EIGRP and return traffic from destination to Source

[mahesh18]

Hi Everyone,

Please see the setup below

Source 172.31.35.x

Destination 172.31.40.x

Source--PC(172.31.35.x)-----Layer 2 switch -----Fw-----R1-----R2----R3----R4----Destination server(Destination 172.31.40.x)

Where R1 to R4 all are running EIGRP.

Fw has static routes only.

There is eigrp routing enabled from R1 to R4 .

For return traffic from destination to PC subnet(172.31.35.x)   R4 and R3 have no route to subnet 172.31.35.0.

R2 has route to Source subnet(172.31.35.x) via R1 but it is static route.

Also R2,R3 and R4 have no interface with IP 172.31.35.x.

i fixed this issue as below

i  add static route on R4  pointing to R3 as next hop and also add static route on R3 with next hop as R2.

Need to know if i had configured R4 with loopback interface IP say 172.31.35.x and advertise that into EIGRP will the return traffic from server to

PC would have worked?

Regards

MAhesh

[Reza Sharifi]

Hi Mahesh,

Assuming the FW is default gateway for subnet 172.31.35.x, the only thing the FW should have is a default route pointing to R1. Than R1 needs to redistribute the connected interface (this is the interface which is the default gateway for subnet 172.31.35.x) into EIGRP.  Now, if EIGRP is running between R1 and R4 and all the peering are correct than R4's route table should have 172.31.35.x subnet in it.  Is that the case?

Thanks,

Reza

[mahesh18]

Hi Reza,

Default gateway for subnet 172.31.35.x is R1.

As R1 has interface with IP 172.31.35.x and if all is good then R4 's routing table should have subnet 172.31.35.x.That was perfect setup.

But under current setup R4 was not learning route 172.31.35.x via EIGRP.

I was thinking if we config loopback  interface on R4 with subnet 172.31.35.x and advertise it into EIGRP then assume that R2 and R1 learns this route via EIGRP.

Now R1 and R2 have two routes to 172.31.35.x one static via fw and other via EIGRP from R1.

In this case R1 and R2 will still use the static route to reach subnet 172.31.35.x right as it has lower AD 1 as compare to D EX 170 right?

With this setup will the return traffic from R4 to subnet 172.31.35.x would have worked?

Regards

MAhesh

[Reza Sharifi]

Hi Maheah,

As R1 has interface with IP 172.31.35.x and if all is good then R4 's routing table should have subnet 172.31.35.x.That was perfect setup.

So, if the default gateway for subnet 172.31.35.x subnet is the R1, than that means the FW is just layer-2?

But under current setup R4 was not learning route 172.31.35.x via EIGRP.

If EIGRP is working properly and you have connected interface for subnet 172.31.35.x redistributed to EIGRP on R1 than R4 should lean that route.

I was thinking if we config loopback  interface on R4 with subnet 172.31.35.x and advertise it into EIGRP then assume that R2 and R1 learns this route via EIGRP.

If EIGRP working correctly, there is no need for this as that subnet should be learned by R4 dynamically through EIGRP.

Now R1 and R2 have two routes to 172.31.35.x one static via fw and other via EIGRP from R1.

In this case R1 and R2 will still use the static route to reach subnet 172.31.35.x right as it has lower AD 1 as compare to D EX 170 right?

If R1 and R2 are both running EIGRP there is no need for static routes.

Can you clarify above points.

Thanks,

[mahesh18]

Hi Reza,

I agree if EIGRP was working correctly then we do not need to advertise subnet 172.31.35 via R4.

For above 2 points by learning point of view just imagine if we config loopback interface on R4 with IP 172.31.35.x and advertise it into EIGRP then assume that R2 and R1 learns this route via EIGRP.

1>Now R1 and R2 have learned route to 172.31.35.x via R3 and R4.Also it has earlier configured static route  via fw to subnet 172.31.35.x.

2>Going with above config will the return traffic coming from R4 reach the PC subnet 172.31.35.x

Regards

MAhesh

But R1 will use static route to forward traffic to 172.31.35.x because of

it has lower AD 1 as compare to D EX 170 right?

2>

[Reza Sharifi · Accepted solution]

Hi Mahesh,

Advertising the same subnet form R1 as well as R4 will cause duplicate IP address/subnet problem.That would confuse the other routers on how to get to 172.31.35.x subnet.  I suggest working on fixing what ever issues you have with EIGRP not seeing routes from R1. It couple be a peering issue or the the subnet not being advertised/redistributed correctly.

HTH

[mahesh18]

Thanks Reza that was what i need to know.

Regards

Mahesh