During IP packet forwarding, after a LPM(ip lookup) done on the FIB, it will fetch the Nexthop ip address and the egress interface to send out the packet. Now, ARP table lookup is done for this nexthop IP. If the entry is not found, ARP request is broadcasted on the outgoing interface, right. My question is, during a MPLS label switching how is the destination MAC address found ? Because we don't do the IP lookup at all at the intermediate LSRs.
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MPLS or not, if ARP is used for IP-to-MAC resolution the ARP broadcast needs to reach the next-hop router interface.– Zac67 ♦May 4, 2022 at 18:16
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@Zac67 To be more specific, my question is, during a MPLS label switching, we don't do a IP lookup, we just swap the labels and send it out. So, in this MPLS switching operation how it will get the information of next-hop IP addresses on which a ARP lookup has to be made ?– HemanthMay 4, 2022 at 18:37
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The only way I could think of is that ,when a LSR receives a Label from downstream router during label distribution, it notes down the source IP address of the IP packet which carries this label. And when this label information is programmed into the ILM, it records the Nexthop IP address as well in the NHLFE pointed by that ILM. But, I'm not sure if my assumption is correct.– HemanthMay 4, 2022 at 18:40
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1 Answer
When a router receives a label via LDP, it records the layer 2 information in the LFIB. This allows the router to construct a layer 2 packet quickly because all the required information (destination MAC address) is there.
answered May 4, 2022 at 18:46
