Does EIGRP need two-way communication ?

[henrybb]

On URL : http://www.cisco.com/en/US/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml#cg7

CISCO said:In many cases, EIGRP converges more quickly than OSPF. But it is important to note that two-way communication is not a requirement in order for neighbors to exchange routing information.

I am in doubt whether it is true.

I think that EIGRP must form an adjacency before exchange route information.

any comments is appreciated. thanks!

[jackyoung]

Check below for the description of EIGRP :

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml

What I believe the two-way communication means the route exchange in the routing protocol. In EIGRP, the router will notify the neighbor only if there is route update. So, it is a single way and the router will not check is there any update from its neighbor. There is only hello packet as keepalive.

Please feel free to comment.

Hope this helps.

[henrybb]

thanks for your reply.

Of course,it is a single way when there any update from its neighbor. BUT,I think it must form an adjacency via two-way communication before they can exchange routing information.

So why CISCO said:

If you assume the default timer values, OSPF sends hello messages every 10 seconds and has a dead interval of 40 seconds (4 * hello). These timers are consistent for OSPF point-to-point and broadcast networks. Because OSPF requires two-way communication in order to form an adjacency, the worse-case failover time is 40 seconds. This is true even if the Layer 1/Layer 2 failure is not pure on a point-to-point connection and leaves a half-baked scenario with which the Layer 3 protocol must deal. Because the detection time of UDLD is very similar to the detection time of an OSPF dead timer expiring (approximately 40 seconds), the advantages of the configuration of UDLD normal mode on an OSPF Layer 3 point-to-point link are limited.

In many cases, EIGRP converges more quickly than OSPF. But it is important to note that two-way communication is not a requirement in order for neighbors to exchange routing information. In very specific half-baked failure scenarios, EIGRP is vulnerable to the black holing of traffic that lasts until some other event brings the routes via that neighbor active. UDLD normal mode can alleviate these circumstances because it detects the unidirectional link failure and error disables the port.

I don't understand what's difference between EIGRP and OSPF. Why cisco said OSPF need two-way communication and EIGRP doesn't.

thanks!

[jackyoung]

Check below OSPF design guide :

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t20

According to my understaning. The adjacency (DR & BDR election) is required in OSPF but not in EIGRP. Where the DR & BDR will be the central point of contact of information exchange with other OSPF routers. So it is a two-way communiction. And EIGRP only require the hello to keepalive. Therefore, according to the article from your message, if there is a failure in OSPF, it may require to elect the DR that is longer time than EIGRP just send the updates to neighbor.

Thanks.

[henrybb]

thanks for your reply.

If network topo is like this:

router A-----linkab-------router B

All of them running EIGRP.

If linkab is uniDirectional , routerA receive hello from routerB and routerB doesn't receive hello from routerA. Do you mean that routerB will send routing information to routerA and routerA will accept it although routerB didn't receive hello from routerA?

thanks!

[henrybb]

You know,if interface of router A which connected router B is passived,neighbor relationship can't make between A and B.

So I don't think that EIGRP don't need two-way communication.

thanks!

[jackyoung]

You are correct that if you passive the interface and there will be no EIGRP hello and update sent fom this interface. So the neighor relationship cannot be made. It is not related to two-way communication. Whenever you passive the interface, any routing protocol will not advertise from it, it is true.

Thanks for devang's explaination, hope he ans. your questions.

[henrybb]

thanks!

So what's your mean when you said " two-way communication" ?

[devang_etcom]

in OSPF you need to have bidirectional connection in order to form the adj. means two ospf running router become full neighbour when they pass with the adj. steps it means they need to have bidirectional communication as you know if the HELLO on both the router not match then they will not become neighboure...

but in case of EIGRP its not the case... here EIGRP is fater in converge because it has SUCCESSOR and FS elected already so if one main path is lost then it can use FS... where as in case of OSPF it elect DR and BDR...yes OSPF have the topology table but when main or you can say best path to the destination will go down then OSPF calculate the SPF algorithm on topology table and find out the other best path so here SPF calculation will take place again so its time consuming in many case.... hope this will help you

rate this post if it helps

regards

Devang

[henrybb]

thanks!

[henrybb]

You said:

in OSPF you need to have bidirectional connection in order to form the adj. means two ospf running router become full neighbour when they pass with the adj. steps it means they need to have bidirectional communication as you know if the HELLO on both the router not match then they will not become neighboure...

I think that it is also correct if it is describing EIGRP. EIGRP also need "HELLO on both the router not match then they will not become neighboure"

I don't think " in case of EIGRP its not the case ". Can you give more detail why EIGRP not the case ?

thanks!

[jackyoung]

According to the EIGRP link that I provided, recap. as below :

"It is possible for two routers to become EIGRP neighbors even though the hello and hold timers do not match. The hold time is included in the hello packets so each neighbor should stay alive even though the hello interval and hold timers do not match."

It is not the same as OSPF. Where the hello is one-way that one router sent the hello out then other routers receive it and knew the sender is alive.

Could you please check both routing protocol doc. then raise out which section that you do not agree or clear then it is earier to explain.

Hope this helps.

[henrybb]

thanks!

I read docs and do a lab.

It is true that EIGRP is one-way communication.It can't become neighbor in passive because "passive-mode" in EIGRP is special.

reference doc:

How Does the Passive Interface Feature Work in EIGRP?

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0a.shtml

[jackyoung]

It is great that you clarified the question and tested it in lan. Thanks for sharing the info. too.

[mbufatihcisco]

Hi All,

As mentioned in the following link (also referred by henrybb) :

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0a.shtml

I have setup a configuration, tested and confirmed that EIGRP does not require two-way communication. What I understand from "two-way" is that one of the EIGRP peers sends Hello packets to form an adjacency and the other does not.

Please let me show you my setup and configuration:

[Router-A] (f0/0) --------------------------------------- (f0/0) [Router-B]

                       .5           (10.10.10.0)       .4

Router-A

router eigrp 7

network 10.10.10.0 0.0.0.255

no auto-summary

Router-B

router eigrp 7

network 10.10.10.0 0.0.0.255

distribute-list 20 out FastEthernet0/0

no auto-summary

!

!

access-list 20 deny   any

!

So, I have configured Router-B to receive EIGRP updates but not to sent any on interface Fa0/0.

The result of this is that Router-A and Router-B formed a successful adjacency from which I figured out that Hello messages exchange is not needed bidirectionally.

Besides, Router-B received the route updates from Router-A.

However, Router-A did not receive any route information from its 'neighbor' Router-B.

This was a really significant test for me to understand one of the major differences between OSPF and EIGRP when they are forming adjacencies.

Please add comments if I miss any point.

Thanks in advance!