OSPF All in One Lab---(BFD & GTSM)

Created Mar 04, 2015 17:43:42Latest reply Mar 10, 2015 11:54:44 2189 1 0 0

OSPF All in One Lab---(BFD&GTSM)

Section VII BFD

Section VII BFD

We’ve talked about FRR before and know it’s used to avoid network interruption for network convergence needs time. Now we’ll talk about BFD, it has similar function as FRR(Fast Reroute).

We should disable frr at first, [RouterA-ospf-1]undo frr.

Routing for Network

 Destination        Cost  Type       NextHop         AdvRouter       Area         0     Stub        1     Transit        10    Transit         1     Stub         2     Inter-area         3     Inter-area        2     Inter-area        2     Inter-area        3     Inter-area

From Routing table of RouterA, we can see that RouterA to RouterB is the main link while RouterA to RouterC to RouterB is backup link.

[RouterA-ospf-1]bfd all-interfaces enable

Now we can see BFD session is established on all interfaces of RouterA.

[RouterA]display ospf bfd session all

          OSPF Process 1 with Router ID

  Area interface's BFD Sessions

 NeighborId:          AreaId:          Interface:GigabitEthernet0/0


 BFDState:up                 rx    :1000             tx       :1000         

 Multiplier:3                BFD Local Dis:8193      LocalIpAdd:

 RemoteIpAdd:        Diagnostic Info:No diagnostic information


  Area interface's BFD Sessions


 NeighborId:          AreaId:          Interface:GigabitEthernet0/0


 BFDState:up                 rx    :1000             tx       :1000         

 Multiplier:3                BFD Local Dis:8192      LocalIpAdd:

 RemoteIpAdd:        Diagnostic Info:No diagnostic information

When I disable GE0/0/0 of RouterA, the routing table of RouterA is as below:

Routing for Network

 Destination        Cost  Type       NextHop         AdvRouter       Area         0     Stub        10    Transit        11    Inter-area


 Routing for ASEs

 Destination        Cost      Type       Tag         NextHop         AdvRouter        11        Type1      1 

        We can see the NextHop becomes immediately, the effort is amazing, which is the same as FRR.

By the way, if we want to change parameters such as Rx(BFD packets interval) and Tx etc, we can configure specific interface, but it’s optional.



In order to avoid illegal OSPF packets, we can use GTSM to define TTL value.

If we want to get familiar with TTL, we can ping RouterA GE0/0/0 from RouterE; through the packet capturing function, we can see ICMP packet as below:

From RouterA to RouterE, the value of TTL(Time to Live) is 255.

From RouterE to RouterA, the value is 253. So we can see that the packet of router itself has TTL value 255, one hop can reduce the value by one. From RouterE to RouterA, there are two routers, so the value is 253 instead.

Once we get GTSM enabled, then we can make sure only OSPF packets from these five Routers can be accepted so that no illegal OSPF packets will pass for their TTL value isn’t in the range.

[RouterA]ospf valid-ttl-hops 3

For the maximum hops is 3 (from RouterA to RouterE)

The valid TTL range of the detected packets is [255 -hops + 1, 255].

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user_2790689  Expert   Created Mar 10, 2015 11:54:44 Helpful(0) Helpful(0)

Thank you for sharing.
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