Tuesday, November 25, 2008

OSPF - Lowest IP address in OSPF is used as forwarding address on ASBR

I was labbing some NSSA today and I was wondering how the OSPF ASBR chose the forward address since it seem to appear on the opposite side of traffic flow. Example:


SW1 is the ASBR (redistributing it's loopback). Traffic was flowing through R6, but the address pointing to R3 was the "forward address". It doesn't really matter as long as the link is in OSPF but it can impact metric calculations since SW1's interface cost to R3 will be included.

I did a short search on GS and RFC2328 but could not find anything. I had a guess it was the lowest IP in OSPF on the router and it turns out that is right:

SW1#show run | sec router ospf
router ospf 1
area 2 nssa
redistribute connected metric-type 1 subnets route-map con2ospf
network area 2
network area 2
network area 2

SW1#show ip int bri
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 YES manual up up
FastEthernet2/0 YES manual up up
Loopback2 YES manual up up
Loopback100 YES manual up up

SW1#show ip ospf database external

OSPF Router with ID ( (Process ID 1)
SW1#show ip ospf database ns
SW1#show ip ospf database nssa-external

OSPF Router with ID ( (Process ID 1)

Type-7 AS External Link States (Area 2)

LS age: 243
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: (External Network Number )
Advertising Router:
LS Seq Number: 80000009
Checksum: 0x8EBF
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 20
Forward Address:
External Route Tag: 0


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