Expiration Date: April 2000 October 1999 File name: draft-ietf-ospf-shortcut-abr-01.txt

Transcription

1 Netwrk Wrking Grup A. D. Zinin Internet Draft AMT Grup Expiratin Date: April 2000 Octber 1999 File name: draft-ietf-spf-shrtcut-abr-01.txt OSPF Shrtcut ABR Enhanced OSPF ABR Behavir draft-ietf-spf-shrtcut-abr-01.txt Status f this Mem This dcument is an Internet-Draft and is in full cnfrmance with all prvisins f Sectin 10 f RFC2026. Internet Drafts are wrking dcuments f the Internet Engineering Task Frce (IETF), its Areas, and its Wrking Grups. Nte that ther grups may als distribute wrking dcuments as Internet Drafts. Internet Drafts are draft dcuments valid fr a maximum f six mnths. Internet Drafts may be updated, replaced, r bsleted by ther dcuments at any time. It is nt apprpriate t use Internet Drafts as reference material r t cite them ther than as a "wrking draft" r "wrk in prgress". The list f current Internet-Drafts can be accessed at The list f Internet-Draft Shadw Directries can be accessed at Abstract OSPF [Ref1] is a link-state intra-dmain ruting prtcl used fr ruting in IP netwrks. Thugh the definitin f the ABR in the current OSPF specificatin des nt require a ruter with multiple attached areas t have a backbne cnnectin, it is actually necessary t prvide successful ruting t the inter-area and external destinatins. If this requirement is nt met all traffic, destined fr the areas nt cnnected t such an ABR r ut f the OSPF dmain, is drpped. The rules f riginating and prcessing Summary-LSAs given in the current OSPF standard [Ref1] can als Zinin [Page 1]

2 result in subptimal inter-area ruting. Thugh all these prblems can be fixed using virtual links, this mem describes an alternative implementatin f the OSPF ABR behavir, which allws the administratr t avid it r, if virtual links are still used, t decrease the number f cnfigured virtual links. This mem als describes pssible situatins where the prpsed implementatin can be used. Acknwledgements The authr wuld like t acknwledge Christian Hpps and Peter Psenak fr their help in finding weak pints in early versins f this dcument, Thmas M. Thmas fr reviewing the preceding versin f it, and James Huang fr pinting ut ptential prblems described in sectin 7. Special thanks g t Jhn My wh cntributed a lt t this dcument and prvided a simpler algrithm representatin, used herein. Table f Cntents 1 Overview Intrductin Mtivatin Descriptin f Shrtcut ABR behavir Prpsed changes t OSPF ABR behavir Changes t Ruter-LSA riginatin Changes t ruting table calculatin Changes t Summary-LSA riginatin Implementatin Details Cmpatibility Deplyment Cnsideratins Necessity f virtual links Change f traffic patterns Optimized inter-area ruting Gradual deplyment f Shrtcut ABRs Ruting lps in transitin perids Security Cnsideratins Appendixes A.1 Ruter-LSA References Authr s Address Zinin [Page 2]

3 1 Overview 1.1 Intrductin An OSPF ruting dmain can be split int several subdmains, called areas, which limit the scpe f LSA flding. A ruter having attachments t multiple areas is called an "area brder ruter" (ABR). The primary functin f an ABR is t prvide its attached areas with Type-3 and Type-4 LSAs (which are used fr describing rutes and ASBRs in ther areas) as well as t perfrm actual inter-area ruting. 1.2 Mtivatin In OSPF flding f Type-1 and Type-2 LSAs is limited t the area brders, s ruters in ther areas must smehw knw hw t reach destinatins and ASBRs residing in different areas. OSPF uses Distance-Vectr (DV) apprach t achieve this gal, i.e., Area Brder Ruters annunce rutes and ASBRs internal t directly cnnected areas in Type-3 and Type-4 Summary-LSAs. If ruters using a DV prtcl annunce nly directly attached netwrks, they must be fully meshed t prvide cmplete ruting infrmatin t each ther. This cnditin cannt always be met, s ruters als annunce the netwrks they heard abut frm their neighbring ruters. This is the main reasn fr lps f ruting updates in DV prtcls, which are slved with such methds as split-hrizn, cunting-t-infinity, triggered updates, and hlddwn-timers. Applicatin f these rules t OSPF inter-area ruting wuld make the cde very cmplex, but since areas in OSPF need nt be fully meshed, ABRs are allwed t reannunce inter-area rutes. In rder t prevent lps f summaries in OSPF, ABRs reannunce nly thse inter-area rutes which are assciated with the backbne area. Summaries frm nn-backbne areas are just nt cnsidered by ABRs. Because interarea rutes are nt reannunced back int the backbne area, the latter functins as a lp-free inter-area ruting infrmatin repsitry. In rder t achieve nrmal ruting t inter-area and ASexternal destinatins, all areas in OSPF must be cnnected t the backbne either physically (via an interface) r lgically (via a virtual link). This is t ensure that all areas are prvided with inter-area rutes frm the backbne. A basic discussin f the disadvantages f the standard inter-area apprach are given in [Ref2] and are applicable t this dcument as well. In additin t that, cnsider anther prblem caused by standard OSPF ABR behavir (Figure 1). Zinin [Page 3]

