ? The answers are absolutely top class.

Thank you! I am honored. Yes, I am working as a full time university teacher... at least for now.

Best regards,
Peter

View solution in original post

Bilal Nawaz · ‎05-27-2015

Q1) yes, passive interface is the solution for that. If you want the opposite, then some kind of filtering would need to be applied at an appropriate point in the network. Care must be taken with this as you may end up breaking some other connectivity. (I actually haven't seen this been done in the working environment)

On this discussion, I'd like to mention that I think of this to "enable interfaces" to be part of the protocol rather than advertising networks, even if it meant doing e.g.

personal preference would be to do individual network statements

network 192.168.1.1 0.0.0.0

network 192.168.2.1 0.0.0.0

With ospf goes to the extent of not having to configure under the routing process, and only enabling on the interface. Personally it is ultimate control on what you do with the protocols and where to enable them.

Please rate useful posts & remember to mark any solved questions as answered. Thank you.

View solution in original post

Peter Paluch · ‎05-27-2015

Hi Koh,

The network command as used in EIGRP (and RIP and OSPF as well) only adds directly connected networks to the routing protocol process but has absolutely no influence on what specific netmasks are going to be advertised with the network addresses, and also has absolutely no relation to any summarization whatsoever.

Basically, consider the network command as defining a range of addresses. Any interface whose own IP address falls under this range will be added to EIGRP and its real subnet/netmask advertised. That's about it.

q1) does the network command in anyway affect / influence the subnet mask that is being send over in route advertisement to another router ?

No, it does not.

q2) is the network command used only to enable interfaces for EIGRP if their IP fall within the network range. By range - should it be classful or classless ?

The classless format using the wildcard mask is considered better because it gives you a more precise control over the networks you will truly advertise. Purists advocate for using the network command as follows:

interface FastEthernet0/0
 ip address 192.0.2.1 255.255.255.128
 no shutdown
!
router eigrp 1
 network 192.0.2.1 0.0.0.0

Note that here, the network command including the wildcard mask basically defines a one-element-long range of addresses, exactly the 192.0.2.1. Because the Fa0/0 has this IP address and thus falls into this range, it will be added to EIGRP and its true network 192.0.2.0/25 will be advertised.

what will be the difference if we put

"network 10.10.0.0" -- what netmask will the router assume to search for interfaces under this network range or

"network 10.10.0.0 0.0.255.255"

Writing network 10.10.0.0 without a wildcard argument will cause the router to replace the address with its classful counterpart 10.0.0.0. This is true for all addresses - if no wildcard mask is given, the configured address will be masked by its classful mask, and the resulting major network (that's the name for this classful address) will be configured instead. As a result, all your directly connected interfaces whose address is 10.x.y.z would be added to EIGRP.

Writing network 10.10.0.0 0.0.255.255 will configure this particular network for EIGRP, meaning that all interfaces having their address as 10.10.x.y would be added to EIGRP.

q2.5) I have only try the following options

10.10.0.1/16 - IP address of interface.
R2(config-router)#network 10.10.0.1 0.0.0.0 -- interface not included in eigrp
R2(config-router)#network 10.10.0.1 -- interface included in eigrp

This means that the network command is not to be use to specify a particular interface to turn on, but the interface which are connected to the network that is being specified in the network command.

So if the 2nd command works "network 10.10.0.1" -- what network is it actually referring to ? 10.0.0.0/8 or 10.10.0.0/16 ?

If the interface was truly configured as 10.10.0.1/16 then the command network 10.10.0.1 0.0.0.0 should have definitely added that interface to EIGRP. If you are saying it has not then you must have had a typo somewhere, or the interface wasn't up/up.

The network 10.10.0.1 command "collapsed" into network 10.0.0.0. If that one caused that interface to be added to EIGRP then its IP address must have been different from 10.10.0.1.

The 2nd command in fact referred to 10.0.0.0/8.

q3) I have asked the above because I see that in EIGRP "network" command, it is optional to include the netmask or wildcard mask and they are recorded as it is inside the running config.

Assuming if I have 2 interfaces fa0/0 and fa0/1 running 10.10.0.1/16 and 10.20.0.1/16 respectively; if i only want to enable fa0/0 for EIGRP. What are the options that I have ?

a) issue network 10.10.0.0 255.255.0.0

b) issue network 10.0.0.0 but set fa0/1 to passive interface

c) but i cannot issue just network 10.0.0.0 - right ? (it will trigger both interface).

You need to clarify what you mean by "enable for EIGRP". Does that mean you just want to advertise that network in EIGRP, or that you want to advertise it and establish EIGRP adjacencies over it with other routers?

Option a) would add the fa0/0 interface to EIGRP and allow establishing adjacencies over it. The network 10.10.0.0/16 would be advertised, the network 10.20.0.0/16 would not be advertised.

Option b) would add both interface to EIGRP. EIGRP adjacencies will be established over fa0/0 only because fa0/1 is a passive interface, meaning that its network will be advertised but no EIGRP packets will be exchanged over it. Both 10.10.0.0/16 and 10.20.0.0/16 networks will be advertised.

