Default routing is very important feature and can be find in each network as last resort mechanism to route packets out of organization to unknow destination. Default origination has few configuration dependency on routing protocol and these will be presented in this post.

OSPF

Let’s start from the most popular IGP protocol. In OSPF default prefix (0/0) can be propagated in two different ways:

Explicitly with default-information originate
Stub Area Border Router (ABR)

To originated 0/0 explicitly we have to issue following command under OSPF process:

R1(config-router)#default-information originate

Once above command has been issued OSPF router will act as Autonomous System Boundary Router (ASBR). Default prefix will not appear in ASBR’s LS database and will not be originated to peers until 0/0 prefix exist in routing table.

To get default network in the routing table we have two options:

Redistribute 0/0 from the another routing protocol (RIP, EIGRP, BGP)
Add static route for 0/0

Default-information originate command has optional keyword – “always” which means originate 0/0 even if no default prefix in routing table exist.

By default network will be propagated as E2 type with metric 1, of course it can be adjusted using metric or metric-type command option.

The second way to originate default is to configure stub area, then ABR will generate 0/0. Please look at OSPF Area Types and LSA Propagation post for details here. Keep in mind that ABR router does not originated 0/0 to standard Not-So-Stubby (NSSA) area, default-information originate or no-summary keyword is needed then.

EIGRP

With EIGRP protocol we have 4 options to generate default route, via:

    network 0.0.0.0
    redistribution
    summarization
    ip default-network

First option is similar to OSPF. Default route needs to exist in routing table and then will be propagated once network 0.0.0.0 command is added under EIGRP process.

R1(config)#router eigrp 1
R1(config-router)# network 0.0.0.0
R1(config-router)#ip route 0.0.0.0 0.0.0.0 null 0

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/409600] via 10.0.12.1, 00:06:32, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
D* 0.0.0.0/0 [90/281600] via 10.0.12.1, 00:05:51, FastEthernet0/0

R2 sees default route as EIGRP internal (AD=90) route with star. Star means default – last resort route will be used if no specific route exist to the specific destination.

Second option is to use redistribute command and take default based on static route or from another routing protocol.

R2#show ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
D*EX 0.0.0.0/0 [170/281600] via 10.0.12.1, 00:00:07, FastEthernet0/0

In this case peers will see default as EIGRP external (AD=170) route with star.

Third option of default route generation is based on the summarization. In EIGRP routes’ summarization is done per interface. It’s very handy option and can be find just in EIGRP.

R1(config)#int fa0/0
R1(config-if)#ip summary-address eigrp 1 0.0.0.0 0.0.0.0

Peers will see default route as EIGRP internal (AD=90).

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
D* 0.0.0.0/0 [90/307200] via 10.0.12.1, 00:00:15, FastEthernet0/0

The last option is using ip default-network command in global configuration mode; additionally prefix needs to be added under EIGRP process. Prefix needs to be classfull network. Of course local interface on router needs to exist and be in up state.

R1(config-if)#int lo1
R1(config-if)#ip add 1.0.0.1 255.0.0.0
R1(config-if)#router eigrp 1
R1(config-router)#network 1.0.0.0

R2#sh ip route
Gateway of last resort is not set
D* 1.0.0.0/8 [90/156160] via 10.0.12.1, 00:00:02, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0

R2 sees 1.0.0.0 subnet as candidate default route and 10.0.12.1 peer will be used as default gateway.

RIP

With RIP protocol we have 4 options to generate default route, via:

network 0.0.0.0
default-information originate
redistribution
ip default-network

First option propagates default route without need to exist in routing table.

R1(config)#router rip
R1(config-router)#no auto
R1(config-router)#network 0.0.0.0

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
R* 0.0.0.0/0 [120/1] via 10.0.12.1, 00:00:02, FastEthernet0/0

Second option is propagates default route the same like default-information originate always in OSPF – prefix does not need to exist in routing table.

R1(config)#router rip

R1(config-router)#version 2
R1(config-router)#no auto
R1(config-router)#network 10.0.0.0
R1(config-router)#default-information originate

R2#sh ip route>
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
R* 0.0.0.0/0 [120/1] via 10.0.12.1, 00:00:02, FastEthernet0/0

Third option is simply redistribution.

