Frame Relay Ip addressing

[boshardy1]

I work in a retail environment. Each of our stores has two PVC's. One to router A and router B. My question is can I put all 4 subinterfaces (2 on store router, 1 each on router A/B) on the same subnet (like a 255.255.255.248 for example). what do I lose/gain by doing this over putting each pair of sub-interfaces on its own subnet.

[gpulos · Accepted solution]

no you cannot do this with routing.

you have 4 separate interfaces and they cannot ALL share the same subnet with this topology.

if you were bridging between all interfaces, then you could have them all in the same subnet. this is probably not recommended in your case but i have no idea what your traffic is like so it might be OK to bridge all interfaces. (also don't know the entire topology from what you've stated)

if you put them all on the same subnet, then you will lose ip connectivity. (the router will tell you it's not possible when you go to address the second interface with the same subnet as the first)

for point-to-point interfaces, keep the subnetting to 255.255.255.252 is possible.

in this scenario you could have ip addressing as follows:

R1-PVC1:

192.168.1.1 255.255.255.252

Ra:

192.168.1.2 255.255.255.252

R1-PVC2:

192.168.1.5 255.255.255.252

Rb:

192.168.1.6 255.255.255.252

see the following link for basic IP Addressing/Subnetting:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

[Richard Burts · Accepted solution]

Thomas

while technically I think that you COULD do this I do not believe that you SHOULD do this. Unlike LAN interfaces where IOS will not permit two interfaces to be configured in the same subnet, I believe that IOS will allow two point to point interfaces (or subintefaces) to be in the same subnet.

I do not understand what you think it would benefit to put both links into the same subnet. I believe that there are benefits to having separate subnets. The main disadvantage that I see in having the same subnet on both links is that the store will see router A and router B as both being reachable through the same subnet. And when the store wants to send something to router A it is as likely to send it to B as it is to send it to A. What will be the impact in your network when B gets packets that should have gone to A?

HTH

Rick

[ilya.varlashkin · Accepted solution]

You cannot put these 4 sub-interfaces into single subnet, however if you're willing to reconfigure then you can put these two PVC into single subnet:

!Rtr_A

! interface serial0

enc frame-relay

ip address 192.168.1.1 255.255.255.248

Configure two other routers the same way (delete sub-interfaces first), then it might be better to reload (to completely clear sub-intf configs).

Do not configure DLCI, routers will use InARP to figure out who is who and where. Note that it's important to have PVC between RTR_A and RTR_B in this case, else communication between them on this subnet will go via remote store router.

You will have to run a dynamic routing protocol in this setup (floating static will not work any longer even with keepalive enabled). If you run OSPF, add following command to each of those serial interfaces:

ip ospf network point-to-multipoint

If you have very many stores (thousand) and determine that OSPF database becomes too big, you could either revert back to 'ip ospf network non-broadcast' and configure 'neighbor' command for each remote store at center site (you don't have configure 'neighbor' on the remote router, but you have to change OSPF network type there). Alternatively you could then use use explicit FR map to specify DLCI/IP mapping _and_ applying 'broadcast' keyword to those maps _and_ changing ospf network type to broadcast.

Hope this helps.