EIGRP is an advanced Distance Vector routing protocol that incorporates traditional Distance Vector features such as split horizon, as well as traditional Link State features such as incremental updates. EIGRP runs directly over IP using protocol number 88 and is a Cisco-proprietary routing protocol. The following sections describe some core characteristics and components that are integral to the operation of EIGRP.
It is important to have a solid understanding of EIGRP in order to effectively troubleshoot any routing problems. Keep in mind that the information provided is simply a summary and recap of the material available in the ROUTE study guide. Please refer to the ROUTE guide for additional information on specific topics and technologies if needed.
Packets
EIGRP uses several different types of packets to exchange routing and control information between EIGRP neighbors. EIGRP uses the following types of packets:
- Hello Packets
- Acknowledgement Packets
- Update Packets
- Query Packets
- Reply Packets
EIGRP sends Hello packets once it has been enabled on a router for a particular network. These messages are used to identify neighbors and once identified, serve or function as a keepalive mechanism between neighbors. EIGRP Hello packets are sent to the link local Multicast group address 224.0.0.10. Hello packets sent by EIGRP do not require an Acknowledgment to be sent confirming that they were received. Because they require no explicit acknowledgment, Hello packets are classified as unreliable EIGRP packets. EIGRP Hello packets have an OPCode of 5.
An EIGRP Acknowledgment (ACK) packet is simply an EIGRP Hello packet that contains no data. Acknowledgement packets are used by EIGRP to confirm reliable delivery of EIGRP packets. ACKs are always sent to a Unicast address, which is the source address of the sender of the reliable packet, and not to the EIGRP Multicast group address. In addition, Acknowledgement packets will always contain a non-zero acknowledgment number. The ACK uses the same OPCode as the Hello Packet because it is essentially just a Hello that contains no information. The OPCode is 5.
EIGRP Update packets are used to convey reachability of destinations. In other words, Update packets contain EIGRP routing updates. When a new neighbor is discovered, Update packets are sent via Unicast so the neighbor can build up its EIGRP Topology Table. In other cases, such as a link cost change, updates are sent via Multicast. It is important to know that Update packets are always transmitted reliably and always require explicit acknowledgement. Update packets are assigned an OPCode of 1.
EIGRP Query packets are Multicast and are used to reliably request routing information. EIGRP Query packets are sent to neighbors when a route is not available and the router needs to ask about the status of the route for fast convergence. If the router that sends out a Query does not receive a response from any of its neighbors, it resends the Query as a Unicast packet to the non-responsive neighbor(s). If no response is received after sixteen attempts, then the neighbor relationship is reset. EIGRP Query packets are assigned an OPCode of 3.
EIGRP Reply packets are sent in response to Query packets. The Reply packets are used to reliably respond to a Query packet. Reply packets are Unicast to the originator of the Query. The EIGRP Reply packets are assigned an OPCode of 4.
NOTE: EIGRP can also use another packet type called the Request packet. This is used in route server applications. EIGRP Request packets can be sent either via Multicast or Unicast, but are always transmitted unreliably. In other words, they do not require an explicit acknowledgment. Route server applications are beyond the scope of the TSHOOT exam.
Neighbor Discovery and Maintenance
EIGRP supports both dynamic and static (manually configured) neighbor discovery. Dynamic neighbor discovery is performed by sending Hello packets to the destination Multicast group address 224.0.0.10. Unlike the dynamic EIGRP neighbor discovery process, static EIGRP neighbor relationships require manual neighbor configuration on the router. When static neighbors are configured, the local router uses the Unicast neighbor address to send packets to these routers to establish a neighbor relationship.
Hello and Hold Timers
EIGRP uses different Hello and Hold timers for different types of media. Hello timers are used to determine the interval rate EIGRP Hello packets are sent. The Hold timer is used to determine the time that will elapse before a router considers an EIGRP neighbor as down. By default, the Hold time is three (3) times the Hello interval.
