Nt 2 Lecture 2 Eigrp

NT 2 Lecture 2 EIGRP

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What is EIGRP?

EIGRP is an advanced distance-vector routing protocol that relies on features commonly associated with link-state protocols. EIGRP uses Link State's partial updates and neighbor discovery. EIGRP's advanced features supports IP, IPX and AppleTalk. EIGRP uses RTP (Reliable Transport Protocol) to transport its routing updates 2

IGRP and EIGRP Metric Calculation - 1 The composite metric is calculated with the following formula:

By default, k1=k3=1 and k2=k4=k5=0. The default composite metric for EIGRP, adjusted for scaling factors, is as follows: 3

IGRP and EIGRP Metric Calculation - 2 BWmin is in kbps and the sum of delays are in 10s of microseconds. Example The bandwidth and delay for an Ethernet interface are 10 Mbps and 1ms, respectively. The calculated EIGRP BW metric is as follows:   

256 × 107/BW = 256 × 107/10,000 = 256 × 10000 = 256000

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Using EIGRP with IGRP

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EIGRP Neighbor Discovery -1

EIGRP routers actively establish relationships with their neighbors, similar to what Link State routers do. EIGRP routers establish adjacencies with neighbor routers by using small hello packets. The Hello protocol uses a multicast address of 224.0.0.10, and all routers periodically send hellos. 6

EIGRP Neighbor Discovery - 2 On hearing hellos, the router creates a table of its neighbors. The continued receipt of these packets maintains the neighbor table To become a neighbor, the following conditions must be met: 

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The router must hear a hello packet or an ACK from a neighbor. The AS number in the packet header must be the same as that of the receiving router. The neighbor’s metric settings must be the same. Each Layer 3 protocol has its own neighbor table.

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Neighbor Discovery - 3

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Neighbor Discovery - 4 By forming adjacencies, EIGRP routers do the following: Dynamically learn of new routes that join their network Identify routers that become either unreachable or inoperable Rediscover routers that had previously been unreachable 9

EIGRP Timers EIGRP updates are set only when necessary and are sent only to neighboring routers. There is no periodic update timer. EIGRP use hello packets to learn of neighboring routes. The holdtime to maintain a neighbor adjacency is three times the hello time. For hello is not received with the holdtime, the neighbor is removed from the table.

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Default Hello Intervals and Hold Time for EIGRP

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What is in the Neighbor Table?

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Dual Terminology - 1 AD (Advertised distance) is the metric that is reported by the neighbor router(s). FD (Feasible Distance) – Feasible distance is the metric that is reported by neighbor router(s), plus the cost associated with the forwarding link from the local interface to the neighbor router(s). When multiple paths exist, the local FD is the lowest-cost metric to a remote network. Feasibility Condition – If the AD from a given neighbor is less than the locally calculated FD, that neighbor meets the criteria to become the feasible successor. 13

Dual Terminology - 2 Successor - A successor is a neighboring router that is currently being used for packet forwarding; it provides the least-cost route to the destination and is not part of a routing loop Feasible successor - A feasible successor is a backup route. Feasible successors provide the next lowest-cost path without introducing routing loops. Feasible successor routes can be used in case the existing route fails. Packets to the destination network are immediately forwarded to the feasible successor, which at that point is promoted to the status of successor

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EIGRP successors and feasible successors - 1

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EIGRP successors and feasible successors - 2

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EIGRP successors and feasible successors - 3

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Dual Example – 1a

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Dual Example – 1b In the previous slide, EIGRP's composite metric is replaced by a link cost to simplify calculations. RTA's topology table includes a list of all routes advertised by neighbors. For each network, RTA keeps the real (computed) cost of getting to that network and also keeps the advertised cost (reported distance) from its neighbor.

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Dual Example – 1c RTY is the successor to network 24, by virtue of its lowest computed cost 31. This value is also the FD to Network 24. RTA follows a three-step process to select a feasible successor to become a successor for Network 24: 

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Determine which neighbors have a reported distance (RD) (=AD) to Network 24 that is less than 31. RTX's RD is 30 < 31, meet FC and is a feasible successor. RTZ's RD is 220 > 31, not meet FC, and cannot be a FS. 20

Dual Example – 2a (a) Is the Destination Network

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Dual Example – 2b In this example, (a) is the destination network, From C point of view, if it goes to (a) via B, the FD is 3 and the AD is 1. Others entries are computed in the same manner. Note in the example that router D does not have a feasible successor identified. The FD for router D to router A is 2 and the AD via router C is 3. Because the AD is larger than the FD, no feasible successor is placed in the topology table.

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Dual Example – 2c Router C has a feasible successor identified because the AD for the next hop router is less than the FD for the successor. How about router E?

