Re: [OSPF] I have a question regarding forwarding address
"Acee Lindem (acee)" <acee@cisco.com> Wed, 13 July 2016 15:29 UTC
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From: "Acee Lindem (acee)" <acee@cisco.com>
To: Abhinay R <abhinay.is2006@gmail.com>
Thread-Topic: [OSPF] I have a question regarding forwarding address
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Date: Wed, 13 Jul 2016 15:29:49 +0000
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Subject: Re: [OSPF] I have a question regarding forwarding address
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Hi Abhinay, From: Abhinay R <abhinay.is2006@gmail.com<mailto:abhinay.is2006@gmail.com>> Date: Wednesday, July 13, 2016 at 11:18 AM To: Acee Lindem <acee@cisco.com<mailto:acee@cisco.com>> Cc: OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>> 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) <acee@cisco.com<mailto:acee@cisco.com>> wrote: Hi Abhinay, Your questions are very implementation specific but since I’m a good guy… From: OSPF <ospf-bounces@ietf.org<mailto:ospf-bounces@ietf.org>> on behalf of Abhinay R <abhinay.is2006@gmail.com<mailto:abhinay.is2006@gmail.com>> Date: Wednesday, July 13, 2016 at 8:11 AM To: OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>> 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♫~♥~∞
- Re: [OSPF] I have a question regarding forwarding… Acee Lindem (acee)
- Re: [OSPF] I have a question regarding forwarding… Abhinay R
- Re: [OSPF] I have a question regarding forwarding… Acee Lindem (acee)
- Re: [OSPF] I have a question regarding forwarding… Acee Lindem (acee)
- [OSPF] I have a question regarding forwarding add… Abhinay R