Background:
In OSPF, there are many loop prevention mechanisms between ABR because the Vector Distance behavior OSPF in multi area design such as INTER-AREA LSA Type 3 loop prevention or P-Bit loop prevention.
Is there a loop prevention mechanism between ASBR?
Let's dive in
Basic configuration of all routers:
R1:
interface FastEthernet0/0
ip address 13.0.0.1 255.255.255.0
ip ospf 1 area 0
no shut
!
router ospf 1
router-id 1.1.1.1
default-information originate always
R2:
interface FastEthernet0/0
ip address 23.0.0.2 255.255.255.0
no shut
!
ip route 0.0.0.0 0.0.0.0 s0/0
!
router ospf 1
router-id 2.2.2.2
!
router eigrp 1
network 23.0.0.0
redistribute static
R3:
interface FastEthernet0/1
ip address 13.0.0.3 255.255.255.0
ip ospf 1 area 0
no shut
!
interface FastEthernet0/0
ip address 23.0.0.3 255.255.255.0
no shut
!
router ospf 1
router-id 3.3.3.3
To allow R3 to access INTERNET, R1 injects a default route using default-information originate always command and R2 injects a default route using redistribute static command.
We can see below that R3 receives an LSA Type 5 default route from R1 with Routing bit set:
R3#show ip ospf database external
OSPF Router with ID (3.3.3.3) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 1133
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000002
Checksum: 0x1B92
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 1
R3#
Now let's allow R1 to access the EIGRP domain by advertising a default route on R3 using the default-information originate always:
R3(config)#router osp 1
R3(config-router)#default-information originate always
We can see below that R3 is still receiving the LSA Type 5 from R1 but notice the missing of the routing bit in this LSA 5:
R3#show ip ospf database exter adv-router 1.1.1.1
OSPF Router with ID (3.3.3.3) (Process ID 1)
Type-5 AS External Link States
LS age: 2008
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000002
Checksum: 0x1B92
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 1
R3#
To explain the missing of the routing bit field in the LSA Type 5's R1 , let's enable debugging:
R3#debug ip ospf spf external
OSPF SPF external debugging is on
R3#
From the debug output we can see that R3 don't install default routes from others since it is originating a default route with always keyword:
R3(config-router)#
*Nov 21 21:19:58.275: OSPF-1 SPF : Schedule partial SPF type 5, LSID 0.0.0.0, adv_rtr 3.3.3.3 area dummy area
*Nov 21 21:19:58.283: OSPF-1 SPF : Service partial SPF, spf instance 41, 0/1/0
*Nov 21 21:19:58.283: OSPF-1 INTER: Process partial summary spf queue
*Nov 21 21:19:58.283: OSPF-1 EXTER: Process partial external spf queue
*Nov 21 21:19:58.287: OSPF-1 EXTER: Process partial spfQ: type 5, LSID 0.0.0.0, mask 0.0.0.0, adv_rtr 3.3.3.3, age 0, seq 0x80000001, area dummy area
*Nov 21 21:19:58.287: OSPF-1 EXTER: Start partial processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0
R3(config-router)#
*Nov 21 21:19:58.291: OSPF-1 EXTER: adv_rtr 1.1.1.1, age 1282, seq 0x80000002, metric 1, metric-type 2, fw-addr 0.0.0.0
*Nov 21 21:19:58.291: OSPF-1 EXTER: We originate default always. Don't install default from others
*Nov 21 21:19:58.295: OSPF-1 EXTER: Start partial processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0
*Nov 21 21:19:58.295: OSPF-1 EXTER: adv_rtr 3.3.3.3, age 0, seq 0x80000001, metric 1, metric-type 2, fw-addr 0.0.0.0
*Nov 21 21:19:58.299: OSPF-1 EXTER: Deleted external route 0.0.0.0/0.0.0.0
*Nov 21 21:19:58.303: OSPF-1 EXTER: Process partial nssa spf queue
Let's see what R1 will do by enabling the debugging:
R1 don't install default route from R3 since it is originating a default route with always keyword.
