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ddutt / cloud-native-data-center-networking

Code repository for the O'Reilly book 'Cloud Native Data Center Networking'
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How can OSPF unnumbered interface get the mac address of its peer route interface? #31

Closed zhaohuabing closed 4 years ago

zhaohuabing commented 4 years ago

Hi Dinesh,

This is actually one OSPF puzzle I'm trying to figure out rather than an issue. Figure 13-4 on page 274 shows the unnumbered OSPF configuration for a two-layer clos topo. The route interfaces at the two ends of the link are in different networks, for example, the ip addresses of the two ends of the link between spine01 and leaf01 are 10.0.0.21/32 and 10.0.0.11/32. So in theory, sipne01 can't get the MAC of leaf01 via ARP because ARP requires that the subnet of IP in the request should be the same of the sender. Similarly, leaf01 can't get the MAC of spine01 either.

So here is my question: how exactly IP-MAC mapping is done in unnumbered OSPF?

Thanks, Huabing

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ddutt commented 4 years ago

Hi Zhao, When you use OSPF unnumbered, you use an option called "onlink" associated with the route. This tells the Linux kernel to ARP even if the address to ARP for is not a connected IP address. Does this make sense?

Dinesh

zhaohuabing commented 4 years ago

Thanks, this puts the last piece of the unnumbered routing puzzle in place for me.