OSPF Metric of > 65535??? WTF!

I was always under the impression the maximum metric of OSPF is 65535.
Anything beyond that wouldn't make it into the routing table. The behavior
would be similar to a RIP route with a hop count of 16 that's considered
unreachable.

I noticed a few routes on our production network that exceeded 65535 mainly
because of network engineer stupidity.

So I made a real simple lab in GNS3. R1 connects to R2 connects to R3. R1, R2,
and R3 have 1.1.1.1, 2.2.2.2, and 3.3.3.3 as loopbacks and all of the links
are maxed out with ip ospf cost 65535.

R1(config-if)#do sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static
route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
     2.0.0.0/32 is subnetted, 1 subnets
O IA 2.2.2.2 [110/131070] via 12.1.1.2, 00:07:10, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/196605] via 12.1.1.2, 00:02:11, FastEthernet0/0
     12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, FastEthernet0/0
     13.0.0.0/24 is subnetted, 1 subnets
O 13.1.1.0 [110/131070] via 12.1.1.2, 00:02:21, FastEthernet0/0

R1(config-if)#do sh ip ospf data summ

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

                Summary Net Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 654
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 2.2.2.2 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000002
  Checksum: 0xEE3D
  Length: 28
  Network Mask: /32
        TOS: 0 Metric: 65535

  Routing Bit Set on this LSA
  LS age: 471
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 3.3.3.3 (summary Network Number)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0xA480
  Length: 28
  Network Mask: /32
        TOS: 0 Metric: 65535

How do I get a routing metric > 65535? Why are both 2.2.2.2 and 3.3.3.3 only
showing 65535 in the DB and higher metrics in the routing table? Isn't the
65535 based on the fact that there are only 16 bits for metric?

What gives???

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Received on Tue Oct 19 2010 - 12:47:25 ART