EIGRP network command follow up / review

Routing & Switching
1

I just wanted to clarify something that’s confused me and I generally just brush over it with regards to EIGRP (and for that matter OSPF) and the network command. I think some comes from mixing in both Physical interfaces and Loopback Int’s and terms “interfaces” and “networks”. When I’m listen to people explain it I hear them using the terms “on that network” and “on that interface” to me it seems they’re using them interchangeably. I just wanted to clarify as it’s now bothering me… For example:

For Example on R1:

Int g1/0:  	IP address 10.1.24.1 255.255.255.252      ! - P2P to another router
Int g2/0:  	IP address 10.1.34.1 255.255.255.252      ! - P2P to another router
!
Int L0:		IP address 172.30.0.0 255.255.255.0
Int L1:		IP address 172.30.1.0 255.255.255.0
Int L2:		IP address 172.30.2.0 255.255.255.0
!
!
Network 10.1.24.1 0.0.0.0 	! - EIGRP is running on that "interface" only, sending and receiving EIGRP updates (both local networks per below and anything received) and hellos. 
!
Network 10.1.34.1 0.0.0.0 	! - EIGRP is running on that "interface" only, sending and receiving EIGRP updates (both local networks per below and anything received) and hellos. 
!
Network 172.30.0.0 0.0.0.255 	! - EIGRP is advertising that "network" on any interface that has the network command for it (10.1.24.1 & 10.1.34.1 per above). 
!
Network 172.30.1.0 0.0.0.255 	! - EIGRP is advertising that "network" on any interface that has the network command for it (10.1.24.1 & 10.1.34.1 per above).
!
Network 172.30.2.0 0.0.0.255	! - EIGRP is advertising that "network" on any interface that has the network command for it (10.1.24.1 & 10.1.34.1 per above).
!
!
OR
!
!
Network 10.0.0.0		! - EIGRP is running on any interface / network in the 10.0.0.0 Class A range
Network 172.30.0.0		! - EIGRP is running on any interface / network in the 172.30.0.0 Class B range
!
!
!
network 0.0.0.0 255.255.255.255      ! - EIGRP is running on every physical interface and advertising those and any loopback int's / Networks on the device
2

Hello Aaron

I understand how this terminology can get confusing. Yes it is important to be very clear about what we’re talking about when we explain topics, and this is indeed an important point.

Keep the following in mind. When EIGRP or OSPF are enabled on a router, there is a special relationship between the interfaces that are participating in the routing protocol and the networks that are being advertised. An advertised network MUST exist on an interface, otherwise it will not be advertised, even if it is configured using the “network” command in the routing protocol. More specifically, the IP address and subnet mask configured on an interface DEFINES the advertised subnet. In order for a particular subnet/network to be advertised, it must exist on an interface and must be defined with a “network” command.

Now in the above description, there is a logic to saying that EIGRP (for example) is running on a particular interface and that it is advertising the network on a particular interface. An interface that is participating in EIGRP has both EIGRP running on it and has its network being advertised. An interface such as a loopback or a passive interface has its network advertised on EIGRP but is not actually running the protocol since it’s not exchanging EIGRP packets.

There really is a nuance to this issue, and it’s great that you brought it up. It keeps us on our toes and helps us to be more careful when expressing and teaching material in using the correct and most clear terminology possible. Thanks for sharing this!

Laz

3

Thank you. Sorry fo the late reply I read this when you posted but wasn’t in a position to reply. Very helpful… As I’m now studying for my CCNP Route the “nuances” are making this all the more fun…:wink:

thanx again

ak

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