Re: Reply to comments -- Comparison between FAR and OSPF - FRR
<liu.bin21 <at> zte.com.cn>
2014-07-23 02:00:47 GMT
Thank you for your comments.
But from what did you get conclusion
of 'research projects'? We not only have a deployment experience but also
strict large-scale system data analysis.
Following is my reply:
'No SPF is required for FRR to provide
protection, restoring traffic flow though sometimes on a suboptimal path.'
--Is FRR only used within domain? If
so, it can be counted as a drawback. And LDP FRR cannot guarantee that
the calculated path is the optimal path, leading to the emergence of new
link congestion. But FAT TREE architecture network is a non-blocking network.
'One commercial CSPF measured about
a decade ago completed in 30-40msec on a test topology of 450. That
was on a 300 MHz or less PPC or Pentium-2.
Todays processors are an order of magnitude
faster, so we could expect (with order NlogN scaling of SPF) to get about
the same SPF time on a topology of 4K or more nodes with no improvements
--As seen from the above CSPF business
application data, there is a linear correlation between the convergence
time and the number of topologies, while it is not sensitive to the FAR.
To compare the FRR and FRR, the working
process of the LDP FRR technology is described as follows:
1) Running LDP protocol in the network,
it works as DU (downstream independent) label distribution + orderly label
control + free label retention. (Disadvantage :additional protocol overhead)
In the above case, there are two paths
from R1 to R5, R5 initiates multi-label mapping message to the upstream.
Eventually, R2 and R3 respectively assign labels to R1 for reaching R5,
among which, the label distributed by R2 is the primary label, the label
distributed by R3 can be used as a backup. (Disadvantage: the irregular
topology leads to complex routing and prone to cause more serious link
2) Specify one equipment port of the
LSR as the backup of another equipment port.
3) Equipment maintenance label forwarding
table: As the port backup has not been implemented, one label forwarding
table has only one next hop and label, and the label is distributed for
FEC by the LDP peer connected to the next hop of the routing of FEC. After
the port backup is implemented, if the next hop of a label forwarding table
is the protected port, add a next hop and label for the entry, and the
label is distributed for FEC by the LDP peer connected to the backup next
hop. (Disadvantage: large protocol database overhead and processing overhead)
4) Equipment maintenance of the working
status of each port (normal/failure).
5) Packets reach the next hop, and are
forwarded to the destination according to the corresponding label forwarding
It can be seen from the above FRR
processing that FRR has the following disadvantages compared to FAR:
1) Additional protocol overhead: For
the protection of links, nodes and paths, it is necessary to set up a backup
LSP respectively, which causes unnecessary overhead and complex protocol
processing; (there is no such protocol overhead for FAR, and because FAR
is based on regular topology, path protection and switching process are
2) Backup LSP failures may exist. As
there is no protection mechanism, it cannot fast reroute when it fails;
(FatTree network architecture has multiple natural selection.)
3) There is a linear correlation between
the convergence time and the number of topologies, while it is not sensitive
to the FAR.
4) LDP FRR cannot guarantee that the
calculated path is the optimal path, leading to the emergence of new link
congestion. But FAT TREE architecture network is a non-blocking network.
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