EIGRP sends Hello packets every 5 seconds on Broadcast, point-to-point serial, point-to-point subinterfaces, and multipoint circuits greater than T1. The default Hold Time is 15 seconds. EIGRP sends Hello packets every 60 seconds on other link types. These include low-bandwidth WAN links less than T1 speed. The default Hold time for neighbor relationships across these links is also three times the Hello interval and therefore defaults to 180 seconds.
The Neighbor Table
The EIGRP Neighbor Table is used by routers running EIGRP to maintain state information about EIGRP neighbors. When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This is applicable to both dynamically discovered neighbors and statically defined neighbors.
Reliable Transport Protocol
EIGRP needs its own transport protocol to ensure the reliable delivery of packets. Reliable Transport Protocol is the protocol that is used by EIGRP to ensure that Update, Query and Reply packets are sent reliably. The use of sequence numbers also ensures that the EIGRP packets are received in the correct order.
Metric Calculation
EIGRP uses a composite metric, which includes different variables. These variables are referred to as the K values. The K values are constants that are used to distribute weight to different path aspects, which may be included in the composite EIGRP metric. The default values for the K values are K1 = K3 = 1 and K2 = K4 = K5 = 0. In other words, K1 and K3 are set to a default value of 1, while K2, K4, and K5 are set to a default value of 0.
Assuming the default K value settings, the complete EIGRP metric can be calculated using the following mathematical formula:
[K1 * bandwidth + (K2 * bandwidth) / (256 – load) + K3 * delay] * [K5 / (reliability + K4)]However, given that only K1 and K3 have any positive values by default, the default EIGRP metric calculation is performed using the following mathematical formula:
[(10⁷ ⁄ least bandwidth on path) + (sum of all delays)] × 256This essentially means that, by default, EIGRP uses the minimum bandwidth on the path to a destination network and the total delay to compute routing metrics. However, Cisco IOS software allows administrators to set other K values to non-zero values to incorporate other variables into the composite metric.
Diffusing Update Algorithm (DUAL)
The Diffusing Update Algorithm is at the crux of the EIGRP routing protocol. DUAL looks at all routes received from neighbor routers, compares them, and then selects the lowest metric (best), loop-free path to the destination network. The best route, which is the route with the lowest metric or Feasible Distance (FD), is then referred to as the Successor route. The Feasible Distance includes both the metric of a network as advertised by the connected neighbor plus the cost of reaching that particular neighbor.
The metric that is advertised by the neighbor router is referred to as the Reported Distance (RD) or as the Advertised Distance (AD). This is that neighbors metric to the destination network. Therefore, the Feasible Distance refers includes the Reported Distance plus the cost of reaching that particular neighbor. The next hop router for the Successor route is referred to as the Successor. The Successor route is placed into the IP routing table and the EIGRP Topology Table. This route points to the Successor, which is the next hop router for the Successor route.
Any other routes to the same destination network that have a lower Reported Distance than the Feasible Distance of the Successor path are guaranteed to be loop-free and are referred to as feasible successor routes. These routes are not placed into the IP routing table; however, they are still placed into the EIGRP Topology Table, along with the Successor routes.
In order for a route to become a Feasible Successor route, it must meet the Feasibility Condition. The Feasibility Condition (FC) occurs only when the Reported Distance to the destination network is less than the Feasible Distance. In the event that the Reported Distance is more than the Feasible Distance, the route is not selected as Feasible Successor. This is used by EIGRP to prevent the possibility of loops.
The Topology Table
The EIGRP Topology Table is populated by EIGRP PDMs acted upon by the DUAL Finite State Machine. All known destination networks and subnets that are advertised by neighboring EIGRP routers are stored in the EIGRP Topology Table. This includes Successor routes, Feasible Successor routes and even routes that have not met the Feasibility Condition.
