From: Chien-Cheng Hsieh (todd123@ms1.hinet.net)
Date: Tue Oct 22 2002 - 12:19:20 GMT-3
Hi, Mamoor
You are great! This do help me understand IRDP priority.
thanks !
Todd
----- Original Message -----
From: "Ahmed Mamoor Amimi" <mamoor@ieee.org>
To: <ccielab@groupstudy.com>
Sent: Monday, October 21, 2002 8:18 PM
Subject: secrets of IRDP
> I was confused why the lower priorty irdp is the preffered. like this :
>
> Rack03r3#sh ip route
> Gateway Using Interval Priority Interface
> 192.168.1.2 IRDP 5 1 Ethernet0
> 192.168.1.1 IRDP 5 2 Ethernet0
>
> Default gateway is 192.168.1.2
>
>
> Here is the theory behind.
>
> Consider a scenrio :
>
> r1---------------------------r2
> |
> r3 (no ip routing)
>
> r1:
> e0
> ip irdp
> ip irdp preference 2
>
> r2:
> e0
> ip irdp
> ip irdp preference 1
>
> As per the Doc CD and RFC the highest value is preffered , i got the
> following result at r3 :
>
> Rack03r3#sh ip route
> Gateway Using Interval Priority Interface
> 192.168.1.2 IRDP 5 1 Ethernet0
> 192.168.1.1 IRDP 5 2 Ethernet0
>
> Default gateway is 192.168.1.2
>
> And by seeing this it shows that lower value is preferred............ this
> is not lower value actaully IRDP dont calculate in decimal normally it
makes
> these values in twos complement and twos complement says "convert the
value
> in binary theninvert the value then add one to it and then convert back to
> decimal.
> So When r3 will recieve packets from r1 and r2 for the irdp
default-gateway
> then it will do as follows
>
> R1 - preference = 1 = 01 <--- in binary
> R2 - preference = 2 = 10 <--- in binary
>
> NOW invert :
> R1 - preference = 1 = 10 <--- in binary
> R2 - preference = 2 = 01 <--- in binary
>
> NOW add one :
> R1 - preference = 1 = 11 <--- in binary
> R2 - preference = 2 = 10 <--- in binary
>
> Convert back to decimal
>
> R1 - preference = 1 = 3 (new value)
> R2 - preference = 2 = 2 (new value)
>
> New values are not visible on the show commands but they are calcualed
from
> TWOs complement formula.
>
>
> REFERENCES :
>
> http://www.ietf.org/rfc/rfc1256.txt?number=1256
> Preference Level[i], The preferability of each Router Address[i]
> i = 1..Num Addrs as a default router address, relative to
> other router addresses on the same subnet.
> A signed, twos-complement value; higher
> values mean more preferable.
>
> http://www.vb-helper.com/tutorial_twos_complement.html#TwosComplement
>
> For a concrete example, consider the value 17 which is 00010001 in binary.
> Inverting gives 11101110. Adding one makes this 11101111. This is the twos
> complement representation of -17. Don't worry if you can't see this
> intuitively. You really need to take the twos complement again to see what
> value this is. Inverting the bits gives 00010000. Adding 1 gives 00010001,
> the representation for 17. All this shows that -(-17) = 17.
>
>
http://127.0.0.1:8080/cc/td/doc/product/software/ios121/121cgcr/ip_r/iprprt1
> /1rdipadr.htm#xtocid1783326
>
> preference number(Optional) Preference value. The allowed range is -231 to
> 231. The default is 0. A higher value increases the router's preference
> level. You can modify a particular router so that it will be the preferred
> router to which others home.
>
>
> -Mamoor
This archive was generated by hypermail 2.1.4 : Tue Nov 05 2002 - 08:35:53 GMT-3