RIP Justification

Scott Morris swm at emanon.com
Thu Sep 30 08:55:17 CDT 2010


   On 9/30/10 12:57 AM, Mark Smith wrote:

On Thu, 30 Sep 2010 14:13:11 +1000
Julien Goodwin [1]<nanog at studio442.com.au> wrote:


On 30/09/10 13:42, Mark Smith wrote:

One of the large delays you see in OSPF is election of the designated
router on multi-access links such as ethernets. As ethernet is being
very commonly used for point-to-point non-edge links, you can eliminate
that delay and also the corresponding network LSA by making OSPF treat
the link as a point-to-point link e.g.

int ethernet0
  ip ospf network point-to-point


If your implementation doesn't support point-to-point mode for an
interface, point-to-multipoint mode on an ethernet would achieve
something somewhat equivalent.

Do any implementations go point-to-point automatically if an ethernet
has a /30 or /31 mask?

Don't know.


   Nope.  Not Cisco anyway.
   NDC-R1-CustA(config)#int f0/0
   NDC-R1-CustA(config-if)#ip addr 10.111.1.1 255.255.255.254
   % Warning: use /31 mask on non point-to-point interface cautiously
   NDC-R1-CustA(config-if)#
   *Sep 30 15:18:22.710: %OSPF-5-ADJCHG: Process 1, Nbr 10.133.1.2 on
   FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or
   detached
   NDC-R1-CustA(config-if)#
   NDC-R1-CustA(config-if)#do sh ip o i f0/0 | i Type|Address
     Internet Address 10.111.1.1/31, Area 0
     Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1
   NDC-R1-CustA(config-if)#
   HTH,
   Scott


   On 9/30/10 12:57 AM, Mark Smith wrote:

On Thu, 30 Sep 2010 14:13:11 +1000
Julien Goodwin [2]<nanog at studio442.com.au> wrote:


On 30/09/10 13:42, Mark Smith wrote:

One of the large delays you see in OSPF is election of the designated
router on multi-access links such as ethernets. As ethernet is being
very commonly used for point-to-point non-edge links, you can eliminate
that delay and also the corresponding network LSA by making OSPF treat
the link as a point-to-point link e.g.

int ethernet0
  ip ospf network point-to-point


If your implementation doesn't support point-to-point mode for an
interface, point-to-multipoint mode on an ethernet would achieve
something somewhat equivalent.

Do any implementations go point-to-point automatically if an ethernet
has a /30 or /31 mask?

Don't know.

If you want to see what interface model OSPF is using, on a Cisco you
use

show ip ospf interface <blah>


The interface type for loopback interfaces can be a bit surprising and
the consequences a bit unexpected if you're intentionally or
otherwise not using a /32 prefix length on one.


Regards,
Mark.

References

   1. mailto:nanog at studio442.com.au
   2. mailto:nanog at studio442.com.au



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