Believe me or not, aside from passing an exam there is another important reason why you should know what is inside the EIGRP packet header. Any hypothesis?

The Job Interview

You thought you know everything when you got the Cisco professional level certification but what happens when the interviewer asked you about what is inside the EIGRP packet header? You memorized all the configurations commands. You know what is BGP route reflector. You know how to do unequal load balancing in EIGRP. You even know how to configure fabric path, ASA firewalls, and do site-to-site VPN. You know everything you did in your laboratory but you forgot what is inside the EIGRP packet header.

“Who is going to ask me this stupid question?”

I guess an interviewer who has a doubt about your skills most especially if you put all your certifications on your resume. Funny, but it is pretty quite true.

“Is the interviewer going to judge me if I forgot what EIGRP packet header contains?”

Uhm, maybe. Depends on many reasons. We can say, the interviewer is using a bottom-up approach. In this way, it saves time and may not continue asking you further questions if you did not know the basic. Or, it can be a warm-up for more heart-pounding questions.

“What if you just forgot and neglect it during the academy session?”

I don’t think the interviewer will be interested in that kind of reason. So, if you were not able to answer, you better pray that the interview will not stop there.

I don’t want to scare you because this is just a legend. It is a traditional story popularly regarded as historical but nobody wants to confirm the truth. Anyhow, it is just my way to open up our “EIGRP Packet Header” discussion.

1. Version – This is the EIGRP header version with the current version of 2. This is a 4-bit field and it is not the same as the TLV version field.
2. Opcode – Remember the EIGRP packet types? This is how EIGRP neighbors know what kind of packet type it is. It is a 4-bit field as well like the version field and below is the equivalent values of message types:
   EIGRP Message TypeOpcode ValueUpdate1Request2Query3Reply4Hello5Reserved6-9SIA Query10SIA Reply11
3. Checksum – this is 24-bit field standard IP checksum. If the packet fails the checksum, the it is discarded.
4. Flags – This is a 32-field that defines special handling of the packet. There are 4 flag bits: INIT flag (0x01), Conditionally Received (CR) flag (0x02), Restart (RS) flag (0x04), and End-of-Table (EOT) flag (0x08). For newly discovered neighbors, the bit is set in the initial UPDATE. The INIT flag instructs the neighbor to advertise its full set of routes. CR flag is that receivers should only accept the packet if they are in Conditionally Received mode. RS flag is set in the HELLO and UPDATE packet. It is an indication that the neighbor is doing a soft restart. This In this way, adjacency is maintained. When EOT flag is set, it indicates that the neighbor has completed sending all updates. This indicates the neighbor can flush all stale routes prior to restart event.
5. Sequence – Every packet sent to the neighbor will have a 32-bit sequence number that is unique to the sender. When the value is set to 0 that means it doesn’t require any acknowledgement. 
6. Acknowledgement – this is another 32-bit field sequence number that is unique to the receiver.
7. Virtual Router ID – This is a 16-bit number that distinguishes the virtual router a packet is associated with. Any value other than listed below, will be discarded:
   
8. Autonomous System – This is the most important part in the EIGRP packet header. This is a 16-bit number which value ranges from 1 – 65535. AS should match on all EIGRP neighbors or else packet will be ignored and there will be no adjacency.

The neighbour, topology, and routing tables are very important in implementing and troubleshooting EIGRP. I often neglect the topology and routing table before because what is important to me is that all neighbours are up. But this is not the case especially if you want to progress your learning to a higher Cisco certification level.

Let’s start with the neighbour table.

If you want to know whether you have established adjacency with neighbouring EIGRP router and also to know the uptime, then you go to the neighbour table.

H – Handle; it is the order in which the EIGRP adjacency is formed. It starts with 0 and so on so forth.

Address – This is your neighbour’s IP address, not the local IP address. Looking at the table it shows that the Nettle router has 2 adjacent neighbours: 192.168.103.3 and 192.168.102.2.

Interface – This is the interface of the neighbour connected.