4 . Area R / \.. / \.. BB /10Mb \ 2Mb.. / \.. \ R1... R Mb \ R4.. Area Figure 1. Subptimal inter-area ruting In this example ruter R2 has a 2Mb link t R5. At the same time R1 has a better link (10 Mbps), but R2 cannt rute traffic ging t area 2 thrugh R1. This is because accrding t [Ref1] R2 is nt allwed t cnsider summary-lsas frm nn-backbne areas and, cnsequently, des nt have rutes cvering destinatins in area 2 via R1. The situatin lks even mre interesting if R4 s ruting table is cnsidered. Since R2 flds summary-lsas frm R1 t R4, ruter R4 will have rutes t the area 2 via R1 (the best path), expecting traffic t g via 10Mbps links. In reality R2 will nt direct traffic t R1, but will frward it via 2Mbps link attached t itself. The last example shws hw the main principle f OSPF---prefer the shrtest path---is brken due t distance vectr apprach used fr inter-area ruting. Again, the prblem can be fixed using the virtual links between R1 and R2 in standard OSPF, but the slutin prpsed in this dcument appears t be mre elegant and invlving n administrative and traffic verhead. Mre sphisticated examples f hw Shrtcut ABR apprach imprves inter-area ruting are given in sectin 6. Zinin [Page 4]

5 2 Descriptin f Shrtcut ABR behavir This sectin describes an alternative implementatin f OSPF ABR behavir, named "Shrtcut ABR". It is an imprvement n standard ABR behavir, based n relaxatin f the restrictins applied t the calculatin f the inter-area rutes. ABRs are allwed t cnsider summary-lsas frm all attached areas (n matter if it is cnnected t the backbne r nt). The ruting lp preventin is dne by restricting riginatin f summary-lsas--- inter-area rutes are readvertised nly if there is a valid summary- LSA fr the destinatin learned frm the backbne. Originatin f summary-lsas fr intra-area rutes is dne as in standard OSPF, described in [Ref1]. The relaxatin f the ruting table calculatin allws ABRs withut a backbne cnnectin t rute traffic between the attached areas, as well as t rute traffic destined fr the backbne and ther areas using the rutes derived frm the summary-lsas in each attached area. This apprach als enables ruter R2 in Figure 1 t rute inter-area traffic via R1. Nte that the prpsed slutin des nt bviate the need f virtual link cnfiguratin in case an area has n physical backbne cnnectin at all. The methd described here imprves the behavir f a ruter cnnecting tw r mre backbne-attached areas. Thugh this dcument is initially riented t prcessing Type-3 LSAs and, cnsequently, is targeted t imprving OSPF inter-area ruting, it s acceptable t apply described methds t Type-4 LSAs, which will lead t imprvement f external ruting in an OSPF dmain. 3 Prpsed changes t OSPF ABR behavir This sectin describes the changes made t the base OSPF described in [Ref1]. 3.1 Changes t Ruter-LSA riginatin The algrithm f Type 1 LSA (ruter-lsa) riginatin is changed t have the Shrtcut ABR annunce its Shrtcut capability in the Ruter-LSA as described in A.1. A Shrtcut ABR must set the S-bit in the Ruter-LSA fr Area A nly if the area A s data structure has ShrtcutCnfigured bit set t TRUE, i.e., the S-bit directly reflects the state f ShrtcutCnfigured flag (see sectin 3.2 fr mre details). As in [Ref1] Shrtcut ABRs identify themselves as ABRs by setting the bit B in their Ruter-LSAs when they have Zinin [Page 5]

