On Thursday, 04/23/2009 at 08:39 EDT, Shimon Lebowitz <shimon...@gmail.com> wrote:
> As far as I can see, I should be able to define connections like this: > > HOME > 10.1.100.21 CTC1 > 10.1.55.21/16 NIC1 > > GATEWAY > ; address 'mask' 1st-hop link-name MTU > 10.1.100.20 HOST = CTC1 4096 > DEFAULTNET 10.1.35.50 NIC1 1024 > > What I mean by this is, that even though NIC1 is defined as having > a mask of /16, I want to also define a specific P2P link which also > starts with "10.1". Since the exact address at the other end of CTC1 > is defined (10.1.100.20), there is no need for TCPIP to use the > subnet mask and assume that this address is part of the network > on NIC1. > > Is this correct, or invalid/illegal? The specification is legal. But is it valid? Since host routes override subnet routes, VM TCP/IP will route correctly. The problem will be with the Outside World. If they view the LAN as 10.1.0.0/16, then they will not route packets for 10.1.100.20 to VM unless they also have a host route. They will simply ARP for 10.1.100.20 and wait for an answer. Since 10.1.100.20 isn't on the LAN, it doesn't hear the question and can't answer. For this reason, it is far better to create a separate /30 subnet (outside of 10.1/16) for the CTC link. MPROUTE can then notify the adjacent routers that you have a link to the 10.2.0.0/30 network. The Outside World will see that IP address as "not local" and will hand it to the router who will deliver it to you. It is just as if you were using a Guest LAN. The icky alternative is to use AssortedParms PROXYarp, causing VM TCP/IP to answer the ARP for 10.1.100.200. This works ok for unicast packets, but does not help with multicasts or broadcasts. FWIW, your MTU of 1024 on NIC1 is too low. I suggest you code it as 0 and let it default to what the value on the LINK statement, which should also be coded as "MTU 0", causing it it to be set to min(LargeEnvelopePoolSize ["LEPS"], 1500). Also, 4K is rather odd. If most traffic is flowing from the Outside World to the adjacent host, then the MTU should be the same as NIC1 - min(LEPS, 1500) - to avoid fragmentation. If it is mostly between VM TCP/IP and the adjacent host, then a large MTU size is better (with correspondingly large LESP) To wit, best practice: DEVICE CTC1 ..... LINK CTC1 .... MTU 4096 (depending) DEVICE NIC1 .... LINK NIC1 .... MTU 0 HOME 10.2.0.1/26 CTC1 (other end must be 10.2.0.2-10.2.0.62) 10.1.55.21/16 NIC1 GATEWAY ; address 'mask' 1st-hop link-name MTU DEFAULTNET 10.1.35.50 NIC1 0 Note that I actually coded a /26 instead of /30 on CTC1. This allows the CTC to be replaced with a Guest LAN some day in the future. If you keep it as /30, then the other end must be 10.2.0.2. Alan Altmark z/VM Development IBM Endicott
