Enhanced Interior Gateway Routing Protocol (EIGRP) is one of the famous routing protocols. It is widely used in networking. In this article, we will discuss types of messages/ packets which is used in EIGRP Protocol. We will also discuss about EIGRP parameter which is required to become EIGRP neighbor.
Quick Overview of EIGRP Messages
Before moving to detailed discussion of EIGRP Messages/Packets, let’s take a quick look on EIGRP Messages:
Hello message is the first message which is sent to a neighbor router in order to become EIGRP neighbor. In EIGRP, Hello message is responsible for:
- Hello message is used for neighbor discovery.
- It is used for Keep alive purpose.
- It is always multicast on every type of links.
- Hello timer is 5 seconds and its periodically send.
- Hold down timer is 15 seconds.
However, several different contents also send in an EIGRP Hello message. Let’s take a look on EIGRP Hello Message which is captured by Wireshark.
According to captured packet, the contents of EIGRP Hello Message:
- Version (v2 by default)
- Opcode 5
- Sequence Number and Acknowledgement number
- Autonomous Number (depends upon the EIGRP process)
- K-Values (by default K1, K3)
- Hold Down Timer
- IOS Version
Update message is sent between neighbors to build topology table and routing table. Basically, OSPF neighbor router shares their prefix to their neighbor with the help of Update Message. Update messages can be Multicast and Unicast. On serial link, update message is Unicast. However, in Ethernet link it can be Multicast and Unicast. The opcode of Update Message is 1. It also contains Autonomous Number. Update messages used RTP (Reliable Transport Protocol) to make packets reliable. Sequence and Acknowledgement number are used in Update Message.
So, Update Message has the following property:
- Can be Unicast/Multicast
- Opcode 1
- Sequence Number and Acknowledgement Number
- Autonomous Number
- Metric (K1, K2, K3, K4, K5)
- Subnet Information
As the name suggests, Query Packets are sent when a successor route fails and there is no feasible successor in the EIGRP topology table. The Router whose lost the route, sends a query message for asking their neighbor that the same route is present or not on their topology table. Maximum delay time is also set in Query Message. It is always Unicast at serial link and Multicast on Ethernet link. Properties of Query Message:
- Opcode 3
- Can be Unicast or Multicast
- Use RTP (Reliable Trunking Protocol)
- Have Sequence and Acknowledge numbers
Reply Message is sent in the response of Query Message. The responding router have an alternate route in their topology table and that router reply the query of the neighbor router. Like Update and Query message, Reply message is also use RTP. They also use sequence and acknowledgement number. Properties of Reply Message:
- Opcode 4.
- Use RTP.
- It is always Unicast Message.
As the name suggests, this message is an acknowledgement for Update, Query and Reply messages. It is not used in the EIGRP Hello message. The opcode of EIGRP Acknowledgement Message is 5.
To become EIGRP neighbor, there are several conditions which must be common. They are:
- Autonomous Number must match
- K-Values must match
- Authentication must match (optional)
- Network-ID must match
In this article we discussed, various EIGRP messages. However, a Hello message is used for neighbor discovery and neighbor formation process. Update, Query and Reply message are used RTP and thus they are reliable messages. We also discussed some parameters which must be common in order to become EIGRP Neighbor.
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