lsa5 forward address issue

[sarahr202]

Hi everybody.

Please consider the following example:

R2---200.200.200.0/24---s0//0--R1-s0/1--------199.199.199.0-----R3.

R2 is not running ospf. while R1 and R2 are running ospf in area 0

R1 config:

router ospf 1

log-adjacency-changes

redistribute static subnets

network 199.199.199.0 0.0.0.255 area 0

network 200.200.200.0 0.0.0.255 area 0

interface Serial0/0

ip address 200.200.200.1 255.255.255.0

interface Serial0/1

ip address 199.199.199.1 255.255.255.0

R1#show ip route

Gateway of last resort is not set

C    200.200.200.0/24 is directly connected, Serial0/0
C    199.199.199.0/24 is directly connected, Serial0/1
C    1.0.0.0/8 is directly connected, Loopback1
S    10.0.0.0/8 [1/0] via 200.200.200.2

The issue is R1 should set forward address as 200.200.200.2 in LSA5. But for some reason, R1 is not doing that.

R1 can ping 200.200.200.2

R1#ping 200.200.200.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 200.200.200.2, timeout is 2 seconds:
!!!!!

Thanks.

Happy Holidays!!

==============================================================

Below is the show running output from R1;

R1#show running-config
Building configuration...

Current configuration : 1313 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R1
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
!
!
!
no ip domain lookup
ip domain name lab.local
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated

archive

log config

  hidekeys

!

interface Loopback1

ip address 1.1.1.1 255.0.0.0

!

!

interface Serial0/0

ip address 200.200.200.1 255.255.255.0

clock rate 2000000

!

!

interface Serial0/1

ip address 199.199.199.1 255.255.255.0

clock rate 2000000

!

router ospf 1

log-adjacency-changes

redistribute static subnets

network 199.199.199.0 0.0.0.255 area 0

network 200.200.200.0 0.0.0.255 area 0

!

ip forward-protocol nd

ip route 10.0.0.0 255.0.0.0 200.200.200.2

!

line con 0

exec-timeout 0 0

privilege level 15

logging synchronous

line aux 0

exec-timeout 0 0

privilege level 15

logging synchronous

line vty 0 4

login

[Peter Paluch · Accepted solution]

Hello Sarah,

Please check the following document:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009405a.shtml#topic1

Specifically, it states that for Cisco's OSPF implementation to set LSA-5 forwarding addresses to a non-zero value, all of the following requirements must be met:

These conditions set the forwarding address field to a non-zero address:
OSPF is enabled on the ASBR's next hop interface AND 
ASBR's next hop interface is non-passive under OSPF AND 
ASBR's next hop interface is not point-to-point AND 
ASBR's next hop interface is not point-to-multipoint AND 
ASBR's next hop interface address falls under the network range specified in the router ospf command. 

In your case, you are using Serial interfaces, i.e. a point-to-point interface. That may be the reason that OSPF does not set the forwarding address to a non-zero value.

Best regards,

Peter

[sarahr202]

Thanks Peter.