Let’s take the following scenario:
R3 is EBGP peering with R2 where there are 3 ways to advertise default route:
1.
Use neighbor x.x.x.x
default-originate
R3 configuration:
router bgp 65003
no
synchronization
bgp
log-neighbor-changes
network 192.168.30.0
neighbor 10.1.23.2 remote-as 65012
neighbor 10.1.23.2 default-originate
no
auto-summary
R3 will advertise default-route to R2 with
or without having default-route in his routing table.
R3:
R3#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
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 not set
C
192.168.30.0/24 is directly connected, Loopback1
10.0.0.0/24 is subnetted, 2 subnets
C
10.1.23.0 is directly connected, FastEthernet0/0
C
10.1.45.0 is directly connected, FastEthernet0/1
R2:
R2#show 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
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 10.1.23.3 to
network 0.0.0.0
B
192.168.30.0/24 [20/0] via 10.1.23.3, 02:51:33
10.0.0.0/24 is subnetted, 2 subnets
C
10.1.12.0 is directly connected, FastEthernet0/0
C
10.1.23.0 is directly connected, FastEthernet0/1
B*
0.0.0.0/0 [20/0] via 10.1.23.3, 00:15:19
2.
Use default-information originate and redistribute
static
R3
configuration:
router bgp 65003
no
synchronization
bgp
log-neighbor-changes
network 192.168.30.0
neighbor 10.1.23.2 remote-as 65012
default-information originate
redistribute static
no auto-summary
!
Ip route 0.0.0.0 0.0.0.0 null0
R3:
R3#show 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
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 0.0.0.0 to
network 0.0.0.0
C
192.168.30.0/24 is directly connected, Loopback1
10.0.0.0/24 is subnetted, 2 subnets
C
10.1.23.0 is directly connected, FastEthernet0/0
C
10.1.45.0 is directly connected, FastEthernet0/1
S*
0.0.0.0/0 is directly connected, Null0
R2:
R2#show 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
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 10.1.23.3 to
network 0.0.0.0
B
192.168.30.0/24 [20/0] via 10.1.23.3, 00:01:57
10.0.0.0/24 is subnetted, 2 subnets
C
10.1.12.0 is directly connected, FastEthernet0/0
C
10.1.23.0 is directly connected, FastEthernet0/1
B*
0.0.0.0/0 [20/0] via 10.1.23.3, 00:01:57
3.
Using network 0.0.0.0
command where R3 does have default-route in his routing table (from static or
any other dynamic routing protocol)
R3
configuration:
router bgp 65003
no
synchronization
bgp
log-neighbor-changes
network 192.168.30.0
neighbor 10.1.23.2 remote-as 65012
network 0.0.0.0
no auto-summary
R3:
R3#show ip ro
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
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 10.1.45.4 to
network 0.0.0.0
C
192.168.30.0/24 is directly connected, Loopback1
10.0.0.0/24 is subnetted, 2 subnets
C
10.1.23.0 is directly connected, FastEthernet0/0
C
10.1.45.0 is directly connected, FastEthernet0/1
O*E2 0.0.0.0/0 [110/1] via 10.1.45.4,
00:00:28, FastEthernet0/1
R2:
R2#show 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
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 10.1.23.3 to
network 0.0.0.0
B
192.168.30.0/24 [20/0] via 10.1.23.3, 00:00:32
10.0.0.0/24 is subnetted, 2 subnets
C
10.1.12.0 is directly connected, FastEthernet0/0
C
10.1.23.0 is directly connected, FastEthernet0/1
B*
0.0.0.0/0 [20/1] via 10.1.23.3, 00:00:32
To summarize this:
Command
|
Default-route in routing table
|
Source
|
Router bgp 65003
Neighbor x.x.x.x default-originate
|
No
|
N/A
|
Router bgp 65003
Default-information originate
Redistribute <protocol>
|
Yes
|
Static, OSPF, RIP, EIGRP
|
Router bgp 65003
Network 0.0.0.0
|
Yes
|
Static, OSPF, RIP, EIGRP, BGP
|
No special command
|
Yes
|
EBGP, IBGP
|
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