OSPF : Forwarding Address in External LSA

[vpalania]

Hi,

1)Iam not clear with forwarding address field in AS-External LSA, if the router in the forwarding address can also able to forward packets to external network, in that case its also an ASBR.So,it should also generate Type-5 LSAs.

2)I saw a configuration, in which the forwarding address points to one of the interface of the orginator of the LSA.Whats the use of it?

Thanks,

Vijay

[pkhatri]

Hi Vijay,

Consider the following setup [which I've copied from RFC2328 - what better source :-) ]:

+

|

+---+.....|.BGP

|RTA|-----|.....+---+

+---+ |-----|RTX|

| +---+

+---+ |

|RTB|-----|

+---+ |

|

+---+ |

|RTC|-----|

+---+ |

|

+

Let's consider what happens when there is no forwarding address:

In the above network, RTA, RTB and RTC run OSPF whereas RTX runs BGP but does not run OSPF. RTA also runs BGP and has a BGP session with RTX. RTA redistributes the BGP routes learned from RTX into OSPF as type-5 LSAs. When RTB and RTC receive these type-5 LSAs, they install these routes with a next hop as RTA. Therefore, every time RTB and RTC send traffic towards a BGP destination, they send the packet to RTA which will then have to send them out again over the same LAN segment to RTX. This is clearly sub-optimal.

That's where the forwarding address (FA) comes in. Now, when redistributing BGP routes into OSPF, RTA sets the FA for these routes to RTX's address, which is on the same segment as RTA. Therefore, when RTB and RTC get these LSAs, they install the routes with a next-hop of RTX, avoiding the extra hop.

OK, that's the background. Now to answer your questions:

1. The router whose address is used as the FA does not even need to run OSPF. Therefore, there is no question of this router having to originate type-5 LSAs at all.

2. If a router uses one of its own addresses as the FA, this allows that router to suggest to other OSPF routers how to get to it in case there are multiple paths to it.

Hope that helps,

Paresh.

[pkhatri]

Whoops.. that figure got mangled somewhat !

Pls see page 142 of RFC2328 for the same figure.

Paresh.

[vpalania]

Hi,

I have got doubt from this statement only,

Statement:By using the AS-external-LSA's forwarding address field, RTA can specify that packets for these destinations be forwarded directly to RTX. Without this feature, Routers RTB and RTC would take an extra hop to get to these destinations.

Doubt : If other OSPF router RTB,RTC can also able to reach RTX, without the need of RTA, then they also have connection to external network.Why not they advertise Type-5 LSA for RTX?

Thanks,

Vijay

[pkhatri]

Hi Vijay,

Just because RTB and RTC can reach RTX does not mean that they should all be running BGP with RTX. However, that does not stop you from configuring all of them with a BGP session to RTX. In that case, all of them would inject Type-5 LSAs for routes learned from RTX. Since this is a duplication of effort, RFC2328 specifies what to do in such a case:

"In this case, RTA and RTB would originate the same set of AS-external-LSAs. These LSAs, if they specify the same metric, would be functionally equivalent since they would specify the same destination and forwarding address (RTX). This leads to a clear duplication of effort. If only one of RTA or RTB originated the set of AS-external-LSAs, the routing would remain the same, and the size of the link state database would decrease. However, it must be unambiguously defined as to which router originates the LSAs (otherwise neither may, or the identity of the originator may oscillate). The following rule is thereby established: if two routers, both reachable from one another, originate functionally equivalent AS-external-LSAs (i.e., same destination, cost and non-zero forwarding address), then the LSA originated by the router having the highest OSPF Router ID is used. The router having the lower OSPF Router ID can then flush its LSA."

Therefore, even if you have multiple routes injecting equivalent type-5 LSas, in the end, you will end up with only one such LSA.

Hope that helps,

Paresh.

[vpalania]

Hi,

I agree with that rule.Only, if we have multiple type-5 LSAs, the forwarding address works,else it should be 0.0.0.0.Are there anyother scenarios ?

What happens if a router recieves Type-5 LSA and it doesn't has any connection to the forwarding address, then it should send data through ASBR.Correct me if I am wrong.

Thanks,

Vijay

[pkhatri]

Vijay,

If a router does not have an intra-area or inter-area route to the forwarding address, the LSA is ignored (section 16.4 (3) of RFC2328). It does not fall back to go through the ASBR.

Paresh.

[vpalania]

Hi,

You are confusing see the below statements:

1)"The router whose address is used as the FA does not even need to run OSPF"

2)"If a router does not have an intra-area or inter-area route to the forwarding address, the LSA is ignored "

How can there be an intra or inter route to a router that doesn't run OSPF.Only external route will be there.

Could your pls clear me.

Thanks,

Vijay

[pkhatri]

Alright consider this:

You have three routers on a LAN segment (with sample IP addresses):

1. RTA (running OSPF) - 192.168.1.1

2. RTB (NOT running OSPF) - 192.168.1.2

3. RTC (running OSPF) - 192.168.1.3

RTA runs BGP with RTB and redistributes the BGP routes into OSPF as type-5s with a FA = 192.168.1.2.

Since RTA runs OSPF on the LAN segment, it injects a route for 192.168.1.0/24 into OSPF (to be more accurate, this route is injected via the network-LSA originated by the DR on the segment).

Therefore, an intra-area route to the forwarding address of 192.168.1.2 exists but the router at 192.168.1.2 does NOT run OSPF.

Does that make a bit more sense ?

Paresh.

[vpalania]

Hi,

This is really helpful.Thanks for your explanation.

I was thinking that the words 'intra or inter route' means they should be available within OSPF.

Thanks,

Vijay