6 mre than ne attached area. 3.2 Changes t ruting table calculatin Shrtcut ABRs maintain tw additinal flags in Area Data Structure fr every nn-backbne area. The flags are named Shrtcut- Cnfigured and ShrtcutCapability. The first flag indicates whether the administratr has cnfigured the area t be Shrtcut. If s the ABR will set the S-bit in its Ruter-LSA fr this area. The secnd flag indicates that all ther ABRs in the areas are als Shrtcut capable. Nte that Shrtcut ABR is allwed t cnsider summary-lsas frm a nn-backbne area nly if ShrtcutCapability flag is set t TRUE. ShrtcutCapability flag, in turn, can be set t TRUE nly if ShrtcutCnfigured flag is als set t TRUE. This means that the area must be cnfigured as Shrtcut n the ABR itself and all ther ABRs. If during the ruting table calculatin a Shrtcut ABR ntices that there is a ABR which is nt Shrtcut-capable in any area, the Shrtcut ABR must clear the ShrtcutCapability flag fr that area, but still annunce the ShrtcutCnfigured flag fr the area in the S-bit f the Ruter-LSA riginated fr this area. Shuld the ABR in questin find that there n ABRs in an area, which are nt Shrtcut-capable, it must set the ShrtcutCapability flag fr that area. T implement this algrithm Steps 1 and 2 in sectin 16.1 f [Ref1] are changed as fllws: Step 1: "Initialize the algrithm s data structures. Clear the list f candidate vertices. Initialize the shrtest-path tree t nly the rt (which is the ruter ding the calculatin). Set Area A s TransitCapability t FALSE and ShrtcutCapability t the value f ShrtcutCnfigured." Step 2: "Call the vertex just added t the tree vertex V. Examine the LSA assciated with vertex V. This is a lkup in the Area A s link state database based n the Vertex ID. If this is a ruter-lsa, and bit V f the ruter-lsa (see Sectin A.4.2) is set, set Area A s TransitCapability t TRUE. If this is a ruter-lsa, and bit B f the ruter-lsa is set Zinin [Page 6]

7 (the ruter is an ABR) and bit S f the ruter-lsa is nt set (the ABR is nt Shrtcut-capable), set Area A s ShrtcutCapability t FALSE. In any case, each link described by the LSA gives the cst t an adjacent vertex. Fr each described link, (say it jins vertex V t vertex W):" ShrtcutCapability flag is used t determine ver which areas the ABR can use shrtcut paths. Shrtcut ABRs are frbidden t cnsider Summary-LSAs frm the areas with ShrtcutCapability flag ff. This methd is intrduced t prevent pssible ruting lps when standard and Shrtcut ABRs are simultaneusly present in an OSPF dmain. The methd als allws fr gradual enabling shrtcutting ver specific nn-backbne areas when and where it is necessary. The algrithm f calculating inter-area rutes is changed t allw the ruter t cnsider the summary-lsas frm attached nnbackbne areas that have ShrtcutCapability flag set t TRUE. This is achieved by applying sectin 16.3 f [Ref1] t such areas. The fllwing changes t 16.3 are made. Paragraph 1 f 16.3 is changed t be as fllws: "This step is nly perfrmed by area brder ruters attached t ne r mre nn-backbne areas that are either capable f carrying transit traffic (i.e., "transit areas", r thse areas whse TransitCapability parameter has been set t TRUE in Step 2 f the Dijkstra algrithm (see Sectin 16.1) r have all ABRs supprting Shrtcut feature (i.e., thse areas whse ShrtcutCapability parameter hasn t been set t FALSE during the Dijkstra algrithm)." Paragraph 4 f 16.3 is changed t be as fllws: "The calculatin prceeds as fllws. All summary-lsas f the areas with TransitCapability r ShrtcutCapability parameter set t TRUE are examined in turn. Each such summary-lsa describes a rute thrugh a nn-backbne area Area A t a Netwrk N (N s address is btained by masking the LSA s Link State ID with the netwrk/subnet mask cntained in the bdy f the LSA) r in the case f a Type 4 summary-lsa, t an AS bundary ruter N. Suppse als that the summary-lsa was riginated by an area brder ruter BR." Zinin [Page 7]

