Re: [OSPF] I have a question regarding forwarding address
Abhinay R <abhinay.is2006@gmail.com> Wed, 13 July 2016 15:18 UTC
Return-Path: <abhinay.is2006@gmail.com>
X-Original-To: ospf@ietfa.amsl.com
Delivered-To: ospf@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 2E4EE12D893 for <ospf@ietfa.amsl.com>; Wed, 13 Jul 2016 08:18:41 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -2.448
X-Spam-Level:
X-Spam-Status: No, score=-2.448 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, HTML_MESSAGE=0.001, NORMAL_HTTP_TO_IP=0.001, RCVD_IN_DNSWL_LOW=-0.7, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (2048-bit key) header.d=gmail.com
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id v1qff70ELGeH for <ospf@ietfa.amsl.com>; Wed, 13 Jul 2016 08:18:38 -0700 (PDT)
Received: from mail-lf0-x22c.google.com (mail-lf0-x22c.google.com [IPv6:2a00:1450:4010:c07::22c]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id D43D612D1D2 for <ospf@ietf.org>; Wed, 13 Jul 2016 08:18:37 -0700 (PDT)
Received: by mail-lf0-x22c.google.com with SMTP id f93so41956731lfi.2 for <ospf@ietf.org>; Wed, 13 Jul 2016 08:18:37 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc; bh=k5iVt+2uK89ssNLNye1iDy7ujvLIvRSdjhDTRTr0/R8=; b=0dm+5I/xmIe9KilUupG/fq5ih4c4Ww4giTnbjXoZ9UBC4fqGPXSjC3IjpDAnn7uX1k I5Th4n98p0Z2c1uI5If36CLVqeBoCKd/lhnWXcWCZ6rS8AWi90oqthoqySWPvcgP9CsT CrRnP7gM3+z2FEZ6Ln0jE3oDPYQREgbTnF6AjsK7UL8ojsRU95cQlH9dClQ8rUcV8hkm xYAmFpdFp46vQgfKIi0hMxd8J/pOsI2055GyCYJ5YYKsdhkqPBDftmUHQhVH/M8YJ+Xy 3dsBz0rz2/YKGK158a53O/579WgTBSuwb2s9gzz54ECjLPHjCkswouMDftQMTN+Gd6P2 vqQg==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:cc; bh=k5iVt+2uK89ssNLNye1iDy7ujvLIvRSdjhDTRTr0/R8=; b=UY6l44NfkXZumgwGds+eqpoMP2B8Jpq74JU8KideOaWTNKEZuvEnkmj4LbW+sXd9hq BfVTCec66Zs6q+er32g1FrI6oUAWBXVuLllIINzrn46vE4h/gy8ing/U+D9XouA3zp3o crUEqwQ/fKXfQCxEQg7LAHxysCX6pJ1Fz5VAihMspxvEXzNcvx2ZzUetUIQ/uWA47lF2 Tm799bjD8whWk6UhvCIgSWJOEigmXyPoX6cgBLATDxZB6ofq0pecyW9qMaOlwDJ/Lau6 1ZLvg8PNLRvAI2eKtJW/TWcI//OneDZSJfJIwfqu9pBPq31q3hfuo8NtzPCxjZ9tAl4T 9XIA==
X-Gm-Message-State: ALyK8tKtYY3t1Vis/avPmOb4M4NZ+yVCIa7nt9lKHKgYZZ+C2Ap6xEJjLisFS/l5RktSEA7KU3C0CzUjNAbzsw==
X-Received: by 10.46.1.166 with SMTP id f38mr4758643lji.25.1468423115960; Wed, 13 Jul 2016 08:18:35 -0700 (PDT)
MIME-Version: 1.0
Received: by 10.25.17.42 with HTTP; Wed, 13 Jul 2016 08:18:35 -0700 (PDT)
In-Reply-To: <D3ABBF93.6B96E%acee@cisco.com>
References: <CAHUNbhbyQH-ALAT7gx55yADsxk4P8v0ZSgiK0bONX=2bMNhSCA@mail.gmail.com> <D3ABBF93.6B96E%acee@cisco.com>
From: Abhinay R <abhinay.is2006@gmail.com>
Date: Wed, 13 Jul 2016 20:48:35 +0530
Message-ID: <CAHUNbhbC0sq2O1XcqWDP-OoO41n8iNQW5OgrCvzgZ3BkNU+H-Q@mail.gmail.com>
To: "Acee Lindem (acee)" <acee@cisco.com>
Content-Type: multipart/alternative; boundary="001a1142bb4a34f753053785e6b1"
Archived-At: <https://mailarchive.ietf.org/arch/msg/ospf/VN6E9PgyGseLEL9bRDcrdS7N-vM>
Cc: "ospf@ietf.org" <ospf@ietf.org>
Subject: Re: [OSPF] I have a question regarding forwarding address
X-BeenThere: ospf@ietf.org
X-Mailman-Version: 2.1.17
Precedence: list
List-Id: The Official IETF OSPG WG Mailing List <ospf.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/ospf>, <mailto:ospf-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/ospf/>
List-Post: <mailto:ospf@ietf.org>
List-Help: <mailto:ospf-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/ospf>, <mailto:ospf-request@ietf.org?subject=subscribe>
X-List-Received-Date: Wed, 13 Jul 2016 15:18:41 -0000
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 ? and is there any scenario where the nexthop will be intra or inter area route, until the nexthop is explicitly set with routemap. 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> wrote: > Hi Abhinay, > > Your questions are very implementation specific but since I’m a good guy… > > From: OSPF <ospf-bounces@ietf.org> on behalf of Abhinay R < > abhinay.is2006@gmail.com> > Date: Wednesday, July 13, 2016 at 8:11 AM > To: OSPF WG List <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 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 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 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