FW: OSPF Metric of > 65535??? WTF!

Hi Mike,

I think you will find that the maximum OSPF metric is actually 2^24. This
is the size of the metric field in the LSA.

R5(config-router)#default-metric ?
  <1-16777214> OSPF default metric

Above shows maximum metric that can still be advertised. BTW; a metric of
0 will also be advertised.

Turn on debug ip ospf lsa-generation, kill an external route, and you will
see the LSA is withdrawn by advertising the poisoned metric, 16777215

R1#debug ip osp lsa-generation
OSPF summary lsa generation debugging is on
R1#
*Oct 19 18:47:02.867: BGP(0): route 158.10.107.0/24 modified
*Oct 19 18:47:02.991: OSPF: Generate external LSA 158.10.107.0, mask
255.255.255.0, type 5, age 3600, metric 16777215, tag 0, metric-type 2, seq
0x80000005

HTH,

Bob Sinclair CCIE 10427 CCSI 30427
CIERS2 Online Instructor
www.tinyurl.com/ciers2online

> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> Mike Kiefer
> Sent: Tuesday, October 19, 2010 1:47 PM
> To: ccielab_at_groupstudy.com
> Subject: 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 - 14:42:53 ART