In conditional default routing, route-map attached under the "default-information originate" command check the matched subnets present in the IP routing table. Once uplink goes down, router removes route from routing table and the originated default route is withdrawn from OSPF domain. Monitoring layer 2 interface status is not accurate indication of end-to-end reachability. Sometimes it might be possible that interface status is showing UP/UP locally but the circuit itself could be down .In such situation router will not remove connected interface subnet or any configure static route to that interface can still cause default route advertisement. A good design is to employ IP SLA tracking which is more reliable than layer 2 interface status; this provides accurate end-to-end reachability information.
It is assumed that reader already has knowledge of OSPF Conditional default routing and IP SLA.
1) Configure IP SLA instance and corresponding track object. 2) Use reliable static routing to configure a static host route for a bogus IP address pointing to null0 interface. As best practices use bogus IP address as link local address because link-local address is intended only for communications within the segment of a local network that a host is connected to. Routers do not forward packets with link-local addresses. Address block for ipv4 is 169.254.0.0/16. 3) Conditionally advertise default route into OSPF based on presence of the static host route.
R1 and R2 runs OSPF in area 0 .One end of the router R1 is connected to service provider router ISP1 through the Ethernet switch Sw1 while other end is connected to R2. If you configured a default conditional route on R1 matching interface prefix 192.168.1.0/24, R1 will advertise default route in OSPF domain i.e.to R2.But the problem here is even if ISP1 link to Sw1 goes down, R1 links to Sw1 will be in UP/UP state and shows connected route 192.168.1.0/24 in its routing table. Hence even if there is no end to end connectivity between R1 and ISP1, R1 still advertise default route to OSPF domain. To avoid such case you need to configure reliable default routing by using IP SLA.
interface FastEthernet0/0 ip address 10.1.1.2 255.255.255.0 duplex auto speed auto
router ospf 100 log-adjacency-changes network 10.1.1.2 0.0.0.0 area 0
interface FastEthernet0/0 ip address 10.1.1.1 255.255.255.0 duplex auto speed auto ! interface FastEthernet0/1 ip address 192.168.1.1 255.255.255.0 duplex auto speed auto ! router ospf 100 log-adjacency-changes network 10.1.1.1 0.0.0.0 area 0
Configure Reliable default routing on R1:
Step1:Configure IP SLA and Tracking object: Configure IP SLA to check reachability of ISP1 interface IP address using ICMP.
Enter configuration commands, one per line. End with CNTL/Z.
Step2:Configure a static host route for a bogus IP address pointing to null0 interface. When we attach track argument to static route, failure of track cause the static route to be remove from routing table.
As per condition, 1) R1 will advertise default route when there is 169.254.1.1/32 prefix present in routing table. 2) R1 will remove 169.254.1.1/32 prefix from routing table when there is failure in tracking object. 3) Tracking object failure depends on the failure of ping response from ISP1 interface.
On R1 tracking object is showing UP
Response Time Reporter 1 reachability
Reachability is Up
2 changes, last change 00:00:51
Latest operation return code: OK
Latest RTT (millisecs) 320
As tracking object is UP static route pointed to null 0 must be in routing Table:
R1#sh ip route | in Null0
S 169.254.1.1 is directly connected, Null0
As configure bogus static route in routing table, R1 will advertise default route in OSPF domain. Let’s verify default route on R2.
R2#sh ip ospf database | in 0.0.0.0
0.0.0.0 192.168.1.1 282 0x80000001 0x001FC4 100
R2#sh ip route ospf
O*E2 0.0.0.0/0 [110/1] via 10.1.1.1, 00:04:48, FastEthernet0/0
If you disable ISP1 interface, R1 interface to Sw1 remain UP/UP but ICMP failure to R1 to ISP1 cause to tracking object failure. Due to same R1 remove static route from routing table that will instruct OSPF process to stop default route advertisement.
*Mar 1 01:38:18.719: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Mar 1 01:38:19.719: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down
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