If I want to summarise routes, I don't want the router I'm advertising to to be connected to the subnet, right?

[dtbullock]

So we have 3 routers as such:

R1 and

                                       /  10.9.8.0 /22

R1 --------------- R2 ---------------- R3 -- 10.9.12.0 /22

                              ?        \ 10.9.16.0 /22

In R1's route table, I want it to know to talk to R2 about that supernet:

   10.9.8.0 /20  'via R2'

But, how should the /30 between R2 and R3 be addressed?

Option A:  R2 and R3 could share a /30 which is a subnet of the aggregated range (eg. 10.9.19.252/30)

    R2's routing table will include:

          10.9.8.0/21 via 10.9.19.253

          10.9.19.252/30 is directly attached

    Which I guess is OK, but to me it says "10.9.80/21 is mostly over at R3 ... except for 10.9.19.253".

Option B: R2 and R3 could share a /30 which is NOT a subnet of 10.9.8.0/21 (eg. 10.40.1.0/30)

    R2's routing table will have

           10.9.8.0/21 via 10.40.1.2/30

           10.40.1.0/30 is directly attached

    Which looks tidier to me.

The question with option B is now, does R1 need to now about the 10.40.1.0/30 subnet and how to get to it? 

Option B1)  Probably not. 

  R1 could send R2 a packet at one of its addresses in the 10.9.8.0/20 range. The /30 is just a local/private convenience for R2 and R3.  So long as we're careful not to re-use the same /30 in the network at any point, we can just pick out a /30 from some 'network-wide pool of /30's'.

Option B2)  Yes of course.

  (but why?)

So, is best-practice, A, B1 or B2?  If B2, why?

Or have I got the wrong end of the stick entirely?

thanks!

[gamalielcruz]

Option B1 for me.

Just make sure that R1 knows that 10.9.8.0/20 goes to R2.

[smehrnia]

Hi,
R1 doesnt need to know about the link between R2 - R3, as long as R1 knows where to pass 10.9.8.0/21 traffic, its fine.

** And now my opinion, if you're using point to point links and you are concerned with your ip addressing and overlaps etc, I'd suggest using IP Unnumbered for the point to point interfaces instead of /30 and cut the hassle, it'll work just fine!

HTH

plz Rate if it helped,

Soroush.

[dtbullock]

I have found a problem with option B1.  If I'm on R3 and try to ping R1, R1 has no route back to me, since by default the 'ping' command will use the interface nearest to the destination, and will thus us its IP address in the /30 address as the src of the IP packet.

ip unnumbered looked like an interesting solution, at first.  But then, it only works on interface types which are NOT mulit-access, and Ethernet interfaces are multi-access.  So you can't "ip unnumbered" a Gi0/0 on a 2911 for example.

Ultimately, only option B2 seems decent. 

Based on pure instinct, I'm going to use a different IP address space for "router topology" versus "data topology" (ie. I think this is a good idea, but can't explain why yet).  So I'll pick some unrelated private address range and apply the usual hierarchical addressing principles.  My 'campuses' are small - I scarcely expect to have more than 2 routers and a diameter of 2 from the edge router at each branch. I could set aside 10.255.224.0/19 as my range for router addresses.   That gives me 13 bits to 'play with'.  So I'll break it up like this:

   0000 0110  . 1111 1111  . 111s ssss . dddd ddrr

where:

   sssss:  is 32 sites

   dddddd: up to 64 /30's per site

   rr: 4 addrs: net, R1, R2, broadcast

  Site 1 (sssss = 00000)

      (ddd=000000)   -   0000 0110 . 1111 1111 . 1110 0000 . 0000 00xx = 10.255.224.0 /30

      (ddd=000001)   -   0000 0110 . 1111 1111 . 1110 0000 . 0000 00xx = 10.255.224.4 /30

      (ddd=000010)   -   0000 0110 . 1111 1111 . 1110 0000 . 0000 00xx = 10.255.224.8 /30

      (etc)

Site 2 (sssss = 00001)

      (ddd=00000)    -   0000 0110 . 1111 1111 . 1110 0000 . 1000 00xx = 10.255.224.128 /30

      (ddd=00001)    -   0000 0110 . 1111 1111 . 1110 0000 . 1000 01xx = 10.255.224.132 /30

      (ddd=00010)    -   0000 0110 . 1111 1111 . 1110 0000 . 1000 01xx = 10.255.224.136 /30

      (etc)

Using this scheme, inter-site routes can be aggregated with a /24.  The 5 bits for 'ddddd' also allows some local aggregation at each site, if warranted.

Athough there are now some additional entries in route tabes for the additional address range, I've gained something: some independence of the "routing infrastructure domain" and the "data traffic domain".

[alexei.fadeev]

have you tried doing 10.9.0.0/22 or 10.0.0.0/21

First of all what protocol are you using? ospf or a eigrp or rip or..... since you have to figureout which one is DR/Main-router

in ospf you could just net 10.9.0.0/21

if eigrp then you can use summary command include every net and then use summary 10.9.0.0/21

every net must be included that you want to make it advertised/been-seen (net x.x.x.x will be seen)if no net(included in net x.x.x.x command then net is unadvertised)  also use of pass int and default-info on a r2 for ospf or......

Also after examining a bit closer your whole ip schema is a bit out of control(planning)