8 Step (3) f the algrithm in 16.3 is changed t be as fllws: "Lk up the ruting table entry fr N. (If N is an AS bundary ruter, lk up the "ruter" ruting table entry assciated with the backbne area). If the rute type is ther than backbne intra-area r inter-area (assciated with any area) then examine the next LSA. In ther wrds, this calculatin updates backbne intra-area rutes fund in Sectin 16.1, inter-area rutes fund in Sectin 16.2 and installs new inter-area rutes if the ABR des nt have a backbne cnnectin." Step (5) f the algrithm in 16.3 is changed t be as fllws: "If this cst is less than the cst ccurring in N s ruting table entry, verwrite N s list f next hps with thse used fr BR, and set N s ruting table cst t IAC. Else, if IAC is the same as N s current cst, add BR s list f next hps t N s list f next hps. If the area assciated with N s ruting table entry is the backbne, then the area and the type f the path (either intra-area r inter-area) must remain unchanged. Otherwise (the ruting table entry des nt exist r the assciated area is nt the backbne), the type f the rute must be set t inter-area and assciated area must be set t the area assciated with the summary-lsa being prcessed." In rder t prevent ruting lps sectins 11.1 and 16.2 f [Ref1] are changed. Sectin 11.1 is restricted t require installatin f discard ruting table entries fr each f the ruter s active area range. S the paragraph 2 f 11.1 shuld be read as fllws: "Befre the lkup begins, "discard" ruting table entries MUST be inserted int the ruting table fr each f the ruter s active area address ranges (see Sectin 3.5). (An area range is cnsidered "active" if the range cntains ne r mre netwrks reachable by intra-area paths.) The destinatin f a "discard" entry is the set f addresses described by its assciated active area address range, and the path type f each "discard" entry is set t "inter-area".[10]" Step (3) f sectin 16.2 is changed t instruct the ABRs t ignre summary defaults received frm stub areas: Zinin [Page 8]

9 "If it is a Type 3 summary-lsa, and the cllectin f destinatins described by the summary-lsa equals ne f the ruter s cnfigured area address ranges (see Sectin 3.5), and the particular area address range is active, then the summary-lsa shuld be ignred. "Active" means that there are ne r mre reachable (by intra-area paths) netwrks cntained in the area range. The summary-lsa shuld als be ignred if it is a summary default (Destinatin ID = DefaultDestinatin, Address Mask = 0x ) and the area it has been received frm is a stub area. This is t prevent pssible ruting lps." 3.3 Changes t Summary-LSA riginatin The algrithm f the summary-lsas riginatin is changed t include an explicit restrictin nt t riginate summary-lsas fr inter-area rutes if the rute t the destinatin is nt assciated with the backbne. Nte that if there are multiple alternative paths t a destinatin, sme f which are via the backbne and the rest are via nn-backbne areas, the area assciated with the crrespnding ruting table entry will remain the backbne area, but the set f next hps will actually direct traffic alng the best path even thrugh nn-backbne areas. If the ABR in questin has n backbne cnnectin, it will nt riginate summary-lsa fr any inter-area rute in any area, because the area assciated with the ruting table entry will never be the backbne area. The ABR will als nt readvertise an inter-area rute frm nnbackbne area if its backbne link state database des nt cntain a summary-lsa r ruter-lsa cvering a specific destinatin. In rder t implement described plicy, the paragraph 2 in sectin f [Ref1] shuld be read as fllws: "... Nte that nly intra-area rutes are advertised int the backbne, while bth intra-area and inter-area rutes are advertised int the ther areas. Als, summary-lsas fr inter-area rutes are riginated if and nly if these rutes are assciated with the backbne area (t prevent lps f summary-lsas)." Zinin [Page 9]

