Thursday, October 10, 2013

EIGRP Is Distance Vector or Hybrid Routing Protocol ? - Let's get rid of an old myth








It’s been a while since I am asking this question in interviews to Routing & Switching candidates that comes from different background like Enterprise and Service Provider and certification level like CCNP, CCIP, CCIE etc.

Yet again I asked same question to couple of people I was interviewing today that had same thoughts as well of EIGRP being Hybrid.
Okay let’s forget the word “Interview” :) and focus on flip side which is that many of our own customers runs EIGRP as IGP in their networks and probably they may ask you similar questions or might have same thought process

Usually when I ask this question the answers comes as “Hybrid”. And of course then I ask people how is it “Hybrid ?”  and people start explaining that It has some features of Distance Vector Protocol and some it borrow from Link state.

And then I ask them again – What are those features that you think it borrows from Link state ?

And usually people replies back as – Hey , It has Topology Table, Routing updates are triggered , neighbor discovery and bla bla bla which comes from Link State background

Now of course one must first understand the differences in terms of EIGRP topology table Vs OSPF Topology Table per say to begin with.

EIGRP has no clue what’s there beyond its connected neighbor. Where in OSPF every router knows about entire topology.

Similarly EIGRP updates don’t go beyond immediate neighbor whereas in Link State (OSPF) every router in area gets the same copy of LSAs and pass them to neighbors without any modification.

Each router in ospf within same area has same LSDB and runs independent SPF calculation whereas EIGRP router just passes the best routes to its neighbor which that neighbor further stores into Topology table and run DUAL to figure out best path.

So perhaps we can call EIGRP as advance distance vector but definitely not Hybrid. And one reason that myth might have become popular because of is many Cisco Press Routing focused books using “Hybrid” terminology for EIGRP.

HTH...
Deepak Arora
Evil CCIE

6 comments:

Michael said...

Agreed. Also forming adjacency is not necessarily a quality of a link-state protocol.
And while we're at it... let's just say that BGP is not a "path-vector" but simply a distance-vector one too.

Michael said...

Agreed. Also forming adjacency is not necessarily a quality of a link-state protocol.
And while we're at it... let's just say that BGP is not a "path-vector" but simply a distance-vector one too.

Waldemar Pera said...

According to you. For years I teach Cisco courses, and always prefer to say that EIGRP is Advanced Distance Vector and isn't a Hybrid protocol. It is more accurate.

Waldemar Pera said...

According to you. For years I teach Cisco courses, and always prefer to say that EIGRP is Advanced Distance Vector and isn't a Hybrid. It is more accurate.

Jadouking said...

Eigrp is more distance vector than a distance vector routing protocol ;) ;)
Eigrp had to implement a not so well taught query/reply mecasim to avoid periodical full routes updates. Yes, this has advantages but brings also issues such as SIA and huge quer/reply propagation domain in big scale implementations.
It is the fastest IGP though. And unequal load sharing is cool.
Composite metric is a joke however.

Jad.

Deepak Arora said...

@ Pera

From chapter 6 (Understanding EIGRP) of Troubleshooting IP Routing Protocols I read this:

“EIGRP is neither a classic distance vector routing protocol nor a link-state protocol—it is a hybrid of these two classes of routing protocol. Like a distance vector protocol, EIGRP gets its update from its neighbors. Like a link-state protocol, it keeps a topology table of the advertised routes and uses the Diffusing Update Algorithm (DUAL) to select a loop-free path.”