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Which Default Route will be preferred in Eigrp

Posted on 2014-01-17
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Last Modified: 2014-02-02
Hi,

I am running EIGRP between two routers R1 and R2. R1 is the one which is connected to other networks and Internet. Instead of passing all those networks to R2, I would like to advertise a default network to R2 so that it can reach all those networks and internet connected to R1.

I have created a default route on R1 and I am redistributing it in Eigrp process. Also, i have configured ip summary-address eigrp 10 0.0.0.0 0.0.0.0 5 on the interface that is connecting to R2.

As per the above scenario which of the below default route should show in R2 and why?
D* 0.0.0.0 0.0.0.0 [5/x] via x.x.x.x
or
D*EX 0.0.0.0 0.0.0.0 [170/x] via x.x.x.x

I hope I have made myself clear. Kindly let me know if any more information is required.

Thanks in advance.
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Question by:Vipin Kumar
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Expert Comment

by:Don Johnston
ID: 39788273
As per the above scenario which of the below default route should show in R2 and why?
D* 0.0.0.0 0.0.0.0 [5/x] via x.x.x.x
or
D*EX 0.0.0.0 0.0.0.0 [170/x] via x.x.x.x

The first one.  Since it has a lower administrative distance of 5.
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Expert Comment

by:Miftaul
ID: 39788335
When there are multiple routes, the route with lowest AD is installed in the routing table.

I agree with donjohnston "D* 0.0.0.0 0.0.0.0 [5/x] via x.x.x.x"  is in the r2 routing table due to the lowest AD of 5.
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Author Comment

by:Vipin Kumar
ID: 39788426
I agree with you, but when I check the Routing Table it is showing D*EX 0.0.0.0 0.0.0.0 [170/x] via x.x.x.x

I dont understand the reason behind it.

Can you help me understand why is it ending up in the routing table?
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Expert Comment

by:Don Johnston
ID: 39788481
Post the output of a "show ip eigrp topology".
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Author Comment

by:Vipin Kumar
ID: 39790375
The output of show ip eigrp topology and show ip route for R2 is mentioned below:

R2#sh ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(10.212.155.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 0.0.0.0/0, 2 successors, FD is 258816
        via 10.212.140.21 (258816/28160), GigabitEthernet3/1
        via 10.212.140.25 (258816/28160), GigabitEthernet4/1
P 10.212.254.15/32, 1 successors, FD is 2816
        via Rstatic (2816/0)
P 10.212.154.0/25, 1 successors, FD is 2816
        via Connected, Vlan302
P 10.212.155.0/24, 1 successors, FD is 2816
        via Connected, Vlan401
P 10.212.152.0/30, 1 successors, FD is 2816
        via Connected, GigabitEthernet3/2
P 10.212.140.20/30, 1 successors, FD is 256256
        via Connected, GigabitEthernet3/1
P 10.212.153.0/24, 1 successors, FD is 2816
        via Connected, Vlan301
P 10.212.140.24/30, 1 successors, FD is 256256
        via Connected, GigabitEthernet4/1
P 10.212.152.8/29, 1 successors, FD is 2816
        via Connected, Vlan101
P 10.212.152.16/28, 1 successors, FD is 2816
        via Connected, Vlan102
P 10.212.152.32/28, 1 successors, FD is 2816
        via Connected, Vlan201
P 10.212.154.128/25, 1 successors, FD is 2816
        via Connected, Vlan104


R2#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 10.212.140.25 to network 0.0.0.0