10 The 6th step f the algrithm given in sectins f [Ref1] must be read as shwn belw: "Else, if the destinatin f this rute is an AS bundary ruter, a summary-lsa shuld be riginated if and nly if the ruting table entry describes the preferred path t the AS bundary ruter (see Step 3 f Sectin 16.4) and it is assciated with the backbne area. If s, a Type 4 summary-lsa is riginated fr the destinatin, with Link State ID equal t the AS bundary ruter s Ruter ID and metric equal t the ruting table entry s cst. Nte: these LSAs shuld nt be generated if Area A has been cnfigured as a stub area." The 7th step f the algrithm given in sectins f [Ref1] must be read as shwn belw: "Else, the Destinatin type is netwrk. If this is an interarea rute and it is assciated with the backbne area, generate a Type 3 summary-lsa fr the destinatin, with Link State ID equal t the netwrk s address (if necessary, the Link State ID can als have ne r mre f the netwrk s hst bits set; see Appendix E fr details) and metric equal t the ruting table cst." Described changes in the ABR behavir allw selectin f mst ptimal paths t inter-area destinatins. Nte that backbne intra-area rutes can be updated with better nn-backbne inter-area ne, thus directing internal backbne traffic alng mre ptimal paths thrugh ther areas. 4 Implementatin Details If the current implementatin f OSPF uses the standard described in [Ref1], then supprt f the prpsed Shrtcut ABR behavir strategy must be implemented as an implicit cnfigurable ptin, allwing t set ShrtcutCnfigured flag fr a given area. Nte that the nature f the changes t OSPF presented in this dcument is s that standard ABR behavir is nt altered until at least ne area is cnfigured as Shrtcut. 5 Cmpatibility ABRs fllwing the apprach described in this dcument are required t annunce their Shrtcut capability fr a given area in Ruter- Zinin [Page 10]

11 LSAs. Since Shrtcut ABRs d nt cnsider Summary-LSAs frm an area when a Shrtcut-incmpatible ABR in such an area is seen, the apprach described in this dcument is cmpatible with standard OSPF described in [Ref1]. 6 Deplyment Cnsideratins This sectin discusses the deplyment details f Shrtcut ABR. 6.1 Necessity f virtual links First f all it shuld be repeated that Shrtcut ABR behavir des nt bviate the need fr virtual links in case an area has n physical backbne cnnectin. The difference with standard OSPF is that the administratr des nt need t cnfigure virtual links thrugh all areas he r she wants the inter-area traffic t g thrugh. A Shrtcut ABR needs a single backbne cnnectin (physical r virtual) t achieve ptimal ruting, since it cnsiders summary-lsas frm all attached areas. 6.2 Change f traffic patterns Use f Shrtcut ABR can lead t changes in the paths inter-area traffic flws take cmparing t thse experienced with standard OSPF. This happens because the Shrtcut ABR apprach allws a ruter t find paths better than it is pssible with the standard OSPF. While standard OSPF tries t frward all inter-area traffic thrugh the backbne area (thugh it des nt guarantee it), the Shrtcut ABR finds best rutes in the dmain even acrss nn-backbne areas. With Shrtcut ABR the backbne area is used as a dedicated place f inter-area ruting infrmatin exchange and inter-area traffic is allwed t crss nn-backbne area brders if such a path is really the best. 6.3 Optimized inter-area ruting Use f Shrtcut ABR imprves inter-area ruting in OSPF dmains by allwing ABRs t cnsider summary-lsas frm all attached area and cnsequently readvertise them int nn-backbne areas. Cnsider an example shw in the Figure 2: Zinin [Page 11]

12 .... Backbne R1 R / 8\ 1/... / \ / 8\ /1 1\. Net N R2... R3... R / \1 1/.. Area Area 1. Area Figure 2. Optimized inter-area ruting In case all ABRs use standard OSPF apprach, ruting t the net N wuld be as fllws: R4 and R5 inject summary-lsas int the backbne R4 als inject a summary-lsa int area 2 R3 is limited t cnsider summary-lsas frm the backbne nly, s it desn t see the alternative path thrugh area 2 and always rutes thrugh the backbne (thugh parallel paths are available) R3 injects summary-lsa fr the inter-area rutes derived frm the backbne summary-lsas and received frm R4 and R5 int Area 2 R2 is nt allwed t cnsider nn-backbne summary-lsas and rutes via serial links t R1, thugh mre ptimal paths d exist If R2, R3, and R4 use Shrtcut ABR apprach inter-area ruting is Zinin [Page 12]

