In another lesson I explained the different EIGRP packets and their function. In this lesson we’ll take a close look at the EIGRP neighbor adjacency to see what exactly happens when EIGRP routers become neighbors. This is what happens when you enable EIGRP on two routers:
We have 2 routers called R1 and R2 and they are configured for EIGRP. As soon as we enable it for the interface they will start sending hello packets. In this example R1 is the first router to send a hello packet.
As soon as R2 receives the hello packet from R1 it will respond by sending update packets that contain all the routing information that it has in its routing table. The only routes that are not sent on this interface are the one that R2 learned on this interface because of split-horizon. The update packet that R2 will send has the initialization bit set so we know this is the “initialization process”. At this moment there is still no neighbor adjacency until R2 has sent a hello packet to R1.
R1 is of course not the only one sending hello packets. As soon as R2 sends a hello packet to R1 we can continue to setup a neighbor adjacency.
After both routers have exchanged hello packets we will establish the neighbor adjacency. R1 will send an ACK to let R2 know he received the update packets. The routing information in the update packets will be saved in the EIGRP topology table.
R2 is anxious to receive routing information as well so R1 will send update packets to R2 who will save this information in its EIGRP topology table.
After receiving the update packets R2 will send an ACK back to R1 to let him know everything is ok.
Want to see what this looks like on a real router? Let’s use the following topology and see what happens: 
This is the topology I’m going to use to configure EIGRP. My goal is to have full connectivity and here are the configurations:
R1(config)#router eigrp 1
R1(config-router)#no auto-summary
R1(config-router)#network 1.1.1.0 0.0.0.255
R1(config-router)#network 192.168.12.0
R1(config-router)#exitR2(config)#router eigrp 1
R2(config-router)#no auto-summary
R2(config-router)#network 2.2.2.0 0.0.0.255
R2(config-router)#network 192.168.12.0
R2(config-router)#exit
Let’s break this one down. Router eigrp 1 will start up EIGRP using AS (autonomous system) number 1. This number has to match on both routers or we won’t become EIGRP neighbors.
No auto-summary is needed because by default EIGRP will behave like a classful routing protocol which means it won’t advertise the subnet mask along the routing information. In this case that means that 1.1.1.0/24 and 2.2.2.0/24 will be advertised as 1.0.0.0/8 and 2.0.0.0/8. Disabling auto-summary will ensure EIGRP sends the subnet mask along.
Network 1.1.1.0 0.0.0.255 mean that I’m advertising the 1.1.1.0 network with wildcard 0.0.0.255. If I don’t specify the wildcard you’ll find “network 1.0.0.0” in your configuration. Does it matter? Yes and no. The same thing applies to “network 2.2.2.0 /24”. It will work but also means that every interface that falls within the 1.0.0.0/8 or 2.0.0.0/8 range is going to run EIGRP. Network 192.168.12.0 without a wildcard mask is fine since I’m using a /24 on this interface which is Class C.
If you are working on a lab and are lazy (like me) you can also type in network 0.0.0.0 which will activate EIGRP on all of your interfaces…if that’s what you want of course.
Let’s do a debug on R2 to see what is going on:
R2#debug eigrp packets ?
SIAquery EIGRP SIA-Query packets
SIAreply EIGRP SIA-Reply packets
ack EIGRP ack packets
hello EIGRP hello packets
ipxsap EIGRP ipxsap packets
probe EIGRP probe packets
query EIGRP query packets
reply EIGRP reply packets
request EIGRP request packets
retry EIGRP retransmissions
stub EIGRP stub packets
terse Display all EIGRP packets except Hellos
update EIGRP update packets
verbose Display all EIGRP packets
<cr>
As you can see we have a LOT of debug options for EIGRP. I want to see the hello packets…
R2#debug eigrp packets hello
EIGRP Packets debugging is on
(HELLO)
R2# EIGRP: Received HELLO on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
Looking good seems we have received a hello packet from R1.
R2# EIGRP: Sending HELLO on FastEthernet0/0
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
And we are sending hello packets to R1 as well.
R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.2 (FastEthernet0/0) is up: new adjacencyR2# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.1 (FastEthernet0/0) is up: new adjacency
You can see we have an EIGRP neighbor adjacency.
R2# EIGRP: Sending HELLO on Loopback0
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
EIGRP: Received HELLO on Loopback0 nbr 2.2.2.2
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0
Hmm interesting it seems R2 is schizophrenic and sending hello packets to its loopback0 interface and also receiving them.
This behavior is normal because the network command does two things:
- Send EIGRP packets on the interface that falls within the network command range.
- Advertise the network that is configured on the interface in EIGRP.
So what do you have to do when you want to advertise a network without sending EIGRP packets on the interface and forming EIGRP neighbors?
Use the passive interface command.
Use the passive interface command.
R2(config)#router eigrp 1
R2(config-router)#passive-interface loopback 0
This will advertise the 2.2.2.0/24 network on R2’s loopback 0 interface without sending EIGRP packets to the loopback.
R2(config)#router eigrp 1
R2(config-router)#passive-interface default
R2(config-router)#no passive-interface fastEthernet 0/0
If you have to configure an ISP router with 50+ interfaces you probably don’t want to type in this command on each of those interfaces. You can also configure passive-interface default and only activate the interfaces you want to run EIGRP on.
