OSPF type 1 LSA

[bellah123]

Will a type 1 include the IP address of an orinigating router's link that is not part of OSPF. Let's say it's running BGP

router 1 ------- ospf ----------- router 2 --------------- BGP

Will router 2 include it's BGP link when sending a type 1 LSA to router 1.

[paul driver]

Hello
As Rtr 2 will be a ASBR between OSPF and BGP, By default the bgp routes will be advertised to R1 as a TYPE 5 LSA. with a forwarding address of 0.0.0.0

However if you advertised the subnet between ospf and bgp into ospf the forwarding address will be changed to the bgp interface>


Note - when redistributing into ospf from bgp this will not be allowed by default unless you apply the redistribute bgp internal in the bgp process.

ABSR

show ip ospf database external

res

Paul

[Masoud Pourshabanian]

Hello,

I am not sure I understood you question correctly or not. I suppose you are distributing BGP routes into OSPF. Router 1 will receive LSA type 5 when you restribute from BGP to OSPF. in this case, next hop of Router 1 will be router 2 interface which is toward Router1. Interface toward BGP will not be used for the next hop.

"orinigating router's link that is not part of OSPF"

There two cases.

1- You do not specify the IP address of link between router2 and BGP under OSPF. In this case, Router2 will not advertise the link to Router1.

2- You specify the link under OSPF configuration, but set that interface as a passive interface. In this case, Router2 advertises the link IP address, but does not participate to find OSPF neighbor on that interface.

In both case, the next hop of R1 will be R2 interface toward R1.

[paul driver]

Hello

@Masoud

"In both case, the next hop of R1 will be R2 interface toward R1."  that maybe the case but the forwarding address within the LSA will change and this can be important point to be aware of.

Even though the next-hop will indeed be the interconnecting router I assumed the question queried the LSA type of OSPF and its relation to BGP redistribution.

As I have stated the forwarding address will change within the Type 5 LSA depending on the advertisement of the subnet between the ospf and bgp processes-

Lets say we have two LSA type 5's to the same destination and 1 LSA type5 has a zero value and the other doesn’t then the metrics for those two lsa's are compared and the lowest of the two is preferred,

Lasty with also redistibuting an external routing process such as BGP into an IGP  These two points I highlighed are nice things to remember when doing such redistribution

res

Paul