13 imprved as shwn belw: R4 and R5 inject summary-lsas int the backbne R4 als inject a summary-lsa int area 2 R3 cnsiders summary-lsas frm bth attached areas and installs the rute thrugh area 2 (it has three rutes in the ruting table---via R5, via R4 thrugh the backbne, and via R4 thrugh area 2) and perfrms traffic sharing between the tw ethernet links. R3 injects summary-lsa fr the inter-area rutes t N (it will be the same as in the previus case, actually) R2 cnsiders summary-lsas frm all attached areas and prefers the rute thrugh area 2 rather than the backbne. 6.4 Gradual deplyment f Shrtcut ABRs Shrtcut ABR behavir is designed in such a way that the administratr can enable shrtcutting thrugh nn-backbne OSPF areas gradually. Since Shrtcut ABRs are allwed t cnsider summaries nly f thse areas that were cnfigured as Shrtcut (ShrtcutCnfigured flag in area data structure is set t TRUE) and whse ShrtcutCapability flag is set t TRUE, it is easy t cntrl which areas will accept additinal inter-area traffic. Fr an area t becme Shrtcut-capable, all ABRs that have links in it must have this area cnfigured as Shrtcut. If a single ABR in an area des nt annunce the S-bit in its Ruter-LSA fr this area, n ther Shrtcut ABRs cnnected t this area will direct inter-area traffic thrugh it (except fr the cases when standard OSPF behavir leads t it). The implementers shuld nte that supprt f a cnfigurable ptin described in sectin 4 is very imprtant fr traffic cntrl and successful deplyment. Zinin [Page 13]

14 7 Ruting lps in transitin perids As it was nted befre standard OSPF ABR behavir uses DV apprach t distribute ruting infrmatin amng the areas. While the basic technique used in OSPF fr this purpse prvides lp-free envirnment, existence f circular virtual link tplgy may lead t temprary ruting lps basically because f the sectin 16.3 that can update backbne rutes with nn-backbne inter-area nes. The ruting lps frmed in such situatins are similar t thse experienced in DV ruting prtcls when the riginatr f a rute lses its cnnectivity t the netwrk, sends messages withdrawing the rute t all neighbrs, but nt all messages manage t get thrugh and the riginatr receives a false update frm an upstream neighbr. An example f such a temprary lp is illustrated in Figure Area Area / 1 R2 1 \.. / \. 10. /. 10. ***************. 1/.. * \1. * _ * R / \ * R * * R \ / Backbne. 1.. \.... /.. \..... / /.. Netwrk N.. \ / Area 3.. Area Figure 3. Sample tplgy with temprary ruting lp In this example ruters R1, R2, R3, and R4 are ABRs. All f them are cnnected t the backbne ring that cnstitutes the backbne area. Nte that the link frm R4 t the backbne ring (marked with asterisks) des nt belng t area 2, but t the backbne area. The cst f backbne intra-area rute between any given tw ABRs is 10, since they are all cnnected via a bradcast segment. The cst f nnbackbne intra-area path frm R1 t R2, frm R2 t R3, and frm R3 t R1 is 1. In this example, it desn t matter what the cst between R3 and R4 is. We are interested in netwrk N residing in area 3. Bth Zinin [Page 14]

