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 ?
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 22.214.171.124 to send updates to other routers or vice-versa.
Please see the EIGRP design guide:
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 ?
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.
"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."
Good Day! Here is a brief summary of EIGRP based on what I've read on Sybex CCNA reviewer.
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
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 126.96.36.199
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
Maximum Transmission Unit (MTU) size - not used but required especially those involving redistribution
Hope this helps,
"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.