EIGRP part 2

In order to create an adjacency with the neighbor, the eigrp discovering process needs to be done first. The process of discovering a neighbor is as below:

1. The originating router sends a Hello packet using multicast to 224.0.0.10
2. The neigboring router will detect the Hello packet and responses with an Update packet using unicast (NOT multicast!). In the same time it will also send an hello packet to the originating router.
3. The originating router receives the update packet and sends an unicast Ack packet to the neighbor router while sending an update packet from its current configuration to the neighbor router in unicast.
4. The neighbor router will send an ack back to the originating router to confirm the receival of the update packet from the originating router.

EIGRP does not require the neighbors to have the same hello and hold time (the same as dead time in ospf) values which is the opposite in ospf, although this difference can create an instability in the connection between the adjacent routers. However, metric configurations (k-weights) and the AS number of the adjacent routers MUST be the same or the connection between the adjacent routers will not happen. Another difference between OSPF and EIGRP is that OSPF has a dead time = 3 x hello time whereas EIGRP has a hold time = 4 x hello time.

The hello packets are by default sent every 5 seconds on a high speed link such as Etherned and FDDI, whereas in low speed links (less than T1 speed such as ATM)

To change the hello time use the command:
(config)#interface
(config-if)#ip hello eigrp

To change the metrics weights use the command:
(config)#router eigrp // 0 is not allowed
(config-router)#metric weights 0


To make a router in an EIGRP network do the following command:
(config)#router eigrp
(config-router)#no auto summary
(config-router)#network // the network can be the primary network or the secondary network (See below)

To show the neighbors use the #show ip eigrp neighbor command

To debug the packets do #debug eigrp packets

EIGRP is also possible when the secondary addresses are applied on the routers. A secondary address is when 2 ip addresses are used on a single physical interface. However, the secondary addresses must be configured properly and the secondary subnets must be the same between the neigboring routers. If the secondary address subnet is configured on a router and the same subnet is configured on the neighboring router as primary, eigrp will not work because the primary addresses of both routers are not in the same subnet. The adjecency will be formed, but on the show ip eigrp neighbor command the adjecency is formed between the primary networks, not the secondary.

To put a secondary address on an interface do the command:
(config)#interface
(config-if)#ip address secondary

EIGRP part 1

EIGRP stands for Enhanced Interior Gateway Routing Protocol with an AD = 90. The EIGRP is an advanced version of IGRP and is cisco proprietary. EIGRP uses a DUAL algorithm, and has a fast convergence, faster than OSPF. EIGRP supports VLSM because EIGRP update packets contain a prefix length for each network. This protocol sends multicasts packets to 224.0.0.10 for dynamic neigbor discovery and uses the IP protocol 88.

EIGRP activates by defaul equal-cost load sharing up to 4 paths, but can be configured to a maximum of 6 paths. The command is:
(config)#router eigrp
(config-router)#maximum-paths

The DUAL (Diffusing Update Algorithm) is an algorithm which calculates the successors (main routes) and the feasible successors (backup route). Therefore, should the successors fail, the feasible successor is instantly ready for use.

EIGRP has 3 tables which is the standard routing table, the EIGRP topology table and the neighbor table. The routing table contains primary routes (successors) only, while the EIGRP topology stores both the successors and the feasible successors. The neighbor table stores information about the EIGRP neigboring routers.

EIGRP's metric is calculated based on 5 k-weights which are in order bandwidth, delay, load, reliability and MTU. However, only bandwidth and delay is set active by default (set to 1) while all other is set inactive (set to 0). The bandwidth and delay can be configured with the command:

(config)#interface
(config-if)#bandwidth
(config-if)#delay

EIGRP has 5 different type of packets which are:

1. Hello packets that are used for neigbor discovery and maintain the connection with the neighbors. This data uses UDP packets and is therefore unreliable.

2. Acknowledgement packets that are used from the originating router to confirm the update received from the neigboring routers. This data uses also UDP packets and is therefore unreliable.

3. Query packets are sent when the router loses its successor route and does not have a feasible successor. This packet uses Reliable Transport Protocol (RTP) and is therefore reliable.

4. Reply packets that is used to response a query packet and can also mean that a new route is found to the destination router. This packet uses RTP and is therefore reliable.

5. Update packets that is sent and received to build a routing table or when a change in the network happens. If the latter happens, this packet will only send the changing routes instead of all the routes from the originating router, thus making EIGRP more resource effecient. This packet is send using unicast to the destination router and uses RTP protocol.

To see these number of packets sent or received use the #show ip eigrp traffic command.

-- 27 January 2011 --