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How Eigrp works

How Eigrp works??

What does mean no auto-summary and passive interface command??

Anyone explain me briefly to understand.

And also explain me how OSPF works??


How Eigrp works


to understand EIGRP I would look at Cisco Doc and for OSPF also at Cisco Doc and at the RFC.

But basically no autosummary means do not automatically summarize the subnet you're going to advertize when crossing a major network boundary, this is now default on latest IOS versions and passive interface means do not establish a neighbour relationship on this link but advertise the subnet anyway.



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How Eigrp works

Hi Dinesh,

Let me give a small scenario to know how that eigrp no-auto summary will work.

Imagine a campus network. the entire network functions within the space internally. you're going to use vlsm to break the network up into different subnets as needed (/30's or /31's for point-to-point links, /23 or /24 for user access, etc.).

routing protocols automatically summarize the networks they advertise (i.e. your router wouldn't care if you had,, and in your routing table, it would advertise one route -- when every router is trying to do this -- they all eventually begin to get confused because they all have routes to the same destination ( but no "more specific" prefix information is carried

And you would use no auto-summary when you require carrying traffic that originates from a subnetted subnet to keep its classless address across the neighbor router. It's usually not an issue when neighbor routers share the same subnetted address space on their interfaces.

Regarding passive interface command:

With EIGRP running on a network, the passive-interface command stops both outgoing and incoming routing updates, since the effect of the command causes the router to stop sending and receiving hello packets over an interface.

You can see the below link for how this works

Hope the above clear and understand you.
Please rate all the helpfull posts.

New Member

How Eigrp works

Thank u very much.

Can u explain me How eigrp works with diagram.

I interview they ask one question.

How eigrp is better than ospf

can u tell me.

How Eigrp works

Hi Dinesh,

See the  below link for EIGRP no auto summary.

Read the below link for EIGRP passive interface.

Hope the above clear and understand you.
Please rate all the helpfull posts.

How Eigrp works

Hello Dinesh,

I would like to add on the interview question - How EIGRP is better than OSPF.

1) My strategy to respond to that question would have been - It's basically a design practice followed by an organization. So, comparing EIGRP with OSPF or vice-versa may not be genuine. But still for the sake of discussion, i would feel below points which gives an edge to EIGRP over OSPF.

  • EIGRP is a Cisco-properietary protocol. So, the level of support we may get in terms of any issues pertaining to this protocol is more ,  Hypothetically   This has it's own disadvantage though, any other non-cisco device wouldn't support this. But if all cisco, then this statement would be fair.
  • EIGRP is a fastest converging protocol (capable to providing sub-second convergence). Well, OSPF as well is capable of providing a sub-second convergence, if tuned correctly).
  • Major edge of EIGRP over OSPF is EIGRP supports unequal cost loadbalancing & route summurization can be done wherever we want in the network.
  • Personally one more thing what i feel is - EIGRP is not a messy routing protocol, what that means is .. I need not worry about the hold-time / dead-interval time etc of it's neighbor. Neighborship forms just without much conditions.

If you look at the above, you wouldn't see much of an advantage apart from summarization & unequal cost load balancing. So, it's frankly unfair to question which is better as per me.



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