Multi-Topology Routing in OSPF

Multi-Topology Routing in OSPF

Multi-topology routing (MTR) introduces the capability of essentially class-based forwarding defined using MQC. A topology is a set of routers and links in a network for which a separate set of routes is calculated. A base topology is the default topology that a router uses to calculate a global routing table. Any additional topologies are called class-specific topologies which carry a specific class of traffic.

Multi-topology OSPF (MT-OSPF) exploits unused TOS (Type of Service) field in OSPF LSAs. MT-OSPF redefines TOS fields to carry MTIDs and their associated metrics.

A Router LSA originated by an MT-OSPF router lists all OSPF links for that router and all the router's neighbors. The modified Router LSA also indicates for each link what topologies the link belongs to (indicated by MTID) and the metric for the link specific to that topology (indicated by MTID metric). LSA types 3, 4, 5 and 7 are also modified to carry MTIDs and their metrics. For each MTID listed in the LSDB, a separate SPF is run, and a separate Routing Information Base (RIB) is maintained. The default (base) topology is indicated by MTID of 0. It consists of all routers and links in the OSPF domain. The class-specific topologies lie within the range 32-127.

The following packet capture shows an OSPF Router LSA with 2 MTIDs (although it shows as TOS):

Sample Scenario

The following network in figure 1 shows a base topology for which OSPF is enabled on all interfaces using network 0.0.0.0 255.255.255.255 area 0 OSPF router configuration command. The metric for base topology is set to 10 for all interfaces using ip ospf metric 10 interface configuration command.

The following output shows that R1 has 2 equal-cost paths to reach destination 4.4.4.4 on R4 via R1-R2-R4 and R1-R3-R4.

Routing to 4.4.4.4 using base topology

R1# show ip route 4.4.4.4

Routing entry for 4.4.4.4/32

  Known via "ospf 1", distance 110, metric 21, type intra area

  Last update from 10.13.1.2 on FastEthernet1/1, 00:03:44 ago

  Routing Descriptor Blocks:

    10.13.1.2, from 4.4.4.4, 00:03:44 ago, via FastEthernet1/1

      Route metric is 21, traffic share count is 1

  * 10.12.1.2, from 4.4.4.4, 00:03:54 ago, via FastEthernet1/0

      Route metric is 21, traffic share count is 1

R1# traceroute 4.4.4.4

Type escape sequence to abort.

Tracing the route to 4.4.4.4

  1 10.12.1.2 140 msec

    10.13.1.2 124 msec

    10.12.1.2 120 msec

  2 10.34.1.2 152 msec

    10.24.1.2 148 msec

    10.34.1.2 172 msec

Enabling MT-OSPF for Critical (Red) and Non-critical (Blue) Topologies

These topologies are overlaid on top of the base topology in figure 1. It is required that for all Critical class, only links R1-R2 and R2-R4 be used with the metric defined in figure 2.

For all non-critical class, only links R1-R2, R2-R3 and R3-R4 be used with metric defined in figure 3.

Step 1. Configuring the unicast topologies for MTR

Configuring topologies

global-address-family ipv4

 topology CRITICAL

  forward-base

 topology NON_CRITICAL

  forward-base

The global-address-family command allows to define the topology. Only IPv4 address-family is supported. The default sub-address family is unicast. The other is multicast. The topology command is entered to label a class-specific topology. An optional all-interfaces command under topology instance configures the instance to use all interfaces on the router.

The forward-base command under topology instance enables the router to look first in the class-specific topology FIB. If a forwarding route is not found, then the router will look in the base topology FIB. This is also called Incremental mode. The default is Strict mode which configures the router to look into class-specific topology FIB only. If the path is not found, the packet is dropped.

Step 2. MTR Traffic Classification

Traffic classification is used to assign different classes of traffic to different topologies. Traffic classification is configured using MQC. Traffic classification is defined globally for each topology rather than on interface.

MTR Traffic Classification

class-map match-any CRITICAL_TRAFFIC

 match ip dscp 46

class-map match-any NON_CRITICAL_TRAFFIC

 match ip dscp af31

 match ip dscp af32

 match ip dscp af33

policy-map type class-routing ipv4 unicast TOPOLOGY_POLICY

 class CRITICAL_TRAFFIC

  select-topology CRITICAL

 class NON_CRITICAL_TRAFFIC

  select-topology NON_CRITICAL

global-address-family ipv4

 topology CRITICAL

  forward-base

 topology NON_CRITICAL

  forward-base

 service-policy type class-routing TOPOLOGY_POLICY

Step 3. Enabling topologies to use interfaces

Since all-interfaces command was not configured under topology instances, topology instances must be enabled on per-interface basis. Topology specific OSPF metric (cost) can be configured here.

