EIGRP Default Routing

Lessons Learned:

EIGRP Default Routing

EIGRP supports default routing in two ways

Candidate default network

--IP default-network (network)

Native advertisement of a 0.0.0.0/0 prefix

Default-information command in EIGRP does not behave the same as other protocols

The default information command in EIGRP simply controls whether the route will accept a default route to whether it can continue to propagate a default route on….

Example:

Under the EIGRP process if you specify “no default-information”, it means the router will not accept an advertisement of a default route.

If we say “no default-information out”  it means that if the router learns a default route from someone else, you will not continue to propagate it on.

IP Default-Network

Originally came from IGRP

Candidate default network is backwards compatible with IGRP

-IGRP does not support the advertisement of 0.0.0.0/0

Default network must be…

-Dynamically learned through EIGRP -Not directly connected

-Classful Network

Limited application due to these restrictions.

Topology:

We can send a default route in via redistribution

EX: on the  BB1 router – let’s been RIP with it and the Edge rtr.

BB1 Router:

router rip

 version 2

 network 11.0.0.0

 network 20.0.0.0

 network 30.0.0.0

 network 192.168.1.0

 no auto-summary

EDGE RTR

router eigrp 500

 network 192.168.1.0

 network 192.168.2.0

 no auto-summary

!

router rip

 version 2

 network 192.168.1.0

 no auto-summary

!

Now on the edge router we would need to do mutual redistribution:

EDGE-RTR(config)#router rip

EDGE-RTR(config-router)#redistribute eigrp 1 metric 1

EDGE-RTR(config-router)#exit

EDGE-RTR(config)#router eigrp 500

EDGE-RTR(config-router)#redistribute rip

EDGE-RTR(config-router)#redistribute rip metric 1 1 1 1 1

Now if we sh ip route on the edge router we have the learned RIP routes.

EDGE-RTR#sh ip route rip

     20.0.0.0/24 is subnetted, 1 subnets

R       20.20.20.0 [120/1] via 192.168.1.10, 00:00:23, FastEthernet0/0

     11.0.0.0/24 is subnetted, 1 subnets

R       11.11.11.0 [120/1] via 192.168.1.10, 00:00:23, FastEthernet0/0

     30.0.0.0/24 is subnetted, 1 subnets

R       30.30.30.0 [120/1] via 192.168.1.10, 00:00:23, FastEthernet0/0

EDGE-RTR#

I can now Tag one of these route as the default route

 (config)#ip default-network 30.30.30.0

This will have the same effect as a default route to the done stream neighbors.

This is the same as advertising a Zero route in EIGRP

So in the BB1 router :

(config)#ip default-network 30.30.30.0

Gateway of last resort is 192.168.1.10 to network 0.0.0.0

     20.0.0.0/24 is subnetted, 1 subnets

R       20.20.20.0 [120/1] via 192.168.1.10, 00:00:21, FastEthernet0/0

     10.0.0.0/24 is subnetted, 2 subnets

D       10.2.1.0 [90/158720] via 192.168.2.3, 00:11:54, FastEthernet0/1

D       10.1.1.0 [90/158720] via 192.168.2.3, 00:11:54, FastEthernet0/1

     11.0.0.0/24 is subnetted, 1 subnets

R       11.11.11.0 [120/1] via 192.168.1.10, 00:00:21, FastEthernet0/0

C    192.168.1.0/24 is directly connected, FastEthernet0/0

C    192.168.2.0/24 is directly connected, FastEthernet0/1

D    192.168.3.0/24 [90/30720] via 192.168.2.3, 00:11:56, FastEthernet0/1

                    [90/30720] via 192.168.2.2, 00:11:56, FastEthernet0/1

     30.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

R       30.30.30.0/24 [120/1] via 192.168.1.10, 00:00:23, FastEthernet0/0

S       30.0.0.0/8 [1/0] via 30.30.30.0

R*   0.0.0.0/0 [120/1] via 192.168.1.10, 00:00:14, FastEthernet0/0

EDGE-RTR#

This will propagate to all downstream routers, with each router setting its next Hop as the default until it reached the BB1 router.

==============================================

Native Default Advertisement

Native 0.0.0.0/0 network can be advertised via..

Static default route to an interface + network 0.0.0.0 under EIGRP process.

Redistribution from static or another protocol

Summarization

All routers are running EIGRP process 500

All interface are participating in EIGRP

BB1’s interfaces:

BB1#sh ip int brief

FastEthernet0/0            192.168.1.10   

Loopback1                  11.11.11.1     

Loopback2                  20.20.20.1     

Loopback3                  30.30.30.1     

Default route on BB2:

Gateway of last resort is not set

First:

On the EDGE Router let’s configure a static default route and point it to interface fa0/0 towards network 30.30.30.0

Then under the EIGRP process we just need configure need to say redistribute static

Result on BB2:

Gateway of last resort is 192.168.3.3 to network 0.0.0.0

D*EX 0.0.0.0/0 [170/33280] via 192.168.3.3, 00:00:05, FastEthernet0/0

BB2#

The only problem is now the route shows up as and External route.

OR

We can set the default route to a next hop value rather just an interface.

Remove the static: ip route 0.0.0.0 0.0.0.0 FastEthernet0/0 30.30.30.1

Replace with: ip route 0.0.0.0 0.0.0.0 FastEthernet0/0  - Just the interface.

Also remove the redistributed route under the process and then advertise the network 0.0.0.0 into EIGRP.

This should advertise a default network to all downstream routers out the FA0/0 interface of the Edge RTR1.

Ex:

router eigrp 500

  network 0.0.0.0

 no auto-summary

Now because we’re not redistributing the route still shows as the candidate default but it’s now an internal route.

Gateway of last resort is 192.168.3.3 to network 0.0.0.0

D*   0.0.0.0/0 [90/33280] via 192.168.3.3, 00:01:15, FastEthernet0/0

BB2#

The only time this will really matter is if you have multiple default routes. One that is an internal route and one that is an external route.  you would always prefer the internal route based on the AD.