2021-05-26 0

Importance of Route-tag in OSPF using VPN-instance

Issue Description

S6720-30C-EI-24S-AC / V200R008C00SPC500
The problem is that all the external routes (imported/redistributed) are in LSDB of S6720 but not installed in vpn-instance routing table.
We configured OSPF in VPN-instance between the 2 switches. On LSW2 we configured a static-route to null0 interface and imported in OSPF.
fd456406745d816a45cae554c788e754 41
Here is the LSDB and routing table of vpn-instance of LSW1:
fd456406745d816a45cae554c788e754 42
fd456406745d816a45cae554c788e754 43

Solution

Using router-tag is the solution in this scenario:
ROUTE-TAG – is a tool to avoid routing loop,  but when we receive the lsdb with same
route-tag, the switch thinks that there may be a loop. So , it will not be delivered to ip routing-table.
We configured route-tag 999 on LSW2.
fd456406745d816a45cae554c788e754 44

After we configured route-tag 999 , the route towards 172.16.1.0 is installed in routing-table
fd456406745d816a45cae554c788e754 45

Please note that all external routes will be checked by route-loop-prevent mechanism.
Routes which satisfy the condition：
1)      Type is External
2)       Ls id  and  Adv rtr are private address
Are seen as potential route loop, even if there are no loop actually.
fd456406745d816a45cae554c788e754 46

To conclude , if we will have configured the same route-tag on both switches (default route-tag is 0) will prevent to put the route in routing table due to loop prevention mechanism.
If we will configure the route-tag(different than default value) just on LSW2 the route will be installed in routing-table of LSW1.

Tags