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

2. 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

3. 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

4. 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

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

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

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

8. 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

9. 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

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

11. 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.

12. 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

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

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

15. 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