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EIGRP Convergence - 1 In the context of routing protocols, convergence refers to the speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology. DUAL results in EIGRP's exceptionally fast convergence. Why? The FS provides the capability to make an immediate switchover to a backup route!

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EIGRP Convergence - 2

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EIGRP Neighbor Tables The most important table in EIGRP is the neighbor table and relationships tracked in the neighbor table are the basis for all the EIGRP routing update and convergence activity. The neighbor table contains information about adjacent neighboring EIGRP routers. A neighbor table is used to support reliable, sequenced delivery of packets. An EIGRP router can maintain multiple neighbor tables, one for each PDM running (e.g., IP, IPX, and AppleTalk) routed protocols.

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EIGRP Packet Types - 1 Hello packets assist in the discovery of EIGRP neighbors. The packets are multicast to 224.0.0.10. An acknowledgment packet acknowledges the reception of an update packet. An acknowledgment packet is a hello packet with no data. Acknowledgment packets are sent to the unicast address of the sender of the update packet.

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EIGRP Packet Types - 2 Update packets contain the routing information of destinations. Update packets are unicast to newly discovered neighbors; otherwise, update packets are multicast to 224.0.0.10 when a link metric changes. Update packets are acknowledged to ensure reliable transmission. Query packets are sent to find feasible successors to a destination. Query packets are always multicast.

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EIGRP Packet Types - 3 Reply packets are sent to respond to query packets. 

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Reply packets provide a feasible successor to the sender of the query. Reply packets are unicast to the sender of the query packet.

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EIGRP Topology Table 1 EIGRP uses its topology table to store all the information it needs to calculate a set of distances and vectors to all reachable destinations. EIGRP maintains a separate topology table for each routed protocol. EIGRP sorts the topology table so that the successor routes are at the top, followed by feasible successors. Entries in the topology table can be in one of two states: active or passive. A passive route is one that is stable and available for use. An active route is a route in the process of being recomputed by DUAL.

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EIGRP Topology Table - 2

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EIGRP Routing Table The routing table contains the routes installed by DUAL as the best loop-free paths to a given destination EIGRP will maintain up to four routes per destination. These routes can be of equal or unequal cost. EIGRP routers maintain a separate routing table for each routed protocol.

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Configuring EIGRP To enable EIGRP and define the autonomous system:  router(config)# router eigrp autonomous-systemnumber This statement identifies the locally connected network that should be advertised to EIGRP neighbors. router(config-router)# network network-number When using serial links, it is important to use the bandwidth on the interface to change the bandwidth used for calculating routing metrics. If you do not change the bandwidth for these interfaces EIGRP assumes the default bandwidth on the link (?) instead of the true bandwidth. router(config-if)# bandwidth kilobits 33

EIGR Route Summarization - 1 EIGRP automatically summarizes routes. The summary routes are advertised at the class A,B, or C boundaries. Autosummarization occurs when a router is on the boundary of different major networks.

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EIGRP Route Summarization - 2 However, if you have discontiguous subnetworks, auto-summarization must be disabled for routing to work properly.

In here, even though RTC is connected only to the subnet 2.1.1.0, it will advertise that it is connected to the entire Class A network, 35 2.0.0.0

EIGRP Route Summarization - 3 By default, EIGRP’s interfaces do not use VLSM. To turn off auto-summarization, use the following command: router(config-router)#no auto-summary

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EIGRP Manual Summarization -1 EIGRP also enables you to manually configure a summary routes on any router a per-interface basis. You first select the interface that will propagate the route summary. Then you can define the summary address with the ip summaryaddress eigrp command. 37

EIGRP Manual Summarization -2

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EIGRP Manual Summarization -3 When configured, RTC will add a summary route to its table, as follows:

The benefits of EIGRP manual summarization:   

support future growth in networks; preserve bandwidth and CPU utilization. flexibility in where apply summarization.

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Unequal Cost Load Balancing -1 EIGRP also supports unequal-cost load balancing. This is achieved by using the variance command ( the same as IGRP we have come across in Lab.)

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Unequal Cost Load Balancing -2

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Unequal Cost Load Balancing -3 If the variance command was configured with a variance (multiplier) of 2; then the best metric is 10 × 2 = 20. These paths would all load-balance traffic from Router F to Network A: F to D to B = 15 F to C to B = 15 F to C to G = 10 One and a half packets would be sent across the path F to C to G for every one packet sent across the other two available paths. 42

Recap: EIGRP New Technologies over IGRP Neighbor discovery and recovery Reliable Transport Protocol Dual Finite State Machine Protocol specific modules

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Recap: EIGRP Advantages Rapid convergence Efficient use of bandwidth  

Partial, bounded updates Minimal consumption of bandwidth when the network is stable

Support for VLSM and CIDR Multiple network-layer support Independence from routed protocols 44