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OSPF and DHCP Server instances

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  • 1.  OSPF and DHCP Server instances

    Posted Apr 25, 2022 03:09 PM
    I am setting up some switches on a bench that I am going to be installing/ replacing others with. I am using OSPF for routing and each switch I am setting up to do their own DHCP services. 

    I have OSPF configured and I am able to ping all the IPs I am using (It is a point to point setup). I have DHCP set up on two switches right now. I have a laptop connected to each switch. Each laptop picks up an IP from the pools I made. 

    I was wondering, would I need to use the redistribute connected command or add the network command for each pool I make so that the two laptops can talk to each other?

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    rford1219
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  • 2.  RE: OSPF and DHCP Server instances

    Posted Apr 26, 2022 04:41 AM
    Hi there

    I assume your two switches do not share the same L2 networks (e.g. the ones where the laptops are connected) and routing is involved to get from one side to the other. If that's the case, you need to advertise the subnets behind each router to the other OSPF routers in order to get end-to-end connectivity between hosts in the networks.
    Whether that be internal OSPF routes ("network x.x.x.x") or external routes ("redistribute XXXX") depends on your OSPF planning and preferences. Either option would work but may be different when it comes to choosing paths out of many.

    In the end you want to see a route to the destination network in your router's routing table ("show ip route" [vrf xxx]).

    I hope this helps.

    Best,
    Thomas

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    Thomas Siegenthaler
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  • 3.  RE: OSPF and DHCP Server instances

    Posted Apr 27, 2022 12:53 PM
    Thank you, I am trying to figure out which would be the better, my understanding is the redistribute is good if you have multiple routing protocols, but the network command gives more control...

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    rford1219
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  • 4.  RE: OSPF and DHCP Server instances

    Posted Apr 28, 2022 01:12 AM
    Well, there is no best practice. This is mainly depending on your design. You need to provide us with some more details about your networks and the goals you want to achieve in order to compare and contrast the two options. There is no right or wrong.

    Generally, "redistribute xxx" commands are mainly - but no exclusively - used to advertise routes coming from other routing procotols such as BGP or static routing into OSPF. A network command, however, advertises prefixes which you have directly on your router (e.g. have a (vlan) interface).
    Redistribution generated Type-5 LSAs and makes your router an ASBR (autonomous system border router). Depending on your design you may or may not want this. Cost/Metric control is different with external routes. Behavior in a multi-area design is also different, especially in cases with stub areas.

    So in sum, for simple networks you can go either way with no notable difference. I suggest you go the "internal" way, meaning you use the "network" statement for "connected" routes. In more complex scenarios you should maybe read through the OSPF theory in more detail and decide afterwards what's the best way for you or provide us with some more insights to your entire network and ask for specific advise.

    Regards,
    Thomas

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    Thomas Siegenthaler
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  • 5.  RE: OSPF and DHCP Server instances

    Posted Apr 28, 2022 11:06 AM
    I was leaning towards the "network" statement. 

    I have several switches that are each running their own DHCP-Server instances. Each one has 5 VLANs/ Pools configured. If I am understanding right, I would need to use the VLAN network ID and wildcard subnet. So for example, if this was one of my pools I setup:

    Interface vlan 99
    ip address 10.99.1.1/24
    exit
    dhcp-server vrf default
    pool vlan99
    range (Start and End) prefix-len 24
    default-router (IP from Interface VLAN 99)
    domain-name example.com
    dns-server #.#.#.#
    lease 00:12:00
    exit

    The "network" statement would be something along the lines of this and I would need it for each pool I make (changing the addressing of course):
    network 10.99.1.0 0.0.0.255



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    rford1219
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  • 6.  RE: OSPF and DHCP Server instances

    Posted Apr 29, 2022 08:53 AM
    Hmmm ... kind of.

    So routing does not necessarily have something to do with DHCP. Whether or not you have DHCP pools configured does not change anything on your router in terms of OSPF. The thing you have to focus on is interfaces (routing interfaces or vlan interfaces) with an IP address. Those you can include or not in OSPF routing either by adding them as a "network" or as a redistributed external route.

    On ArubaOS-CX platform there is no "network" statement. The thing you have to do with similar effect is to go to your interface (e.g. int vlan 99) and type in "ip ospf 1 area 0.0.0.0" assuming you are running OSPF process 1 and want to add this interface to area 0.0.0.0 (0).

    Please refer to the IP routing guide for any details:
    https://www.arubanetworks.com/techdocs/AOS-CX/10.08/PDF/ip_route_6300-6400-83xx.pdf

    Regards,
    Thomas

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    Thomas Siegenthaler
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  • 7.  RE: OSPF and DHCP Server instances

    Posted May 02, 2022 03:53 PM
    Got it, that makes sense, I was thinking a different Vendor...

    It turns out the firewall on the laptops were blocking it... I changed it and everything is good, thank you for all the help!

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    rford1219
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  • 8.  RE: OSPF and DHCP Server instances

    Posted May 03, 2022 02:07 AM
    Well that sounds more like a layer2 issue or a firewall installed on the laptop which blockes traffic coming from a different subnet.

    Do you see the laptop's MAC address on the switch? Do you have an ARP entry of the laptop's MAC address on the router with the vlan interface?
    Please also make sure that forward routing (e.g. from your ping source to the laptop) and also back routing (e.g. from the laptop to the ping source's address) is properly set up on all the routers in between.

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    Thomas Siegenthaler
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