| Routing Area Working Group | P. Sarkar, Ed. |
| Internet-Draft | H. Gredler |
| Intended status: Informational | S. Hegde |
| Expires: April 30, 2015 | C. Bowers |
| Juniper Networks, Inc. | |
| October 27, 2014 |
LFA selection for Multi-Homed Prefixes
draft-psarkar-rtgwg-multihomed-prefix-lfa-00
This document shares experience gained from implementing algorithms to determine Loop-Free Alternates for multi-homed prefixes. In particular, this document provides explicit inequalities that can be used to evaluate neighbours as a potential alternates for multi-homed prefixes. It also provides detailed criteria for evaluating potential alternates for external prefixes advertised by OSPF ASBRs.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [RFC2119].
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The use of Loop-Free Alternates (LFA) for IP Fast Reroute is specified in [RFC5286]. Section 6.1 of [RFC5286] describes a method to determine loop-free alternates for a multi-homed prefixes (MHPs). This document describes a procedure using explicit inequalities that can be used by a computing router to evaluate a neighbour as a potential alternate for a multi-homed prefix. The results obtained are equivalent to those obtained using the method described in Section 6.1 of [RFC5286]. However, some may find this formulation useful.
Section 6.3 of [RFC5286] discusses complications associated with computing LFAs for multi-homed prefixes in OSPF. This document provides detailed criteria for evaluating potential alternates for external prefixes advertised by OSPF ASBRs, as well as explicit inequalities.
The following set of inequalities can be used to evaluate LFAs for multi-homed prefixes.
This document proposes the following set of LFA inequalities for selecting the most appropriate LFAs for multi-homed prefixes (MHPs). They can be derived from the inequalities in [RFC5286] combined with the observation that D_opt(N,P) = Min (D_opt(N,PO_i) + cost(PO_i,P)) over all PO_i
Link-Protection:
D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,S) +
D_opt(S,PO_best) + cost (PO_best,P)
Link-Protection + Downstream-paths-only:
D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(S,PO_best) + cost (PO_best,P)
Node-Protection:
D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +
D_opt(E,PO_best) + cost (PO_best,P)
Where,
S - The computing router
N - The alternate router being evaluated
E - The primary next-hop on shortest path from S to
prefix P.
PO_i - The specific prefix-originating router being
evaluated.
PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P.
Cost (X,P) - Cost of reaching the prefix P from prefix
originating node X.
D_opt(X,Y) - Distance on the shortest path from node X to node
Y.
Figure 1: LFA inequalities for MHPs
To compute a valid LFA for a given multi-homed prefix P, a computing router S shall evaluate for each alternate neighbor N, one of the above set of inequalities, once for each remote node that originated the prefix. If the inequality is satisfied by any neighbour N for any remote prefix-originating node, router S shall choose neighbour N, as one of the valid LFAs for the prefix P.
Redistribution of external routes into IGP is required in case of two different networks getting merged into one or during protocol migrations. External routes could be distributed into an IGP domain via multiple nodes to avoid a single point of failure. During LFA calculation, alternate LFA next-hops to reach the best ASBR could be used as LFA for the routes redistributed via that ASBR. When there is no LFA available to the best ASBR, it may be desirable to consider the other ASBRs (referred to as alternate ASBR hereafter) redistributing the external routes for LFA selection as defined in [RFC5286] and leverage the advantage of having multiple re-distributing nodes in the network.
LFA evaluation for multi-homed external prefixes in IS-IS is similar to the multi-homed internal prefixes. Inequalities described in sec 2 would also apply to multi-homed external prefixes as well.
Loop free Alternates [RFC 5286] describes mechanisms to apply inequalities to find the the loop free alternate neighbour. For the selection of alternate ASBR for LFA consideration, additional rules have to be applied in selecting the alternate ASBR due to the external route calculation rules imposed by [RFC 2328]. This document also defines the inequalities defined in RFC [5286] specifically for the alternate loop-free ASBR evaluation.
The process to select an alternate ASBR is best explained using the rules below. The below process is applied when primary ASBR for the concerned prefix is chosen and there is an alternate ASBR originating same prefix.
1. If RFC1583Compatibility is disabled
1a. if primary ASBR and alternate ASBR are intra area
non-backbone path go to step 2.
1b. If primary ASBR and alternate ASBR belong to
intra-area backbone and/or inter-area path go
to step 2.
1c. for other paths, skip the alternate ASBR and
consider next ASBR.