R1(config)#ip route 0.0.0.0 0.0.0.0 Null0
R1(config)#router rip
R1(config-router)# redistribute static metric 5

The last option is similar to ip default-network in EIGRP but interesting thing – does not need add classfull network under RIP configuration process.

R1(config)#int lo1
R1(config-if)# ip add 1.0.0.1 255.0.0.0
R1(config-if)# ip default-network 1.0.0.0

The output of show ip route command is also different – instead of classful network with star showing pure 0.0.0.0/0

R2#*Mar 10 23:39:27.912: RIP-DB: redist 0.0.0.0/0(metric 1, last interface FastEthernet0/0) to RIP
*Mar 10 23:39:27.912: RIP-DB: network_update with 0.0.0.0/0 succeeds
*Mar 10 23:39:27.912: RIP-DB: adding 0.0.0.0/0 (metric 1) via 10.0.12.1 on FastEthernet0/0 to RIP database
*Mar 10 23:39:27.912: RIP-DB: add 0.0.0.0/0 (metric 1) via 10.0.12.1 on FastEthernet0/0
*Mar 10 23:39:27.916: RIP-DB: Adding new rndb entry 0.0.0.0/0
*Mar 10 23:39:27.916: RIP-DB: Created rip ndb summary entry for 0.0.0.0/0
*Mar 10 23:39:27.916: RIP-DB: Adding new rndb entry 0.0.0.0/0
*Mar 10 23:39:31.113: RIP-DB: network_update with 0.0.0.0/0 succeeds
*Mar 10 23:39:31.113: RIP-DB: adding 0.0.0.0/0 (metric 1) via 10.0.12.1 on FastEthernet0/0 to RIP database

BGP

We have covered all IGP protocols. Let’s take a closer look at BGP.

With BGP protocol we have 3 options to generate default route, via:

    default-information originate
    network 0.0.0.0
    default-originate to specific neighbor

First option is similar to OSPF and EIGRP but with one difference. Besides 0/0 needs to exist in routing table additionally has to be redistributed to BGP routing from static or any other dynamic routing protocol. Just one important note – 0/0 prefix is not visible in BGP table until default-information originate command will be issued, strange but true. Let’s test it.

R1(config)#ip route 0.0.0.0 0.0.0.0 Null0
R1(config)#ip route 2.2.2.2 255.255.255.255 Null0
R1(config)#router bgp 1
R1(config-router)# redistribute static
R1(config-router)#exit
R1#sh ip route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
2.0.0.0/32 is subnetted, 1 subnets
S 2.2.2.2 is directly connected, Null0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 is directly connected, Null0

R1#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 2.2.2.2/32 0.0.0.0 0 32768 ?

As you can see no 0/0 prefix in BGP table, let’s add key command.

R1(config)#router bgp 1

R1(config-router)#default-information originate

R1(config-router)#do sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 0.0.0.0 0 32768 ?
*> 2.2.2.2/32 0.0.0.0 0 32768 ?

Here we are! Confirmed that R2 is getting route.

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
2.0.0.0/32 is subnetted, 1 subnets
B 2.2.2.2 [200/0] via 10.0.12.1, 00:00:19
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
B* 0.0.0.0/0 [200/0] via 10.0.12.1, 00:00:05

Second option, use of network 0.0.0.0 under BGP requires 0/0 prefix in routing table too – the same like with first one but network command assure existence default network in the BGP table and propagation to all neighbors, so no need to redistribute into BGP table.

R1(config)#router bgp 1
R1(config-router)#network 0.0.0.0
R1(config-router)#ip route 0.0.0.0 0.0.0.0 null 0

R2#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*>i0.0.0.0 10.0.12.1 0 100 0 i
R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
B* 0.0.0.0/0 [200/0] via 10.0.12.1, 00:01:08

Third option is usfull and allows to select to which neighbors to send 0/0 prefix without need of filtering. This option does not need to have 0/0 in routing table to originate default.

R1(config)#router bgp 1
R1(config-router)# neighbor 10.0.12.2 default-originate
R2#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*>i0.0.0.0 10.0.12.1 0 100 0 i

As you see there is some dependency in default route generation. It’s good to know it.

NetContractor

Default routing orignation in OSPF, EIGRP, RIP and BGP

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