Option c) would do something you don't want to do according to your description: it would advertise both 10.10.0.0/16 and 10.20.0.0/16 and allow EIGRP adjacencies to be established both over fa0/0 and fa0/1.

Best regards,
Peter

Assuming if I have 2 interfaces fa0/0 and fa0/1 running 10.10.0.1/16 and 10.20.0.1/16 respectively; if i only want to enable fa0/0 for EIGRP. What are the options that I have ?

a) issue network 10.10.0.0 255.255.0.0

b) issue network 10.0.0.0 but set fa0/1 to passive interface

c) but i cannot issue just network 10.0.0.0 - right ? (it will trigger both interface).

- See more at: https://supportforums.cisco.com/discussion/12517846/eigrp-network-command-adding-interfacesnetwork-eigrp-routing#sthash.lnSlVVLg.dpuf

SJ K · ‎05-27-2015

You are right Peter.

I miss type the interface ip for 10.10.0.1 to 10.10.10.1.

The command ip address 10.10.0.1 0.0.0.0 works.

============================================

Just a side question for the below
Assuming if I have 2 interfaces fa0/0 and fa0/1 running 10.10.0.1/16 and 10.20.0.1/16 respectively; if i only want to enable fa0/0 for EIGRP. What are the options that I have ?
a) issue network 10.10.0.0 255.255.0.0

Option a) would add the fa0/0 interface to EIGRP and allow establishing adjacencies over it. The network 10.10.0.0/16 would be advertised, the network 10.20.0.0/16 would not be advertised

Q1) What if I want a network to be advertised, but doesn't want its interfaces to be doing any route advertisement ? -> the solution will be passive interface - right ?

If i want the opposite, I do not want the network to be advertise, but i want its interfaces to carry route advertisements -> what should be done ? (route filter list ?)

=============================================

Omg, there are simply too many gurus here .

Are you guys working as lecturers in university or something ? The answers are absolutely top class.

P.S. I am digesting the EIGRP discussion on another thread and i am going 0_0~~.

Regards,
Noob

Peter Paluch · ‎05-27-2015

Hi Koh,

Q1) What if I want a network to be advertised, but doesn't want its interfaces to be doing any route advertisement ? -> the solution will be passive interface - right ?

I am not entirely sure what you mean by "doesn't want its interfaces to be doing any route advertisement". Can you perhaps rephrase your question in different words?

Before you ask, though, keep in mind that EIGRP can only advertise networks that are either directly connected or learned by EIGRP (let's ignore redistribution for now). If you want to advertise a directly connected network then you must use a network command that covers the IP address of the interface whose network you want to advertise. If, in addition, you want that interface to disallow any EIGRP communication and thus prevent any remote routes from being sent out and received from that interface, you would use the passive-interface as well.

You do not do anything to advertise a network you have already learned over EIGRP yourself. These are advertised further over all non-passive EIGRP-enabled interfaces by default (again, let's forget about split horizon for now).

Are you guys working as lecturers in university or something ? The answers are absolutely top class.

Thank you! I am honored. Yes, I am working as a full time university teacher... at least for now.

Best regards,
Peter

SJ K · ‎05-27-2015

Hi Peter, Bilal,

Thanks for your replies.

Hi Peter,

Apologies for the bad english and confusion. For Q1, what i meant actually is that if R1 fe0/1 is on network 10.10.10.0/24, i want network 10.10.10.0/24 to be advertise, but I do not want R1 fe0/1 to be use to transport or receive any network advertisement.

And for the opposite question, it means that I want the interface fe0/1 to carry network advertisement, but i do not want the network it is currently connect to 10.10.10.0/24 to be advertised through any interfaces at all.

So as per what Bilal and you mentioned is to use passive interface for Q1) and filter list for Q2.

Am i right ;)

Regards,
Noob

Jon Marshall · ‎05-27-2015

q1) yes you make the interface passive so no EIGRP neighborships are formed via that interface but the network is still advertised out to other EIGRP peers on other interfaces.

The opposite question is not quite as clear.

If you want to stop that network being advertised to any other EIGRP peers then you can use a distribute list and filter that subnet. That would stop any EIGRP neighbors on different interfaces from getting an advertisement for that subnet.

However the peer you share that subnet with will also have an interface with an IP from that subnet so it will be advertising that subnet to it's peers.

So you need to pick your point in the network where you want to do the filtering as Bilal mentioned.

Jon

SJ K · ‎05-27-2015

Hi Jon,

Good to see your reply. Clear understood. Thanks!

Regards,
Noob

Bilal Nawaz · ‎05-27-2015

Q1) yes, passive interface is the solution for that. If you want the opposite, then some kind of filtering would need to be applied at an appropriate point in the network. Care must be taken with this as you may end up breaking some other connectivity. (I actually haven't seen this been done in the working environment)

On this discussion, I'd like to mention that I think of this to "enable interfaces" to be part of the protocol rather than advertising networks, even if it meant doing e.g.

personal preference would be to do individual network statements

network 192.168.1.1 0.0.0.0

network 192.168.2.1 0.0.0.0

With ospf goes to the extent of not having to configure under the routing process, and only enabling on the interface. Personally it is ultimate control on what you do with the protocols and where to enable them.

Please rate useful posts & remember to mark any solved questions as answered. Thank you.