Hold – this is the hold time value in seconds. This tells how long the router will hold a neighbour if it doesn’t receive a hello. There are two default values: 15 seconds and 180 seconds.  The 15 seconds default value is from the 5-second-hello packet on high bandwidth links like PPP, Ethernet, HDLC, point-to-point ATM and frame-relay subinterfaces, and others greater than T1 circuits. The 180 seconds value is from the 60-second hello packet of slower T1 circuits.

Uptime – It tells how long the local router has established adjacency to its neighbour router.

SRTT – is the acronym for Smooth Round Trip Time. Its value is in milliseconds and tells how long an EIGRP is to be sent to the neighbour and for the local router to receive an acknowledgement of that packet. The table, it is showing us the value of 66. It is ok unless the value is 0 because the value of 0 indicates that there are no acknowledgement packets being received.

RTO – short for Retransmission Timeout whose value is also in milliseconds. It tells how long the packet will be retransmitted from the retransmission queue of the neighbour.

Q Cnt – short for Queue Count. It is the number of EIGRP packets whether it is an update packet, query packet or reply packet that the software is waiting to send. On the table, it is showing 0 which means it is good as there are no pending packets in the queue. There is some sort of unidirectional or congestion issue if the Q Cnt value is not decrementing because that means nothing is being received or acknowledged.

Seq Num – it is the sequence number. It is the last used sequence number in which the EIGRP received the last packet.

EIGRP is Cisco’s baby. It works on all Cisco devices. It is an advanced distance vector routing protocol that has some link state features. Like RIP, it has the hop count feature. Routes that reached the maximum hop cunt will be tagged as unreachable. Although it is not used as its metric, it limits the EIGRP AS when routing to a remote network. The default hop count is 100 and the value can vary between 1 – 255. And, like OSPF it does not send the whole routing table when there is a routing change.

In addition, EIGRP supports Classless Inter-Domain Routing (CIDR) and variable length subnet masking. Like OSPF and RIP v2, routes are not summarized on classful boundaries. However, be careful with not putting the “no auto-summary” command under the router process. The “no auto-summary” command disables or prevents automatic summarization of subnet routes into network-level routes.

Remember that EIGRP uses Diffusing Update ALgorithm (DUAL) as its algorithm in every routing decisions, prevents routing loops, and it is used to respond to changes in the routing topology. It is developed by J.J. Garcia-Luna-Aceves and uses three tables for the route calculation: Neighbor table, Topology table, and Routing table.

Lastly, internal EIGRP has an administrative distance (AD) value of 90, the external has a value of 170, and the summary has a value of 5.

EIGRP RELATED TOPICS:

1. EIGRP Metrics
2. The EIGRP Neighbor Table
3. The EIGRP Topology Table
4. The EIGRP Routing Table
5. EIGRP Packet Types
6. EIGRP Neighbor Adjacency
7. EIGRP Successor, Feasible Successor and Feasibility Condition
8. EIGRP Packet Header
9. EIGRP Show Commands

EIGRP CONFIGURATION:

1. EIGRP Configuration
2. EIGRP Redistribution
3. Configuring “no auto-summary” eigrp summarization
4. EIGRP Authentication
5. EIGRP Network Advertisement
6. EIGRP Passive Interface
7. EIGRP Unequal Path Cost Load Balancing
8. Changing Administrative Distance

So what are IGP and EGP?

IGP is composed of routing protocols that are used within the autonomous system while EGP is used between different autonomous system. IGP usually runs within the LAN while EGP mostly used in WAN. EGP is the protocol run by ISPs to connect to customer’s edge device and protocol used by the internet. It is used to interconnect different autonomous system.

Interior Gateway Protocols:

1. Routing Information Protocol (RIP)
2. Open Shortest Path First (OSPF)
3. Enhanced Interior Gateway Routing Protocol (EIGRP)
4. Intermediate System to Intermediate System (IS-IS)

Exterior Gateway Protocol:

1. Border Gateway Protocol (BGP)

Believe me, if you missed it, you’ll get to know it on the other day.