Hi Abhinay,
From: Abhinay R <[email protected]<mailto:[email protected]>>
Date: Wednesday, July 13, 2016 at 11:18 AM
To: Acee Lindem <[email protected]<mailto:[email protected]>>
Cc: OSPF WG List <[email protected]<mailto:[email protected]>>
Subject: Re: [OSPF] I have a question regarding forwarding address
Hi Acee,
According to "12.4.4. AS-external-LSAs" where forwarding address
example is given, I see that it states forwarding address will just help to
skip a extra hop to destination.
And don't you think if forwarding address is set to inter area nexthop, then if
that inter area route gets deleted. it will take time for that change to reach
the ASBR and then ASBR to compute the change and originate new External LSA
with no forwarding address.and this need to be flooded across the AS and new
route need to be installed in them, which will be time consuming. So don't you
think that forwarding address must be set if the next hop is part of connected
network ?
No - the OSPF specification sets no such criteria. Although one might ask why a
router in the area with the next-hop is not redistributing the route.
and is there any scenario where the nexthop will be intra or inter area route,
until the nexthop is explicitly set with routemap.
I would not be a proponent of routing-policy which allows direct manipulation
of the next-hop.
Note that the example you cite, is just an example. The specification doesn’t
limit forwarding address advertisement to this use-case. Some implementations,
do limit advertisement to the most common use cases but this is not limited by
the OSPF specification.
Thanks,
Acee
P.S. If I were to be designing OSPFv4, I would most likely omit the forwarding
address feature (along with virtual links).
12.4.4.1. Examples of AS-external-LSAs
Consider once again the AS pictured in Figure 6. There
are two AS boundary routers: RT5 and RT7. Router RT5
originates three AS-external-LSAs, for networks N12-N14.
Router RT7 originates two AS-external-LSAs, for networks
N12 and N15. Assume that RT7 has learned its route to
N12 via BGP, and that it wishes to advertise a Type 2
metric to the AS. RT7 would then originate the
following LSA for N12:
; AS-external-LSA for Network N12,
; originated by Router RT7
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 5 ;AS-external-LSA
Link State ID = N12's IP network number
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
metric = 2
Forwarding address = 0.0.0.0
Moy Standards Track [Page 140]
?
RFC 2328 OSPF Version 2 April 1998
In the above example, the forwarding address field
has been set to 0.0.0.0, indicating that packets for
the external destination should be forwarded to the
advertising OSPF router (RT7). This is not always
desirable. Consider the example pictured in Figure
16. There are three OSPF routers (RTA, RTB and RTC)
connected to a common network. Only one of these
routers, RTA, is exchanging BGP information with the
non-OSPF router RTX. RTA must then originate AS-
external-LSAs for those destinations it has learned
from RTX. 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.
Note that when the forwarding address field is non-
zero, it should point to a router belonging to
another Autonomous System.
A forwarding address can also be specified for the
default route. For example, in figure 16 RTA may
want to specify that all externally-destined packets
should by default be forwarded to its BGP peer RTX.
The resulting AS-external-LSA is pictured below.
Note that the Link State ID is set to
DefaultDestination.
; Default route, originated by Router RTA
; Packets forwarded through RTX
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 5 ;AS-external-LSA
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB
exchange BGP information with RTX. In this case,
Moy Standards Track [Page 141]
?
RFC 2328 OSPF Version 2 April 1998
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. Flushing an LSA
is discussed in Section 14.1.
+
|
+---+.....|.BGP
|RTA|-----|.....+---+
+---+ |-----|RTX|
| +---+
+---+ |
|RTB|-----|
+---+ |
|
+---+ |
|RTC|-----|
+---+ |
|
+
Figure 16: Forwarding address example
Thanks & Regards,
Abhinay R
On Wed, Jul 13, 2016 at 7:29 PM, Acee Lindem (acee)
<[email protected]<mailto:[email protected]>> wrote:
Hi Abhinay,
Your questions are very implementation specific but since I’m a good guy…
From: OSPF <[email protected]<mailto:[email protected]>> on behalf of
Abhinay R <[email protected]<mailto:[email protected]>>
Date: Wednesday, July 13, 2016 at 8:11 AM
To: OSPF WG List <[email protected]<mailto:[email protected]>>
Subject: [OSPF] I have a question regarding forwarding address
Hi All,
I have a question on when forwarding address is set in external LSA?
Suppose I have a route from RIP say 100.1.1.0/24<http://100.1.1.0/24> with
nexthop 10.1.1.1 in routing table and I want to redistribute that route into
OSPF, then I feel below conditions must be vaild to set 10.1.1.1 as forwarding
address in the external LSA that OSPF originates for
100.1.1.0/24<http://100.1.1.0/24> network.
These conditions set the forwarding address field to a non-zero address:
OSPF is enabled on the ASBR's next hop interface AND
Although, it would be a strange topology where it weren’t enabled, it only has
to be reachable within the OSPF routing domain as an intra or inter area route.
ASBR's next hop interface is non-passive under OSPF AND
This should have no bearing on whether the forwarding address is advertised.
There could be a better path through the OSPF routing domain independent of
whether the interface is passive.
ASBR's next hop interface is not point-to-point AND
ASBR's next hop interface is not point-to-multipoint AND
Although the likelihood of the forwarding address providing a better path is
increased for multi-access networks, it is not necessary.
ASBR's next hop interface address falls under the network range specified
in the router ospf command.
Again, the next hop only has to be reachable within the OSPF routing domain as
an intra or inter area route.
Hope the Helps,
Acee
Any other conditions besides these set the forwarding address to 0.0.0.0.
If I have 10.1.1.0/24<http://10.1.1.0/24> network as intra or inter route in
OSPF routing table at the router that imported RIP route, do I originate
External LSA with 10.1.1.1 set as forwarding address ?
There is a confusion that if the redistributed network nexthop is present in
routing table as intra inter or connected route, we need to set it as
forwarding address. Please clarify.
While we receive a External LSA with forwarding address set, then we see if
there is a connected, intra or inter route to that nexthop and then use it to
compute the route nexthop. This is clear.
Thanks & Regards,
Abhinay R
--
~♥~♫AbHiNaY♫~♥~∞
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