     10.0.0.0/8 is variably subnetted, 11 subnets, 6 masks
S       10.212.254.15/32 [1/0] via 10.212.152.1
C       10.212.154.0/25 is directly connected, Vlan302
C       10.212.155.0/24 is directly connected, Vlan401
C       10.212.152.0/30 is directly connected, GigabitEthernet3/2
C       10.212.140.20/30 is directly connected, GigabitEthernet3/1
C       10.212.153.0/24 is directly connected, Vlan301
C       10.212.140.24/30 is directly connected, GigabitEthernet4/1
C       10.212.152.8/29 is directly connected, Vlan101
C       10.212.152.16/28 is directly connected, Vlan102
C       10.212.152.32/28 is directly connected, Vlan201
C       10.212.154.128/25 is directly connected, Vlan104
D*EX 0.0.0.0/0 [170/258816] via 10.212.140.25, 1w4d, GigabitEthernet4/1
               [170/258816] via 10.212.140.21, 1w4d, GigabitEthernet3/1
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Expert Comment

by:Don Johnston
ID: 39790563
The default route doesn't show in the topology table.

Need to see the config of R1
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Expert Comment

by:Miftaul
ID: 39790569
This topology table is from router2, we wont see default route here. We need to see the topology table and the config of R1.
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Expert Comment

by:Don Johnston
ID: 39790593
This topology table is from router2, we wont see default route here.
Actually, the default route is in the topology table. Just not the one we're looking for.
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Expert Comment

by:Miftaul
ID: 39790604
I meant to see something like below which isn't there
P 0.0.0.0/0, 1 successors, FD is 28160
        via Rstatic (28160/0)
        via Summary (28160/0), Null0

Open in new window

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Miftaul earned 500 total points
ID: 39790631
Your routing table shows like below, where we want to see the summary route with D* and not the redistributed route with D*EX. You current routing table in R2 is something like below
R2#sh ip route

Gateway of last resort is 10.10.20.1 to network 0.0.0.0

     3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
     4.0.0.0/32 is subnetted, 1 subnets
D       4.4.4.4 [90/156160] via 10.10.30.2, 00:09:40, FastEthernet1/0
     10.0.0.0/24 is subnetted, 2 subnets
C       10.10.20.0 is directly connected, FastEthernet0/0
C       10.10.30.0 is directly connected, FastEthernet1/0
D*EX 0.0.0.0/0 [170/30720] via 10.10.20.1, 00:00:14, FastEthernet0/0

Open in new window

Where we want to see the summary route ,
D*   0.0.0.0/0 [90/30720] via 10.10.20.1, 00:00:07, FastEthernet0/0

Open in new window

It’s not happening because Default route has lower AD than summary route and in R1, default route is first installed in the routing table than the summary route and gets redistributed. That’s why we see the redistributed route in R2 routing table. The "Show IP Eigrp Topology" looks like below for the default route on R1
P 0.0.0.0/0, 1 successors, FD is 28160
        via Rstatic (28160/0)
        via Summary (28160/0), Null0

Open in new window

If we want to see the summary route in the R2 routing table like D*, we need to manipulate the default route on the R1 with a higher AD than the summary route. Say for example "IP Route 0.0.0.0 0.0.0.0  x.x.x.x 250".
Now the summary route has lower Administrative Distance than the default route and gets injected to the routing table. The "Show IP Eigrp Topology" looks like below for the default route on R1
P 0.0.0.0/0, 1 successors, FD is 28160
        via Summary (28160/0), Null0

Open in new window

And now the routing table on R2 has the
Gateway of last resort is 10.10.20.1 to network 0.0.0.0
     3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
     4.0.0.0/32 is subnetted, 1 subnets
D       4.4.4.4 [90/156160] via 10.10.30.2, 00:10:48, FastEthernet1/0
     10.0.0.0/24 is subnetted, 2 subnets
C       10.10.20.0 is directly connected, FastEthernet0/0
C       10.10.30.0 is directly connected, FastEthernet1/0
D*   0.0.0.0/0 [90/30720] via 10.10.20.1, 00:00:07, FastEthernet0/0

Open in new window

We have to increase the Administrative distance for the default route in R1 to something higher than the administrative distance of summary route.
Router seems to advertise summary address before looking into the routing table and doing redistribution.
or remove the "redistribute static" from Eigrp process "R1(config-router)#no redistribute static". This removes the redistribution in R1 and summary route gets installed in R2.
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