eigrp

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Jan 22nd, 2007
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Hi all, I know eigrp is a hybrid , what do they class it as on ccna, link state ? and does it get updates via unicast from all the other routers like ospf etc, or does it only learn from its directly attatched neighbours ?

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Amit Singh Mon, 01/22/2007 - 05:29
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Hi Carl,


EIGRP is hybrid but it exhibits the link state property as well. When the EIGRP routers becomes neighbor, they send the entire routing table to each other (distance vector property) and after that only the routing changes are forwarded (Link state property). It uses a multicast IP 224.0.0.10 to send updates to other routers or vice-versa.


Please see the EIGRP design guide:


http://cisco.com/en/US/tech/tk365/tk207/tsd_technology_support_sub-protocol_home.html


HTH,

-amit singh

carl_townshend Mon, 01/22/2007 - 06:08
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So it only sends the full table once then ? and then after that does it only forwards its changes via multicast, is this the same as ospf etc ?

Amit Singh Mon, 01/22/2007 - 06:09
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Yes.Please go through the EIGRP design guide for better understanding of it.


-amit singh

ruwhite Tue, 01/23/2007 - 10:05
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EIGRP is a distance vector protocol, it's not a hybrid.... If you know of a test or cisco doc's that say it's a hybrid, please, please email me with a link or information about where to find it.


:-)


Russ

kofflerg Thu, 01/25/2007 - 11:27
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"Cisco Systems's EIGRP is one of the most feature-rich and robust routing protocols to ever be developed. Its unique combination of features blends the best attributes of distance vector protocols with the best attributes of link-state protocols. The result is a hybrid routing protocol that defies easy categorization with conventional protocols."


from http://www.cisco.com/en/US/tech/tk1330/tsd_technology_support_technical_reference_chapter09186a00807598ad.html#wp1020605


George

ruwhite Fri, 01/26/2007 - 05:45
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Thanks for the reference.... I'll get that one fixed.


:-)


Russ

albert.remo Wed, 01/24/2007 - 21:31
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Hi!


Good Day! Here is a brief summary of EIGRP based on what I've read on Sybex CCNA reviewer.


Description

classless protocol that has characteristics of both distance-vector and link-state protocols (hyrbrid routing protocol)

uses the concept of autonomous system to share routing information

includes subnet mask in its route updates thus allow the use of VLSM

sends information about network plus the cost of reaching them from the perspective of the advertising router

synchronizes routing tables between neighbors at startup then sends updates only when topology changes occur


Advantages

Support for IP, IPX, and AppleTalk via protocol-dependent modules


Each EIGRP PDM will maintain a separate series of tables containing the routing information that applies to a specific protocol


Considered classless (same as RIPv2 and OSPF)

Support for VLSM/CIDR

Support for summaries and discontiguous networks

Efficient neighbor discovery

Communication via Reliable Transport Protocol (RTP)

Best path selection via Diffusing Update Algorithm (DUAL)

Uses a series of tables to store information about its environment

>Neighborship table - records information about routers with whom neighborship relationships have been formed

>Topology table - stores the route advertisements about every route in the internetwork received from each neighbor

>Route table - stores the routes that are currently used to make routing decisions

can provide unequal cost load balancing of up to four links by default


Conditions for Neighbor Discovery

Hello or ACK received

AS numbers match

Identical metrics (K values)


Feasible distance - best metric along all paths to a remote network. Found in the routing table.

Reported distance - metric of a remote network reported by a neighbor

Neighbor table - state information about adjacent neighbors stored in RAM. Organized by sequence numbers forwarded by neighbors

Topology table - contains all destinations advertised by neighboring routers

Feasible successor - is a path whose reported distance is less than the feasible distance, and is considered a backup route. Up to 6 feasible successors in the topology but the one with the best metric is placed in the routing table.

Successor - is the best route to a remote network seen on topology table and routing table


Reliable Transport Protocol (RTP)

A proprietary protocol used by EIGRP to manage the communication of messages between routers

When EIGRP sends multicast traffic, it uses the Class D address 224.0.0.10

reliable multicast - eigrp process to learn neighbors using multicast. If a neighbor does not respond, it will send a unicast to resend the same data. After 16 unicast attempts, the neighbor is declared dead


Diffusing Update Algorithm (DUAL)

usedby EIGRP for selecting and maintaining the best path to each remote network. This algorithm allows for the following

* Backup route determination if one is available

* Support of Variable-Length Subnet Masks

* Dynamic route recoveries

* Queries for an alternate route if no route can be found


EIGRP Metrics - any combination of four

Bandwidth

Delay

Load

Reliability

Maximum Transmission Unit (MTU) size - not used but required especially those involving redistribution


Hope this helps,

Albert

ruwhite Fri, 01/26/2007 - 05:50
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"classless protocol that has characteristics of both distance-vector and link-state protocols (hyrbrid routing protocol)"


I think the reason people believe this is because EIGRP establishes neighbor adjacencies--but what makes a DV protocol DV is that you only send what you're using, rather than flooding everything.


You could, in theory, make a version of RIP that uses neighbor adjacencies, reliable transport, and only sends updates when needed, rather than periodically, but it would still use Bellman-Ford, and hence, it would still be DV.


:-)


Russ

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