Hi all authors of segment routing,
When we did research on segment routing and found some new issues. Could you
help clarify if the analysis is right and
how will they be taken into account in the design of SR.
This is the first issue.
(Non-SR)
S11-----------S12----------S13---------S14
| | | |
| | | |
| | | |
S21-----------S22----------S23---------S24
As above topoloby, there are 8 nodes and assume all the metics of the links are
1 and VPNs are deployed
on S11/S21/S14/S24.
If LDP is used as the tunnel for VPN on S11/S14, since LDP can support the
ordered mode. The LSP for the S14 will be
setup as the order S14->S13->S12->S11 for distribution of label mapping. And
the shortest path for routes
to S14 is also S11->S12->S13->S14. So the end-to-end LSP to S14 is setup. If
one node (e.g. S13) does not
support LDP, according to LDP ordered mode, the end-to-end LSP cannot setup
since S12 does not receive the label
mapping from the exact nexthop of the route, S13 and it will not distribute the
label mapping to S11. And the
result is that the VPN on S11 cannot take the LSP since the LSP cannot setup on
S11.
If SR-BE path is used as the tunnel for VPN on S11/S14 and assume the S13
cannot support the SR, according to my
understanding, there will be an SR-BE path for the destination S14 which is
interrupted at S12. This is similar as
the independent mode of LDP. If this VPN takes this SR-BE Path at S11, the VPN
traffic will be dropped at S12.
If the analysis is right. I have two questions:
1. How will SR avoid such risk? Some enhancement on SR or just leave it to the
local implmentation (For example,
LSP ping is firstly used to check the connectivity)?
2. Assume LDP is already deployed on all nodes in the network to bear VPN
traffic. When SR-BE path is adopted in the
network to replace LDP LSP, since there is the possible risk proposed by
interrupted SR-BE path and I do not think it
is impossible to carefully determine the upgraded nodes. So the only choice is
to upgrade all nodes to support SR all
at once. is that right that the incremental deployment of SR in such scenario
is difficult to adopt?
Regards,
Zhenbin(Robin)
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