Metrics Size in OSPF and IS-IS ..


Each interface in the link state protocols in given a metric or a cost, which is advertised with the link state information in the LSP or the LSA. The SPF algorithm uses this metric to calculate the cost and the nexthop to a destination. Metrics used are generally the inverse of bandwidth, thus a large bandwith capacity link would have a lower metric

IS-IS

o ISO10589 specified the metric to be 6 bits in size. Therefore, the metric value could only range from 0-63. This metric information was carried in Neighbor Reachbility TLV and the IP reachability TLV. Since it was only 6 bits wide, it was called the “narrow metric“. The maximum path metric MaxPathMetric supported is 1023. This in theory brought down the complexity of the SPF algorithm from O(nlog n) to O(n). But this isn’t a significant motivation any more since the CPUs are really fast these days. The metric size apparently was kept small to optimize search while doing the SPF. It also allowed two types of metrics – External and Internal.

o Soon , the “narrow” metric range was found to be too small for certain networks and new TLVs (Extended IP and Extended Neighbor Reachability) were introduced to carry larger metrics as part of the Traffic Engineering document. These were called Wide Metrics. The MaxLinkMetric value now is 0xFFFFFFand the MaxPathMetric, 0xFE00000.

The Extended IP reachability TLV allows for a 4 byte metric, while the Extended Neighbor reachability TLV allows for 3 bytes metric size. This is to enable the metric summarized across levels/domains to be as large as 0xFFFFFFFF while the link metric itself is no larger than 0xFFFFFE. If a metric value of 0xFFFFFF is used the prefix is not used in SPF computation.

o The original specification defined 4 kinds of narrow metrics – delay, default, error and expense. These were never implemented and all implementations only support the default metric. In ISO 10589, the most significant bit of the 8 bit metrics was the field S (Supported) which defined if the metric was to be considered or not. Confusingly, as most ISO documents are, an implementation was supposed to set this bit to 1 if it wanted to indicate that the metric was unsupported. Since only the default metric is used, the implementations must always set this bit to 0 when using the narrow metrics. Later RFC 2966 used this bit in the default metric to mark L1 routes that have been leaked from L1 to L2 and back down into L1 again.

Current implementations must generate the default metric when using narrow metrics and should ignore the other three metrics when using narrow metrics.

OSPFv2

o It allows a link to have a 2 byte metric field in the Router LSA which implies a maximum metric of 0xFFFF.

o The Summary, Summary ASBR, AS-External and NSSA LSAs have a 3 byte metric value. A cost of 0xFFFFFF (LSInfinity) is used to tell the destination described in the LSA is unreachable.

o AS-External and NSSA LSAs allow two metric types, Type 1 and Type 2 which are equivalent to IS-IS Internal and External metrics. The type 1 considers the cost to the ASBR in addition to the advertised cost of the route while the latter uses just the advertised cost while calculating the routes during the SPF.

o The scheme thus allows for links to be configured with a metric no larger than 0xFFFF, while allowing cost of destinations injected across areas/levels to be as large as 0xFFFFFE.

OSPFv3

o It allows similar metric size for the Router LSA as in OSPFv2.

o It allows similar metric sizes for Intra Area Prefix LSA, Inter Area Prefix LSA, AS-External LSA and NSSA LSA as in OSPFv2. The value and significance of LS Infinity is valid here too.

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About Manav Bhatia

Manav Bhatia is a SDN/NFV dataplane architect at Ionos Networks and has co-authored several IETF standards on routing protocols, BFD, IPv6, security, etc. He is also a member of IETF Routing Area Directorate where he helps the Area Directors review the IETF standards for their impact on the Routing Area. View all posts by Manav Bhatia

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