15 ABRs (R3 and R4) can reach this netwrk. Suppse R3 can reach it via a path with cst 1, while R4 via a path with cst 100. Bth ruters annunce summary-lsas fr netwrk N int the backbne area. If all ABRs fllw the standard ABR behavir, and there are n virtual links in the dmain, R1 and R2 will install inter-area rutes t netwrk N thrugh the backbne area. If virtual links are established between R1 and R2, R2 and R3, and R3 and R1, then ruters R1 and R2 will chse mre ptimal paths thrugh areas 4 and 2 crrespndingly, accrding t the algrithm described in 16.3 f [Ref1]. Als, R1 and R2 will annunce summary-lsas with cst 2 int area 1, since interarea rutes t netwrk N in their ruting tables are still backbneassciated. The same happens when R1, R2, and R3 are Shrtcutcapable ABRs and agree t use areas 2 and 4 fr shrtcutting. Nw assume R3 lses its cnnectivity t area 3. After the ruting table is recalculated, R3 has an inter-area rute t netwrk N thrugh the backbne area via R4 with cst 110. R3 withdraws its summary-lsa cvering netwrk N advertised int the backbne by flding crrespnding MaxAge LSA (premature aging, as described in 14.1 f [Ref1]) and updates areas 2 and 4 with new versin f summary-lsa, cntaining the cst f 110. Nw assume that R2 successfully receives and installs the new LSA, while R1 des nt (due t packet drps r ther ptential prblems). R2 recalculates the ruting table and installs the inter-area rute via R1, because R1 did nt withdraw its summary-lsa with cst 2. After the new rute is installed int R2 s ruting table, R2 riginates a summary-lsa fr netwrk N with cst 3 int area 2. R3 uses this LSA t calculate the rute t N via R2 and annunces a new summary-lsa with cst 4 int area 4. This LSA is used by R1. A lp is frmed. Nte that R1, R2, and R3 will nt use the backbne path, because it will always be updated by a nn-backbne path with smaller metric. Described lping stps when the cst f nn-backbne inter-area path t netwrk N becmes greater than the cst f backbne-assciated path, that is greater than 110. The lp described abve is nt Shrtcut ABR-specific, i.e., it can be seen even with standard ABR behavir when virtual links create a circle. Hwever, this dcument encurages administratrs t cnfigure areas as shrtcut and thus ptentially increases the prbability f such a temprary lp. The authr wuld like t emphasize that 1) such ruting lps are hardly t be seen in real netwrks with prper dmain design, and 2) even if such a lp frms, it is temprary, the netwrk finally cnverges and prper rutes are installed. There exist several methds t minimize the prbability f described ruting lp frmatin and t prvide faster cnvergence in case a lp has frmed, but specificatin f these methds is utside the scpe f this dcument and will be delayed until the prblem becmes apparent. Zinin [Page 15]

16 8 Security Cnsideratins Shrtcut ABR behavir specified in this dcument des nt raise any security issues that are nt already cvered in [Ref1]. 9 Appendixes A.1 Ruter-LSA An OSPF ruter riginates a ruter-lsa int each f its attached areas. The ruter-lsa describes the state and cst f the ruter s interfaces t the area. The cntents f the ruter-lsa are described in detail in Sectin A.4.2 f [Ref1]. One mre flag has been added t the ruter-lsa, called bit S belw. This flag indicates whether the area has been cnfigured as Shrtcut n the ABR. Nte that all ABRs in an area must annunce the S-bit this area t be used in shrtcutting LS age Optins Link State ID Advertising Ruter LS sequence number LS checksum length rtype 0 # links Link ID P E Link Data R Type # TOS TOS 0 metric # L # TOS 0 metric I T N O... K S S TOS 0 metric The ruter LSA Zinin [Page 16]

17 * * * S W V E B The rtype field The fllwing defines the flags fund in the rtype field. Each flag classifies the ruter by functin: bit B. When set, the ruter is an area brder ruter (B is fr brder). These ruters frward unicast data traffic between OSPF areas. bit E. When set, the ruter is an AS bundary ruter (E is fr external). These ruters frward unicast data traffic between Autnmus Systems. bit V. When set, the ruter is an endpint f an active virtual link (V is fr virtual) which uses the described area as its Transit area. bit W. Used in MOSPF [Ref3], when set, the ruter is a wild-card multicast receiver. These ruters receive all multicast datagrams, regardless f destinatin. Inter-area multicast frwarders and inter-as multicast frwarders are smetimes wild-card multicast receivers (see [Ref3] fr mre details). bit S. When set, the ruter is a Shrtcut-capable ABR and intends t use the area fr shrtcutting prvided that all ther ABRs in this area agree n that (als annunce the S-bit int this area). See sectins 2 and 3 fr mre details. 10 References [Ref1] J. My. OSPF versin 2. Technical Reprt RFC 2328, Internet Engineering Task Frce, ftp://ftp.isi.edu/inntes/rfc2328.txt. [Ref2] Zinin, Lindem, Yeung. Alternative OSPF ABR Implementatins. Wrk in prgress, Internet Engineering Task Frce. draftietf-spf-abr-alt-00.txt Zinin [Page 17]

18 [Ref3] J. My. Multicast Extensins t OSPF. Internet Engineering Task Frce, Authr s Address Alex D. Zinin AMT Grup 8 Maly Znamensky per., bld. 1 Office 3b Mscw, Russia zinin@amt.ru Zinin [Page 18]