R1#debug ip ospf spf external
OSPF SPF external debugging is on
R1#
*Nov 21 21:19:58.855: OSPF-1 SPF : Schedule partial SPF type 5, LSID 0.0.0.0, adv_rtr 3.3.3.3 area dummy area
*Nov 21 21:19:58.859: OSPF-1 SPF : Service partial SPF, spf instance 43, 0/1/0
*Nov 21 21:19:58.859: OSPF-1 INTER: Process partial summary spf queue
*Nov 21 21:19:58.863: OSPF-1 EXTER: Process partial external spf queue
*Nov 21 21:19:58.863: OSPF-1 EXTER: Process partial spfQ: type 5, LSID 0.0.0.0, mask 0.0.0.0, adv_rtr 3.3.3.3, age 1, seq 0x80000001, area dummy area
*Nov 21 21:19:58.867: OSPF-1 EXTER: Start partial processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0
*Nov 21 21:19:58.867: OSPF-1 EXTER: adv_rtr 1.1.1.1, age 1281, seq 0x80000002, metric 1, metric-type 2, fw-addr 0.0.0.0
*Nov 21 21:19:58.871: OSPF-1 EXTER: Start partial processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0
*Nov 21 21:19:58.871: OSPF-1 EXTER: adv_rtr 3.3.3.3, age 1, seq 0x80000001, metric 1, metric-type 2, fw-addr 0.0.0.0
*Nov 21 21:19:58.875: OSPF-1 EXTER: We originate defau
R1#lt always. Don't install default from others
*Nov 21 21:19:58.875: OSPF-1 EXTER: Process partial nssa spf queue
The LSA Type 5 advertised by R3 does not have a routing bit set therefore it is not considered for SPF calculation:
R1#show ip osp database external adv-router 3.3.3.3
OSPF Router with ID (1.1.1.1) (Process ID 1)
Type-5 AS External Link States
LS age: 476
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0xE0C5
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 1
R1#
As a result R1 will never install a default route in its routing table:
R1#sh ip route 0.0.0.0
% Network not in table
R1#
R3 will install a default eigrp route through R2 because the LSA Type 5 advertised by R1 is ignored:
R3#show ip route eigrp | beg Gate
Gateway of last resort is 23.0.0.2 to network 0.0.0.0
D*EX 0.0.0.0/0 [170/156160] via 23.0.0.2, 00:00:25, FastEthernet0/0
R3#
To conclude : in order to prevent routing loop an OSPF router that originates a default route with always keyword will never accept other LSA Type 5 0.0.0.0 learned from another OSPF router.
Now what happen when R1 advertises a default route without the always keyword ?
Let's configure a static default route with an Administrative Distance 111 and we advertise it using the default-information originate command and enable debugging:
R1(config)#do debug ip ospf spf external
OSPF SPF external debugging is on
R1(config)#
R1(config)#ip route 0.0.0.0 0.0.0.0 lo0 111
R1(config)#router ospf 1
R1(config-router)#default-information originate
*Nov 21 22:05:45.839: OSPF-1 MON : Schedule Full SPF in area 0, change in LSID 1.1.1.1, LSA type R
R1(config-router)#
*Nov 21 22:05:50.847: OSPF-1 EXTER: Started Building Type 5 External Routes
*Nov 21 22:05:50.847: OSPF-1 EXTER: Start processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0
*Nov 21 22:05:50.847: OSPF-1 EXTER: adv_rtr 3.3.3.3, age 798, seq 0x80000002, metric 1, metric-type 2, fw-addr 0.0.0.0
*Nov 21 22:05:50.851: OSPF-1 SPF : Add better path to LSA ID 0.0.0.0, gateway 13.0.0.3, dist 1
*Nov 21 22:05:50.851: OSPF-1 SPF : Add path: next-hop 13.0.0.3, interface FastEthernet0/0
*Nov 21 22:05:50.855: OSPF-1 EXTER: Route update succeeded for 0.0.0.0/0.0.0.0, next-hop FastEthernet0/0/13.0.0.3
*Nov 21 22:05:50.855: OSPF-1 EXTER: Started Building Type 7 External Routes
R1(config-router)#
*Nov 21 22:05:50.859: OSPF-1 EXTER: Entered External route sync for area dummy area
*Nov 21 22:05:50.859: OSPF-1 EXTER: Entered External route sync for area dummy area
*Nov 21 22:05:50.863: OSPF-1 EXTER: Entered NSSA route sync for area 0
*Nov 21 22:05:50.863: OSPF-1 EXTER: Entered NSSA route sync for area 0
R1(config-router)#do undebug all
All possible debugging has been turned off
R1(config-router)#
Let's verify the routing table of R1, of course now R1 installs the default LSA Type 5 learned from R3 in the routing table because the routing bit is set in the LSA Type 5's R3:
R1(config-router)#do show ip route | inc 0.0.0.0
Gateway of last resort is 13.0.0.3 to network 0.0.0.0
O*E2 0.0.0.0/0 [110/1] via 13.0.0.3, 00:07:38, FastEthernet0/0
R1(config-router)#
Notice the routing bit set on this LSA Type 5 originated from R3 so it allows R1 to install a default route:
R1#show ip ospf data external adv-router 3.3.3.3
OSPF Router with ID (1.1.1.1) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 1012
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000002
Checksum: 0xDEC6
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 1
R1#
Now how R3 treats the LSA Type 5 advertised by R1 ?
