Hello community,
I have a question that has been puzzling me for a while and I was hoping to find some answers.
In our environment we are running OSPF. On the network devices pointing towards the internet we have "default-information originate", so that all routers know how to reach the internet.
Running the command "sh ip ospf database external 0.0.0.0" on one of our backbone routers we get:
"sh ip ospf database external 0.0.0.0
OSPF Router with ID (x.x.x.x) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 653
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000A01
Checksum: 0x3AB2
Length: 36
Network Mask: /0
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 0
Routing Bit Set on this LSA
LS age: 631
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 1.1.1.2
LS Seq Number: 80000A01
Checksum: 0x34B7
Length: 36
Network Mask: /0
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 0
LS age: 888
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 2.2.2.1
LS Seq Number: 8000AFDC
Checksum: 0x7462
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 0
LS age: 927
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 2.2.2.3
LS Seq Number: 80000EE2
Checksum: 0x472A
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 0
LS age: 814
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 3.3.3.1
LS Seq Number: 80000234
Checksum: 0xBBA5
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 3489667727
As you can see routers 1.1.1.1 and 1.1.1.2 advertise 0.0.0.0 as E1 type links. Routers 2.2.2.1, 2.2.2.2 and 3.3.3.1 advertise 0.0.0.0 as E2 type links. In normal operation 0.0.0.0 shows up in the routing table either via 1.1.1.1 or 1.1.1.2. My question involves the failure of 1.1.1.1 and 1.1.1.2. In practice we have seen that if both 1.1.1.1 and 1.1.1.2 fail, then 0.0.0.0 is advertised via 2.2.2.1 or 2.2.2.2 (which is what we want!). But why are these routes preferred in contrast to 3.3.3.1, since all have the same metric-type E2 ????
We do not with 0.0.0.0 to be learnt via 3.3.3.1, so just to be on the safe side, I will change the metric-type of 2.2.2.1 and 2.2.2.2 to E1 with a metric of 20, so that the network prefers 1.1.1.x first and in case of a double failure, 2.2.2.x will advertise the default route.
I would be grateful if someone could shed some light!
