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FAQ-How Does ISIS Distinguish Between Shortcut and FA and How Does It Decide Whether to Advertise Tunnel Information to Other Routes In MPLS TE

Publication Date:  2012-07-27 Views:  35 Downloads:  0
Issue Description

Q:

How does ISIS distinguish between shortcut and FA and how does it decide whether to advertise tunnel information to other routes In MPLS TE? 

Alarm Information
Null
Handling Process
 A:
1. Whether ISIS enables the shortcut or FA mode on the local equipment decides whether the tunnel information is advertised through LSPs.
2. For the shortcut mode, the tunnel information does not exist in LSPs and is thus not advertised. For the FA mode, the routing information of the tunnel is advertised through LSPs.
3. For the FA mode, check ISIS LSDB information on the equipment:
[PE1-Tunnel0/0/0]dis isis lsdb local verbose
+NBR ID          P1          COST: 10
+NBR ID          PE2        COST: 1
You can view a neighbor PE2 whose cost value is 1. Add TLV to PE2, which floods the LSA. That is why other routers realize that there is a tunnel connecting PE2 with PE1.
4. For the shortcut mode, check ISIS LSDB information on the equipment:
[PE1-Tunnel0/0/0]dis isis lsdb local verbose
+NBR ID          P1           COST: 10
You can view that no TLV is added to the neighbor PE2. Therefore, PE2 has nothing to flood to other neighbors. That is why other routers cannot use the TE tunnel.
View ISIS spf-tree information.
[PE1-Tunnel0/0/0]dis is spf-tree
>PE1.00          T/-/-/-         P1.00          10         C/-/-/-/-/-/-
                                            PE2.00        1           C/-/-/-/-/-/-
You can view that, in the local SPF tree, PE2 is taken as being directly connected with PE1, participating in the local route calculation. Therefore, the local router can use this TE tunnel. 
 
Root Cause
Null
Suggestions
Null

END