From the debug output we can see that R1 sets the MAXAGE in its LSA Type 5 to 3600 and the max metric to 16777215, this prevent R3 to install the default route in order to avoid a routing loop:
R3#debug ip osp spf external
OSPF SPF external debugging is on
R3#
*Nov 21 21:54:52.227: OSPF-1 SPF : Detect MAXAGE in LSA type 5, LS ID 0.0.0.0, from 1.1.1.1
*Nov 21 21:54:52.227: OSPF-1 SPF : Detect generic change in LSA type 5, LSID 0.0.0.0, from 1.1.1.1 area 0
*Nov 21 21:54:52.231: OSPF-1 SPF : Schedule partial SPF type 5, LSID 0.0.0.0, adv_rtr 1.1.1.1 area dummy area
*Nov 21 21:54:52.235: OSPF-1 SPF : Service partial SPF, spf instance 48, 0/1/0
*Nov 21 21:54:52.235: OSPF-1 INTER: Process partial summary spf queue
*Nov 21 21:54:52.235: OSPF-1 EXTER: Process partial external spf queue
*Nov 21 21:54:52.239: OSPF-1 EXTER: Process partial spfQ: type 5, LSID 0.0.0.0, mask 0.0.0.0, adv_rtr 1.1.1.1, age 3600, seq 0x80000003, area dummy area
*Nov 21 21:54:52.239: OSPF-1 EXTER: Start partial processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0
*Nov 21 21:54:52.243: OSPF-1 EXTER: adv_rtr 1.1.1.1, age 3600, seq 0x80000003, metric 16777215, metric-type 2, fw-addr 0.0.0.0
*Nov 21 21:54:52.243: OSPF-1 EXTER: We originate default always. D
R3#on't install default from others
*Nov 21 21:54:52.247: OSPF-1 EXTER: Start partial processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0
*Nov 21 21:54:52.247: OSPF-1 EXTER: adv_rtr 3.3.3.3, age 139, seq 0x80000002, metric 1, metric-type 2, fw-addr 0.0.0.0
*Nov 21 21:54:52.251: OSPF-1 EXTER: Process partial nssa spf queue
*Nov 21 21:54:52.691: OSPF-1 SPF : Detect generic change in LSA type 1, LSID 1.1.1.1, from 1.1.1.1 area 0
*Nov 21 21:54:52.695: OSPF-1 MON : Schedule Full SPF in area 0, change in LSID 1.1.1.1, LSA type R
R3#
Below we can see that R1 does not install an LSA Type 5 in its LSDB even if it is configured to redistribute a default route:
R1#show ip osp data external self-originate
OSPF Router with ID (1.1.1.1) (Process ID 1)
R1#
R3#show ip route | inc 0.0.0.0
Gateway of last resort is 23.0.0.2 to network 0.0.0.0
D*EX 0.0.0.0/0 [170/156160] via 23.0.0.2, 00:39:46, FastEthernet0/0
R3#
