Add a screenshot of Wireshark; do not say 'pop' but rather 'decrement' because the Segment List remains intact
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2026-02-21 15:22:14 +01:00
parent 95146624c2
commit 770ac9ce71
2 changed files with 17 additions and 4 deletions

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@@ -354,15 +354,25 @@ packet.
is expected to go.
1. on vlan 21, because the first segment wants the packet to go to `vpp0-2`. and `vpp0-1` is
acting as a transit router (just normally using IPv6 FIB lookup to pass it along)
1. on vlan 21 again, because when `vpp0-2` got it, it popped the SRH list, and sent it to the
second segment, which is onwards to `vpp0-1`.
1. on vlan 21 yet again, because when `vpp0-1` got it, it popped the SRH list, and sent it to the
third and final segment, which is onwards to `vpp0-3`.
1. on vlan 21 again, because when `vpp0-2` got it, it decremented the SRH _Segments Left_ from 2
to 1, and sent it to the second segment, which is onwards to `vpp0-1`.
1. on vlan 21 yet again, because when `vpp0-1` got it, it decremented the SRH _Segments Left_ from
1 to 0, and sent it to the third and final segment, which is onwards to `vpp0-3`.
1. on vlan 22, because `vpp0-2` is a transit router, using its FIB to pass it along to `vpp0-3`,
which decapsulates it with End.DX2 and sends it as an L2 packet on Gi10/0/3.
1. coming out of vlan 43 (between `vpp0-3:Gi10/0/3` and `host0-1`), where it is simply an IPv4
packet again.
Some folks find it easier to visualize packets by looking at Wireshark output. I grabbed one of the
packets from the wire, and here's what it looks like:
{{< image width="100%" src="/assets/vpp-srv6/wireshark.png" alt="Wireshark SRv6 packet with SRH" >}}
The screenshot shows the packet observed on step 4 above - it is coming from `vpp0-0`'s loopback address and
destined to the End localsid on `vpp0-1`, and I can see that the SRH has the list of 3 Segments in
reversed order with the `Address[0]` is the last one, a _LocalSID_ on `vpp0-3` that is an End.DX2. I
can also see that _Segments Left_ is set to 1.
VPP has a few relevant dataplane nodes:
1. ***sr-pl-rewrite-encaps-l2***: This node encapsulates ethernet at the ingress point by steering
packets into an _SR Policy_ named by its _Binding Segment ID_

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