Multi Topology Routing (MTR) for OSPF draft-psenak-mt-ospf-00.txt

MTR Goals Support multiple independent topologies over one physical topology Influence the path certain type of traffic takes over the network to reach the destination Traffic separation across the network links are dedicated to certain types of traffic prevent a link to be used for some type of traffic incongruent topologies for unicast and multicast

OSPF MTR extension Extend OSPF to support multiple topologies: advertise multiple topology specific metrics compute multiple topology specific SPFs compute prefix reachability for each topology

Advertising Topology Metrics RFC1583 supported TOS based routing later removed from the spec LSA format did not change Only TOS0 metric used today Redefined TOS to MT-ID and TOS metrics to MT-ID metrics Used in Router LSA, Summary LSAs External LSAs to describe the metric for multiple topologies

Advertising Topology Metric – Router LSA | LSA Header | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |*|*|*|N|W|V|E|B| 0 | #links | | Link ID | | Link Data | | | Type | # MT-ID | metric | | MT-ID | 0 | MT-ID metric | | ... |

Advertising Topology Metric – Summary LSAs | LSA Header | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | | | 0 | metric | | MT-ID | MT-ID metric | | ... |

Advertising Topology Metric – External LSAs | LSA Header | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | | |E| 0 | metric | | Forwarding Address | | External Route Tag | |E| MT-ID | MT-ID metric | | ... | |E| MT-ID | MT-ID metric | | ... |

Advertising Topology Metric MTID range 0 – 127 0 reserved for base topology Explicit metric advertisement if the topology metric for a certain link/prefix is not advertised, that link/prefix does not participate in that topology this has changed compared to RFC1583

Topology Specific SPF & Route Calculation SPF for an area is computed for each topology independently, using only metrics belonging to that topology Network LSA has no metric and is used by all topologies During inter-area and external route calculation for each topology, only metric belonging to that topology is considered

Adjacencies Single adjacency formed even when link participates in multiple topologies OSPF control packets are sent using the base topology as today

Excluding the Link/Prefix from Base Topology How to exclude link/prefix from base topology: In Summary/External LSA set the TOS0 metric to 0xFFFFFF (compatible with the RFC2328) In Router LSA area must be in the MTR Enabled mode in which case TOS0 metric in Router LSA is ignored. MT#0 metric is used instead and is optional.

Interoperability with non-MTR Aware Routers If there is no need to exclude link from the base topology, there is no interoperability problem MTR unaware routers will only participate in base topology When there is a need to exclude the link from the base topology, area MTRoutingCapability must be set to Enabled on all routers in the area: MT- bit (old T-bit) set in Hello packets If received Hello does not have MT-bit set, it’s ignored TOS0 metric is not used, MT#0 metric used instead

WG draft proposal for draft-psenak-mt-ospf-00.txt to be accepted as a OSPF WG document

RST-372 © 2002, Cisco Systems, Inc. All rights reserved. 14 14 14

Comparison Between MTR and TBR TBR (TOS Based Routing) can’t exclude selectively link from a single TOS unspecified TOS metrics defaults to the TOS 0 metric “Unreachable” metric would have to be defined inefficient if link participate in few topologies, where other topologies would have to be advertised as “Unreachable” MTR include only metrics for those topologies that are active on the link