Enabling topology instances on interfaces on R1 and R2

R1 router

interface Loopback 0

 ip address 1.1.1.1 255.255.255.255

 topology ipv4 unicast CRITICAL

 topology ipv4 unicast NON_CRITICAL

interface Fa 1/0

 ip address 10.12.1.1 255.255.255.0

 ip ospf cost 10

 topology ipv4 unicast CRITICAL

  ip ospf cost 20

 topology ipv4 unicast NON_CRITICAL

  ip ospf cost 50

interface Fa 1/1

 ip address 10.13.1.1 255.255.255.0

 ip ospf cost 10

 R2 router

interface Fa 1/0

 ip address 10.12.1.2 255.255.255.0

 ip ospf cost 10

 topology ipv4 unicast CRITICAL

  ip ospf cost 20

 topology ipv4 unicast NON_CRITICAL

  ip ospf cost 50

interface Fa 1/1

 ip address 10.24.1.1 255.255.255.0

 ip ospf cost 10

 topology ipv4 unicast CRITICAL

  ip ospf cost 20

interface Fa 2/0

 ip address 10.23.1.1 255.255.255.0

 ip ospf cost 10

 topology ipv4 unicast NON_CRITICAL

  ip ospf cost 30

Enabling topology instance on interfaces on R3 and R4

R3 router

interface Fa 1/0

 ip address 10.13.1.2 255.255.255.0

 ip ospf cost 10

interface Fa 1/1

 ip address 10.23.1.2 255.255.255.0

 topology ipv4 unicast NON_CRITICAL

  ip ospf cost 30

interface Fa 2/0

 ip address 10.34.1.1 255.255.255.0

 topology ipv4 unicast NON_CRITICAL

  ip ospf cost 30

 R4 router

interface Loopback 0

 ip address 4.4.4.4 255.255.255.255

 topology ipv4 unicast CRITICAL

 topology ipv4 unicast NON_CRITICAL

interface Fa 1/0

 ip address 10.24.1.2 255.255.255.0

 ip ospf cost 10

 topology ipv4 unicast CRITICAL

  ip ospf cost 20

interface Fa 1/1

 ip address 10.34.1.2 255.255.255.0

 ip ospf cost 10

 topology ipv4 unicast NON_CRITICAL

  ip ospf cost 30

Step 4. Activating MTR using OSPF

All the topologies defined in global-address-family must be activated in OSPF for it to send the metrics for a specific topology in LSAs. Also, each topology must be assigned a topology ID (tid) in OSPF from the range 32 - 127. The following configuration must be applied to all routers:

Activating MTR using OSPF

router ospf 1

 network 0.0.0.0 255.255.255.255 area 0

 address-family ipv4

  topology CRITICAL tid 32

  topology NON_CRITICAL tid 33

Verification

The following output shows the LSDB on R1 router. Notice the Router LSAs received have metric for specific topologies along with MTIDs. Also note that from router LSAs, R1 figures out which advertising routers are reachable on which topology. Hence, R3 is not reachable on CRITICAL topology.

LSDB on R1

R1# show ip ospf database router

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

  LS age: 527

  Options: (No TOS-capability, DC)

  LS Type: Router Links

  Link State ID: 1.1.1.1

  Advertising Router: 1.1.1.1

  LS Seq Number: 80000008

  Checksum: 0x1299

  Length: 76

  Number of Links: 3

    Link connected to: a Stub Network

     (Link ID) Network/subnet number: 1.1.1.1

     (Link Data) Network Mask: 255.255.255.255

      Number of MTID metrics: 2

       TOS 0 Metrics: 1        ! TOS 0 is metric for base topology

       MTID 32 Metrics: 1      ! metric for CRITICAL topology (MTID 32)

       MTID 33 Metrics: 1      ! metric for NON_CRITICAL topology (MTID 33)

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.13.1.1

     (Link Data) Router Interface address: 10.13.1.1

      Number of MTID metrics: 0

       TOS 0 Metrics: 10

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.12.1.1

     (Link Data) Router Interface address: 10.12.1.1

      Number of MTID metrics: 2

       TOS 0 Metrics: 10

       MTID 32 Metrics: 20

       MTID 33 Metrics: 50

  LS age: 987

  Options: (No TOS-capability, DC)

  LS Type: Router Links

  Link State ID: 2.2.2.2

  Advertising Router: 2.2.2.2

  LS Seq Number: 8000000F

  Checksum: 0x3CCE

  Length: 76

  Number of Links: 3

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.23.1.1

     (Link Data) Router Interface address: 10.23.1.1

      Number of MTID metrics: 1

       TOS 0 Metrics: 10

       MTID 33 Metrics: 30

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.24.1.1

     (Link Data) Router Interface address: 10.24.1.1

      Number of MTID metrics: 1

       TOS 0 Metrics: 10

       MTID 32 Metrics: 20

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.12.1.1

     (Link Data) Router Interface address: 10.12.1.2

      Number of MTID metrics: 2

       TOS 0 Metrics: 10

       MTID 32 Metrics: 20

       MTID 33 Metrics: 50

  Adv Router is not-reachable in topology CRITICAL with MTID 32

  LS age: 985

  Options: (No TOS-capability, DC)

