Hi Subramanya, In general a router implements the weak host model, see https://datatracker.ietf.org/doc/html/rfc1122#section-3.3.4 In the weak host model the source address selected is independent of the outgoing interface.
> I'm trying to ping, and I observe that there is a mismatch between the > interface taken by the packet to go out and the source IP in the packet (when > there are ecmp routes). > The issue is that when I try to ping, and there are ECMP routes, the > interface it is going out and the source IP value filled in the IP header is > different, and also packet is not going out of the desired interface, if I > use the "source" option in ping.(even though IP is correct). > > Suppose, there are two sub-interfaces configured this way: > > TwentyFiveGEthernet0/7/0.13 (up): > L3 10.10.10.1/29 ip4 table-id 1 fib-idx 1 > > TwentyFiveGEthernet0/8/0.14 (up): > L3 10.10.10.17/29 ip4 table-id 1 fib-idx 1 > > And hardware Addresses are this way: > > TwentyFiveGEthernet0/7/0 3 up TwentyFiveGEthernet0/7/0 > Link speed: 25 Gbps > Ethernet address fa:16:3e:xx:xx:x1 > > TwentyFiveGEthernet0/8/0 4 up TwentyFiveGEthernet0/8/0 > Link speed: 25 Gbps > Ethernet address fa:16:3e:xx:xx:xx > > > > show ip fib output is as follows: > 0.0.0.0/0 > unicast-ip4-chain > [@0]: dpo-load-balance: [proto:ip4 index:31 buckets:2 uRPF:199 to:[0:0]] > [0] [@5]: ipv4 via 172.19.171.113 TwentyFiveGEthernet0/7/0.1328: mtu:1500 > next:6 a47b2cf5dbf9fa163ecda406810005300800 > [1] [@5]: ipv4 via 172.19.171.121 TwentyFiveGEthernet0/8/0.1329: mtu:1500 > next:7 a47b2cfaf0edfa163eb0de2f810005310800 > > > Now, suppose I ping using interface TwentyFiveGEthernet0/7/0.13, [ ping > 8.8.8.8 source TwentyFiveGEthernet0/7/0.13 ] > it is filling the IP of that interface which is 10.10.10.1, but the > outgoing packet is going out of interface TwentyFiveGEthernet0/8/0.14 and the > mac and vlan is also taken of TwentyFiveGEthernet0/8/0.13, Are you saying that VPP sends a packet out on 0.14 with a source MAC from 0.13? If that's the case, that's clearly a bug. > Also, using the option "table-id"[ping 8.8.8.8 table-id 1], the same issue > is seen, where there is a mismatch of the source ip filled in packet and the > packet is taking a different interface to go out. Are you saing the source address chosen is not part of table 1? If so that is also a bug with the source address selection done by ping. > So I'm guessing that once the ping plugin fills the source IP address in > the packet, and when the packet gets to the next node i.e. "ip4-lookup" node, > it is not considered strictly to go out of the interface with which the user > has specified in ECMP case. The ping "source" command doesn't specify outgoing interface, it specifies which interface the SAS algorithm should try to get a source address from. Suggestions for improvements? We could possibly add an option that would allow you to set outgoing interface in case of ECMP. Or even a toggle to select strong host model too. Best regards, Ole
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