The Topology Table allows all EIGRP routers to have a consistent view of the entire network. It also allows for rapid convergence in EIGRP networks. Each individual entry in the Topology Table contains the destination network and the neighbor (or neighbors) that have advertised the destination network. Both the Feasible Distance and Reported Distance are stored in the Topology Table. The EIGRP Topology Table contains the information needed to build a set of distances and vectors to each reachable network, including:
- The lowest bandwidth on the path to the destination network
- The total or cumulative delay to the destination network
- The reliability of the path to the destination network
- The loading of the path to the destination network
- The minimum Maximum Transmission Unit (MTU) to the destination network
- The Feasible Distance to the destination network
- The Reported Distance by the neighbor router to the destination network
- The route source (only external routes) of the destination network
Stub Routing
Stub routing is an EIGRP feature primarily designed to conserve local router resources, such as memory and CPU, and improve network stability. The stub routing feature is most commonly used in hub-and-spoke networks. This feature is configured only on the spoke routers. When configured on the spoke router, the router announces its stub router status using a new TLV in the EIGRP Hello messages. When the hub router receives the Hello packet from the spoke router, one of two things happens:
- If the hub router is running a newer version of software, upon receiving the Hello packet with the new TLV, the router will not query the stub router about the status of any prefixes. This is the default mode of operation in current Cisco IOS software versions.
- If the hub router is running a version of software less than 12.0(7)T, upon receiving the Hello with the new TLV, the router will ignore this field because it does not understand it. The router will send Query packets the stub router if it needs information about a route or routes. However, the stub router will respond with a message of inaccessible to any queries received from the hub router. This method allows for backward compatibility with older versions of software while retaining stub routing functionality.
When the stub routing feature is enabled on the spoke router, the stub router will only advertise specified routes to the hub router. The router will not advertise routes received from other EIGRP neighbors to the hub router. Cisco IOS software allows administrators to select the type of routes that the stub router should advertise to the hub router; however, by default, the stub router will advertise connected and summary routes only. The EIGRP stub routing feature provides 4 advantages when implemented in hub-and-spoke networks. These advantages are:
- It prevents sub-optimal routing from occurring within hub-and-spoke EIGRP networks
- It prevents stub routers with low-speed links from being used as transit routers
- It eliminates EIGRP Query storms, allowing the EIGRP network to convergence faster
- It reduces the required amount of configuration commands on the stub routers
EIGRP Route Summarization
Route summarization reduces the amount of information that routers must process, which then allows for faster convergence within the network. Summarization also restricts the size of the area that is affected by network changes by hiding detailed topology information from certain areas within the network. And finally, summarization is used to define a Query boundary for EIGRP. EIGRP supports two types of route summarization. These two types are:
- Automatic route summarization
- Manual route summarization
By default, automatic route summarization is in effect when EIGRP is enabled on the router. This is implemented using the auto-summary command. This command allows EIGRP to perform automatic route summarization at Classful boundaries.
Unlike EIGRP automatic summarization, EIGRP manual route summarization is configured and implemented at the interface level using the ip summary-address eigrp [ASN] [network] [mask] [distance] [leak-map <name>] interface configuration command. By default, an EIGRP summary address is assigned a default administrative distance value of 5. This default assignment can be changed by specifying the desired administrative distance value as specified by the distance keyword.
By default, when manual route summarization is configured, EIGRP will not advertise the more specific route entries that fall within the summarized network entry. The leak-map <name>keyword can be configured to allow EIGRP route leaking wherein EIGRP allows specified specific route entries to be advertised in conjunction with the summary address. Those entries that are not specified in the leak map are still suppressed.
EIGRP Unequal Cost Load Sharing
In addition to equal cost load balancing capabilities, EIGRP is also able to perform unequal cost load sharing. This unique ability allows EIGRP to use unequal cost paths to send outgoing packets to the destination network based on weighted traffic share values. Unequal cost load sharing is enabled using the variance <multiplier> router configuration command.
The <multiplier> is an integer between 1 and 128. A multiplier of 1, which is the default, implies that no unequal cost load sharing is being performed. This default setting is also illustrated in the output of the show ip protocols command. If any other value is used, EIGRP will load share across the successor route as well as any other route with a route metric at least x that of the successor metric. By default, routes that do not meet the Feasibility Condition are excluded from this calculation.