  LS Type: Router Links

  Link State ID: 3.3.3.3

  Advertising Router: 3.3.3.3

  LS Seq Number: 80000005

  Checksum: 0x2F4D

  Length: 68

  Number of Links: 3

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.34.1.1

     (Link Data) Router Interface address: 10.34.1.1

      Number of MTID metrics: 1

       TOS 0 Metrics: 10

       MTID 33 Metrics: 30

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.23.1.1

     (Link Data) Router Interface address: 10.23.1.2

      Number of MTID metrics: 1

       TOS 0 Metrics: 10

       MTID 33 Metrics: 30

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.13.1.1

     (Link Data) Router Interface address: 10.13.1.2

      Number of MTID metrics: 0

       TOS 0 Metrics: 10

  LS age: 457

  Options: (No TOS-capability, DC)

  LS Type: Router Links

  Link State ID: 4.4.4.4

  Advertising Router: 4.4.4.4

  LS Seq Number: 80000007

  Checksum: 0x68EF

  Length: 76

  Number of Links: 3

    Link connected to: a Stub Network

     (Link ID) Network/subnet number: 4.4.4.4

     (Link Data) Network Mask: 255.255.255.255

      Number of MTID metrics: 2

       TOS 0 Metrics: 1

       MTID 32 Metrics: 1

       MTID 33 Metrics: 1

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.34.1.1

     (Link Data) Router Interface address: 10.34.1.2

      Number of MTID metrics: 1

       TOS 0 Metrics: 10

       MTID 33 Metrics: 30

    Link connected to: a Transit Network

     (Link ID) Designated Router address: 10.24.1.1

     (Link Data) Router Interface address: 10.24.1.2

      Number of MTID metrics: 1

       TOS 0 Metrics: 10

       MTID 32 Metrics: 20

The following output shows the topology routing table entry for 4.4.4.4. Notice the metric to reach 4.4.4.4 from R1 for CRITICAL topology is 41 while the metric for NON_CRITICAL topology is 111.

Routing table entries for 4.4.4.4 for specific topologies

R1# show ip route topology CRITICAL 4.4.4.4

Routing Table: CRITICAL

Routing entry for 4.4.4.4/32

  Known via "ospf 1", distance 110, metric 41, type intra area

  Last update from 10.12.1.2 on FastEthernet1/0, 03:02:59 ago

  Routing Descriptor Blocks:

  * 10.12.1.2, from 4.4.4.4, 03:02:59 ago, via FastEthernet1/0

      Route metric is 41, traffic share count is 1

R1# show ip route topology NON_CRITICAL 4.4.4.4

Routing Table: NON_CRITICAL

Routing entry for 4.4.4.4/32

  Known via "ospf 1", distance 110, metric 111, type intra area

  Last update from 10.12.1.2 on FastEthernet1/0, 03:03:01 ago

  Routing Descriptor Blocks:

  * 10.12.1.2, from 4.4.4.4, 03:03:01 ago, via FastEthernet1/0

      Route metric is 111, traffic share count is 1

Now notice the path taken for traffic to destination 4.4.4.4 with DSCP value 46 i.e. for traffic belonging to CRITICAL topology. It takes the optimal path R1-R2-R4.

Traceroute to 4.4.4.4 using CRITICAL topology

R1# traceroute topology CRITICAL ip

Target IP address: 4.4.4.4

Source address: 1.1.1.1

DSCP Value [0]: 46

Numeric display [n]:

Resolve AS number in (G)lobal table, (V)RF or(N)one [G]:

Timeout in seconds [3]:

Probe count [3]:

Minimum Time to Live [1]:

Maximum Time to Live [30]:

Port Number [33434]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Type escape sequence to abort.

Tracing the route to 4.4.4.4

  1 10.12.1.2 144 msec 136 msec 152 msec

  2 10.24.1.2 156 msec 168 msec *

Now notice the path taken for traffic to destination 4.4.4.4 with DSCP value 26 (DSCP AF32) i.e. for traffic belonging to NON_CRITICAL topology. It takes the path R1-R2-R3-R4.

Traceroute to 4.4.4.4 using NON_CRITICAL topology

R1# traceroute topology NON_CRITICAL ip

Target IP address: 4.4.4.4

Source address: 1.1.1.1

DSCP Value [0]: 26

Numeric display [n]:

Resolve AS number in (G)lobal table, (V)RF or(N)one [G]:

Timeout in seconds [3]:

Probe count [3]:

Minimum Time to Live [1]:

Maximum Time to Live [30]:

Port Number [33434]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Type escape sequence to abort.

Tracing the route to 4.4.4.4

  1 10.12.1.2 108 msec 120 msec 156 msec

  2 10.23.1.2 180 msec 140 msec 148 msec

  3 10.34.1.2 220 msec 200 msec *

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