Why configure EIGRP neighbors

[Kevin Melton]

I have a client whom is using EIGRP between a few of the devices.  Some of the devices have "neighbor" configuration statements within the config of their EIGRP process.  Others do not.  What I see is that even the devices which do not have "neighbor" statements in their EIGRP process in the config DO establish neighbor adjacencies.

What is then the purpose of configuring "neighbors" in the EIGRP process, if they will form neighbor adjacencies anyhow?

Thanks

kevin

[Reza Sharifi · Accepted solution]

Q. What does the neighbor statement in the EIGRP configuration section do?

A. The neighbor command is used in EIGRP in order to  define a neighboring router with which to exchange routing information.  Due to the current behavior of this command, EIGRP exchanges routing  information with the neighbors in the form of unicast packets whenever  the neighbor command is configured for an interface. EIGRP stops  processing all multicast packets that come inbound on that interface.  Also, EIGRP stops sending multicast packets on that interface.

The ideal behavior of this command is for EIGRP to start sending  EIGRP packets as unicast packets to the specified neighbor, but not stop  sending and receiving multicast packets on that interface. Since the  command does not behave as intended, the neighbor command should be used carefully, understanding the impact of the command on the network.

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a008012dac4.shtml#ten

HTH

[JohnTylerPearce]

Reza that snwer from Cisco is kind of confusing. It says....

Due to the current behavior of this command, EIGRP exchanges routing information with the neighbors

in the form of unicast packets whenever the 'neighbor' command is configured for an interface. EIGRP

STOPS processing all multicast packets that come inbound on that interface. Also, EIGRP stops

sending multicast packets on that interface.

Then you have

The ideal behavior of this command is for EIGRP to start sending EIGRP packets as unicast packets to the

specified neighbor, but not stop sending and receiving multicast packets on that interface.

According to the top part, if you run the command 'neighbor x.x.x.x' under router eigrp AS, it sends packets

via unicast, but does not process multicast packets send and received from that interface. But the other senetance

almost appears as the opposite..

What does "ideal behavior" of this command mean?

Does this mean that ideally this is how you would want it to work, but with Cisco the 'neighbor x.x.x.x' command under

the router eigrp process, it doesn't work that way, it just enables unicast and kills MC?

[Ryan Newell]

I am curious to know the reason why the command is used. I know it is particularly helpful when your layer 2 protocol does not support broadcast packets in a default configuration. For example with frame relay. Many CCIE lab scenarios I have encountered will restrict the use of broadcast keyword but requiring peering of IGPs. The neighbor command becomes very useful then because the packet is now unicast instead of multicast. Could you please describe your topology? Is there any reason you could think of that unicast delivery would be required?

Regards,

Ryan

[Kevin Melton]

thanks for the answer Reza.  Things that did not make sense to me earlier do now.  I should not have configured the "neighbor command" where I did not need it.  by doing so, it stopped some Multicast updates that I otherwise would have seen.

I have removed the "neighbor" statements I added last night, and subsequently the networkis back up again.

Thank You.

[Mohamed Sobair]

Hello Guys,

The Main purpose of using the Passive Command in a routing Protocol like (RIP, Eigrp and OSPF) is to provide additional Security features.

To elaborate more, the neigbor command in Eigrp prevents the hello packets which contains the (version, Code, Acknowledgemnt, Sequence , TLVs ...etc) to be intercepted in a broadcast media and thus preventing an attacker to form an Eigrp neighbor relationship and exchange routing information on the link.

I know there is additional Security feature that could be used to prevent rouque adjacencies, like authentication for example, but still when you Specify a Neighbor, The Packet is ONLY Sent as Unicast to that Neighbor including routing Protocol update, Unlike when its Sent as Multicast.

In a tight Security environment, its a restriction to run this command for all routing protocol , this of cours plus routing protocol authentication , port security ...etc.

Regards,

Mohamed

[fb_webuser]

to be simple here.it would be used where multicast and broadcaste

is not allowed...because as you know this config send unicast to neighbor rather than periodic multcast hello packets.If there is no scenario like that,this command would not be required.Client does mistake,our job is to correct them..HTH

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Posted by WebUser Mukti Chandwani

[lucentmoon]

Eigrp neighbor command also increases the TTL to greater than 1.  This is for things like forming neighbor adjacency from spoke to spoke through hub, whereas multicast ttl is always a value of 1.  Rip neighbor command functions the same way in this respect.  Not sure if ospf is the same way.