[Day 8] 5. EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP)

Type: Hybrid routing protocol, combining the best features of both distance-vector and link-state protocols.
Developed by: Cisco Systems.
Open Source: Initially a proprietary Cisco protocol, EIGRP was later made available to the public under an open standard.

Comparison with OSPF

|Feature|EIGRP (Enhanced Interior Gateway Routing Protocol)|OSPF (Open Shortest Path First)| |---|---|---| |Multicast Address|224.0.0.10|224.0.0.5 (All OSPF routers), 224.0.0.6 (All DR routers)| |Hello Interval|5 seconds|10 seconds| |Update Interval|60 seconds|N/A (OSPF uses LSAs for updates)| |Transport Protocol|RTP (Reliable Transport Protocol) - Similar to TCP; sends acknowledgements for reliable delivery.|No dedicated transport protocol, uses IP.| |Metric Calculation|Bandwidth, Delay, Load, Reliability, and MTU (Maximum Transmission Unit).|Cost (based on bandwidth).| |Protocol Dependent Module (PDM)|EIGRP uses PDM, enabling it to support multiple types of routing protocols (e.g., IP, IPX).|OSPF is designed specifically for IP routing and doesn't support PDM.| |Backup Path (Immediate Backup)|Supports Immediate Backup for fast failover and resilience.|Does not have an immediate backup path.| |Load Balancing|Supports Unequal Cost Load Balancing (UCLB), allowing for more efficient use of network resources.|Only supports Equal-Cost Multi-Path (ECMP) load balancing.|

Unique Features of EIGRP:

Immediate Backup: EIGRP supports a mechanism for configuring a backup route, which can immediately take over if the primary route fails. This feature is unique to EIGRP.
Unequal Cost Load Balancing: EIGRP supports load balancing across unequal-cost paths, unlike OSPF, which only supports equal-cost load balancing. This allows more efficient use of network bandwidth by leveraging multiple paths with different metrics.

Metric (Distance) in EIGRP:

The metric in EIGRP is calculated based on several factors:
1. Bandwidth of the link.
2. Delay introduced by the link.
3. Load and Reliability of the link.
4. MTU (Maximum Transmission Unit).
This composite metric allows EIGRP to make more intelligent routing decisions compared to other protocols, which typically only use one metric like cost (OSPF) or hop count (RIP).

Basic Configuration of EIGRP:

To configure EIGRP on a router, you must specify the Autonomous System (AS) number. This AS number should match across all routers that will be exchanging routes.

Example configuration:

Enable EIGRP with the AS number (e.g., 10):

bash
A(config)# router eigrp 10

Configure networks to be included in EIGRP routing:
```
bash
A(config-router)# network 172.16.0.0
```
- The AS number (
```
10
```
  in this case) must be the same on all routers in the EIGRP network to ensure that they can exchange routing information.
- The network command is used to define which interfaces participate in EIGRP.

Additional Notes:

The AS (Autonomous System) number in EIGRP uniquely identifies a group of routers that share the same routing information within a specific administrative domain. It ensures that routers in different AS numbers do not exchange EIGRP routing information, maintaining separate routing domains. The AS number acts as a boundary for EIGRP communication, making it essential for defining the scope of the routing protocol.
RTP (Reliable Transport Protocol) in EIGRP ensures that routing updates are reliably transmitted to all EIGRP neighbors, similar to TCP, by sending acknowledgements after packets are received. This is in contrast to protocols like OSPF, where no such acknowledgement mechanism is implemented.
PDM (Protocol Dependent Module) is a unique feature of EIGRP, which allows it to support multiple network layer protocols (IP, IPX, AppleTalk). This modular approach is one of the reasons EIGRP can be so flexible and scalable.