Let’s look at a debug of some EIGRP update packets. I’m going to clear the EIGRP neighbor adjacency to see what it looks like:
R2#debug eigrp packets update
EIGRP Packets debugging is on
(UPDATE)R1#clear ip eigrp neighbors
Let’s reset the EIGRP neighbor adjacency on R1.
R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.2 (FastEthernet0/0) is down: Interface Goodbye received
You can see that R1 is being nice and at least telling R2 that it’s deleting the neighbor adjacency.
R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.2 (FastEthernet0/0) is up: new adjacency
R1 is back in business and our EIGRP neighbor adjacency has been reestablished.
R2#
EIGRP: Enqueueing UPDATE on FastEthernet0/0 nbr 192.168.12.1 iidbQ un/rely 0/1 peerQ un/rely 0/0
EIGRP: Received UPDATE on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x1, Seq 13/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Sending UPDATE on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x1, Seq 13/13 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
Here you can see R2 is putting an update packet in its queue for R1. R2 is also receiving an update packet from R1 and afterwards sending its own update packet. You’ll see a couple of update packets flying back and forth.
R2#debug eigrp packets ack
EIGRP Packets debugging is on
(ACK)
We can also check out some acknowledgments by debugging ack packets.
R1#clear ip eigrp neighbors
We’ll reset the EIGRP neighbor adjacency so the update packets are resent.
R2a#
EIGRP: Received ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/17 idbQ 0/0 iidbQ un/rely 0/1 peerQ un/rely 0/1
EIGRP: Received ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/18 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Enqueueing ACK on FastEthernet0/0 nbr 192.168.12.1
Ack seq 18 iidbQ un/rely 0/0 peerQ un/rely 1/2
EIGRP: Sending ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/18 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 1/2
EIGRP: Enqueueing ACK on FastEthernet0/0 nbr 192.168.12.1
Ack seq 19 iidbQ un/rely 0/0 peerQ un/rely 1/2
EIGRP: Sending ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/19 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 1/2
EIGRP: Received ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/20 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
These are the ack packets in response to some of the update packets. If you want to see the whole process we can combine some debugging.
R2#debug eigrp packets
EIGRP Packets debugging is on
(UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY, SIAREPLY)
This is how you enable debugging for all EIGRP packets.
R1#clear ip eigrp neighbors
Reset the EIGRP neighbor adjacency again…
R2#
EIGRP: Sending HELLO on FastEthernet0/0
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
EIGRP: Received HELLO on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.1 (FastEthernet0/0) is up: new adjacency
R2#
EIGRP: Enqueueing UPDATE on FastEthernet0/0 nbr 192.168.12.1 iidbQ un/rely 0/1 peerQ un/rely 0/0
EIGRP: Sending HELLO on FastEthernet0/0
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/1
EIGRP: Requeued unicast on FastEthernet0/0
EIGRP: Received UPDATE on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x1, Seq 25/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Sending UPDATE on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x1, Seq 25/25 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Received ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/25 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Enqueueing UPDATE on FastEthernet0/0 iidbQ un/rely 0/1 serno 1-2
EIGRP: Enqueueing UPDATE on FastEthernet0/0 nbr 192.168.12.1 iidbQ un/rely 0/0 peerQ un/rely 0/0 serno 1-2
EIGRP: Sending UPDATE on FastEthernet0/0
AS 1, Flags 0x8, Seq 26/0 idbQ 0/0 iidbQ un/rely 0/0 serno 1-2
EIGRP: Enqueueing UPDATE on FastEthernet0/0 nbr 192.168.12.1 iidbQ un/r
R2#ely 0/1 peerQ un/rely 0/1
EIGRP: Requeued unicast on FastEthernet0/0
EIGRP: Received ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/26 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/2
EIGRP: FastEthernet0/0 multicast flow blocking cleared
EIGRP: Sending UPDATE on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x8, Seq 27/25 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Received ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/27 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Received UPDATE on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x8, Seq 26/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
EIGRP: Enqueueing ACK on FastEthernet0/0 nbr 192.168.12.1
Ack seq 26 iidbQ un/rely 0/0 peerQ un/rely 1/0
EIGRP: Sending ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/26 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 1/0
EIGRP: Enqueueing UPDATE on FastEthernet0/0 iidbQ un/rely 0/1 serno 9-9
EIGRP: Enqueueing UPDATE on FastEthernet0/0 nbr 192.168.12.1 iidbQ un/rely 0/0 peerQ un/rely 0/0 serno 9-9
EIGRP: Received UPDATE on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x8, Seq 27/27 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
EIGRP: Enqueueing ACK on FastEthernet0/0 nbr 192.168.12.1
Ack seq 27 iidbQ un/rely 0/0 peerQ un/rely 1/1
EIGRP: Sending ACK on FastEthernet0/0 nbr 192.168.12.1
AS 1, Flags 0x0, Seq 0/27 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 1/1
This will show you the entire process of sending hello packets to each other, becoming EIGRP neighbors, exchanging updates and sending acks. That’s the end of my story. I hope this has been helpful for you to understand the EIGRP neighbor adjacency process. If you have any questions or comments please leave a message.
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