2. If cost type (type1/type2) advertised by alternate
ASBR same as primary
2a. If not same skip alternate ASBR and consider next ASBR.
3. If cost type is type1
3a. If cost is same, program ECMP
3b. else go to step 5.
4 If cost type is type 2
4a. If cost is different, skip alternate ASBR and
consider next ASBR
4b. If type2 cost is same, compare type 1 cost.
4c. If type1 cost is also same program ECMP.
4d. If type 1 cost is different go to step 5.
5. If route type (type 5/type 7)
5a. If route type is same, check route p-bit,
forwarding address field for routes from both
ASBRs
match. If not skip alternate ASBR and consider
next ASBR.
5b. If route type is not same, skip ASBR
and consider next ASBR.
6. Apply inequality on the alternate ASBR.
Figure 2: Rules for selecting alternate ASBR in OSPF
When "RFC1583compatibility" is set to disabled, OSPF[RFC2328] defines certain rules of preference to choose the ASBRs. While selecting alternate ASBR for loop evaluation for LFA, these rules should be applied and ensured that the alternate neighbour does not loop the traffic back.
When there are multiple ASBRs belonging to different area advertising the same prefix, pruning rules as defined in RFC 2328 section 16.4.1 are applied. The alternate ASBRs pruned using above rules are not considered for LFA evaluation.
If there are multiple ASBRs not pruned via rules defined in 3.2.2, the cost type advertised by the ASBRs is compared. ASBRs advertising Type1 costs are preferred and the type2 costs are pruned.If two ASBRs advertise same type2 cost, the alternate ASBRs are considered along with their type1 cost for evaluation.If the two ASBRs with same type2 as well as type1 cost, ECMP FRR is programmed.If there are two ASBRs with different type2 cost, the higher cost ASBR is pruned.The inequalities for evaluating alternate ASBR for type 1 and type 2 costs are same, as the alternate ASBRs with different type2 costs are pruned and the evaluation is based on equal type 2 cost ASBRS.
When RFC1583Compatibility is set to enabled, multiple ASBRs belonging to different area advertising same prefix are chosen based on cost and hence are valid alternate ASBRs for the LFA evaluation.
Type 5 routes always get preference over Type 7 and the alternate ASBRs chosen for LFA calculation should belong to same type.Among Type 7 routes, routes with p-bit and forwarding address set have higher preference than routes without these attributes. Alternate ASBRs selected for LFA comparison should have same p-bit and forwarding address attributes.
The alternate ASBRs selected using above mechanism described in 3.2.1, are evaluated for Loop free criteria using below inequalities.
Link-Protection:
F_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,S) +
F_opt(S,PO_best) + cost (PO_best,P)
Link-Protection + Downstream-paths-only:
F_opt(N,PO_i)+ cost(PO_i,P) < F_opt(S,PO_best) +
cost (PO_best,P)
Node-Protection:
F_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +
F_opt(E,PO_best) + cost (PO_best,P)
Where,
S - The computing router
N - The alternate router being evaluated
E - The primary next-hop on shortest path from S to
prefix P.
PO_i - The specific prefix-originating router being
evaluated.
PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P.
cost(X,Y) - External cost for Y as advertised by X
F_opt(X,Y) - Distance on the shortest path from node X to Forwarding
address specified by ASBR Y.
D_opt(X,Y) - Distance on the shortest path from node X to node Y.
Figure 3: LFA inequality definition when forwarding address in non-zero
Link-Protection:
D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,S) +
D_opt(S,PO_best) + cost (PO_best,P)
Link-Protection + Downstream-paths-only:
D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(S,PO_best) + cost (PO_best,P)
Node-Protection:
D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +
D_opt(E,PO_best) + cost (PO_best,P)
Where,
S - The computing router
N - The alternate router being evaluated
E - The primary next-hop on shortest path from S to
prefix P.
PO_i - The specific prefix-originating router being
evaluated.
PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P.
cost(X,Y) - External cost for Y as advertised by X.
D_opt(X,Y) - Distance on the shortest path from node X to node Y.
Figure 4: LFA inequality definition for type1 and type 2 cost
Thanks to Alia Atlas and Salih K A for their useful feedback and inputs.
N/A. - No protocol changes are proposed in this document.
This document does not introduce any change in any of the protocol specifications. It simply proposes additional inequalities for selecting LFAs for multi-homed prefixes.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC5286] | Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008. |