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Beginner

EIGRP asymmetric Routing loops

Dear All,

Below are the scenario explanations:

# I have Four locations and each location have two cisco routers and connected using point to point leased line and have HSRP running on them.

# All the routers are learning routes using single EIGRP dynamic routing protocol.

# I have redistributed all the static routes into EIGRP.

# In this network all the router have full network topology database.

 

So here are some problems what i have observed:

1. I want traffic should pass between two directly connected site via connected point to point links(Primary or secondary link) not using any other links/router however when i checked routing table of DC and DR they are not using/preferring their directly connected link but coming via site 1 or site 2.

2. Other strange issue is if i am tracing 192.168.2.10 from site 1 with Host A then it is going via directly connected link and application is working fine but when i trace same 192.168.2.10 from Host B in same segment as of Host A it is going via site 2 and application is not working. As per routing table it should go via directly connected network.

 

I have an idea in my mind but do not know how to execute it. Please let me know if below mention thought can be executable or not.

** As i said earlier all the router have static routes pointing towards their LAN switch and I am redistributing these routes into Eigrp then EIGRP transferring all the routing database to directly connected network. So here what i want is DC should send its static routes to DR, Site 1 and Site 2 but DR, Site 1 or Site 2 should not tell any other routers about that routing. This should happen for all the router if they send their own routing instead of learned routing table then my problem can be solved.

 

Please refer attached network diagram any helpful input would be really appreciable.

 

 

Scenario.JPG

7 REPLIES 7
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VIP Mentor

Hello,

 

it is difficult to give any meaningful advice without seeing the configs. Can you post those (of all 8 routers)?

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Hall of Fame Master

Hello @kai.ny111 ,

in order to create in EIGRP primary paths and secondary paths the best way is to manipulate the delay of interfaces on secondary links.

In EIGRP metric delay is cumulative so if you do something like

 

interface type x/y

desc secondary path

delay 10000

 

on all secondary links both directions on all 8 routers,  the EIGRP behaviuor should change and should start to be similar to the desired behaviour.

 

If this is not enough to solve your issues you should post your configurations as already suggested.

 

Hope to help

Giuseppe

 

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Hi Giuseppe

 

As you can see from diagram primary router have 2 or 3 active links connected on them. Even if traffic from LAN coming at primary router it have 2 or 3 path to reach the destination out of these paths 1 is directly connected and other 2 is going via other sites. So I don't think after setting delay  on secondary link it will work.

I want all the destination should reach via  directly connected routes only.

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Thank you for taking out your valuable time and spending it on my issue. Pls check the attached configuration file.

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Hello @kai.ny111 ,

you have tried to manipulate the bandwidth parameter on some links on routers in DC and DR.

 

However, the following notes apply:

a) in EIGRP composite metric the bandwidth component is inversely proportional to the lowest bandwidth link on the path

As a result of this changes in bandwidth have to be carefully performed.

b) it is appropriate to change the bandwidth to make it reflect the true speed of a WAN link for example 90 Mbps, but this should be done using the same bandwidth value on both ends of the same link. For example DC-Router1 has modified bandwidth values towards R1-site1 and towards R1-site2 but those routers do not have a corresponding bandwidth statement set at  the other end of each link.

In my opionion this is the root cause of the asymmetric routing you see in your network depending on direction/versus links with modified reduced bandwidth are involved or not making EIGRP to take surprising decisions.

 

So as a first step I would suggest to configure appropriate bandwidth values (the same value) at both ends of each WAN link.

Later, to implement primary and secondaty paths you can use an increased delay on secondary paths again in a symmetric way ( same increased delay value on both end of each secondary link)

 

Hope to help

Giuseppe

 

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@Giuseppe Larosa 

 

I'll work on bandwidth modification suggestion.

 

As i earlier described my all the routers have HSRP running on them and so from LAN all the traffic only comes to primary router where 2-3 links with different link speed is connected with other sites. So if i add some delay on DC router R1 then all the traffic will pass via other link which i don't want.

 

Is it possible to stop eigrp from exchanging complete topology table apart from their own routing?

 

scenario with speed.JPG

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Hello @kai.ny111 ,

>> So if i add some delay on DC router R1 then all the traffic will pass via other link which i don't want.

I agree my suggestion is to add delay on links of secondary routers like DC R2 and so on in a symmetric way.

 

>> I'll work on bandwidth modification suggestion.

This should improve the EIGRP routing behaviour.

 

>> Is it possible to stop eigrp from exchanging complete topology table apart from their own routing?

yes this is possibile with EIGRP stub routing , but EIGRP stub routing is recommended for spoke sites and would require appropriate configuration of leak maps to cover all possible fault cases.

In your network scenario routers in site1 and in site2 could be configured as EIGRP stub router with redistribution of static and with appropriate leak route maps to allow to accomodate for faults on primary link if HSRP has not transitioned.

router eigrp 80

eigrp stub static

 

Don't test this in a a production network.

Making routers in site1 and site2 EIGRP Stub would avoid passing on the other site to reach a DC prefix.

 

Hope to help

Giuseppe