To avoid having to rebuild the LDP session altogether, you can protect it. When the LDP session between two directly connected LSRs is protected, a targeted LDP session is built between the two LSRs. When the directly connected link does go down between the two LSRs, the targeted LDP session is kept up as long as an alternative path exists between the two LSRs.
For the protection to work, you need to enable it on both the LSRs. If this is not possible, you can enable it on one LSR, and the other LSR can accept the targeted LDP Hellos by configuring the command mpls ldp discovery targeted-hello accept.
Reference: https://www.ccexpert.us/mpls-network/mpls-ldp-session-protection.htmlOr from the referenceat https://www.ciscolive.com/c/dam/r/ciscolive/us/docs/2019/pdf/5eU6DfQV/TECMPL-3201.pdfTroubleshooting LDP IssuesProblem:
I. When a link flaps (for a short time),
…S olution:
+ When LDP session supported by link hello is setup, create a targeted hello to protect the session.
B. Increase a hold-timer to protect the session:
Correct: Increasing the hold-timer allows the session to remain up longer during intermittent link flaps, reducing the frequency of session resets and label binding exchanges. This can stabilize the connection and reduce the impact of a flapping link.
D. Create link dampening on links to protect the session:
Correct: Link dampening is a mechanism that suppresses the effects of flapping links by temporarily penalizing an interface that goes up and down frequently. This helps in stabilizing the network by reducing the frequent exchange of label bindings and other control plane activities.
D and E:
To avoid having to rebuild the LDP session altogether, you can protect it. When the LDP session between two directly
connected LSRs is protected, a targeted LDP session is built between the two LSRs. When the directly connected link
does go down between the two LSRs, the targeted LDP session is kept up as long as an alternative path exists
between the two LSRs. For the protection to work, you need to enable it on both the LSRs. If this is not possible, you
can enable it on one LSR, and the other LSR can accept the targeted LDP Hellos by configuring the command mpls ldp
discovery targeted-hello accept.
Reference:
https://www.ccexpert.us/mpls-network/mpls-ldp-session-protection.html
https://www.ciscolive.com/c/dam/r/ciscolive/us/docs/2019/pdf/5eU6DfQV/TECMPL-3201.pdf
Troubleshooting LDP Issues
Problem:
I. When a link flaps (for a short time),
...
Solution:
+ When LDP session supported by link hello is setup, create a targeted hello to protect the session.
For me these are the closest:
B) Increase a hold-timer to protect the session.
E) Create targeted hellos to protect the session.
E) already explained by others.
https://www.ciscolive.com/c/dam/r/ciscolive/us/docs/2019/pdf/5eU6DfQV/TECMPL-3201.pdf
When link is down, the targeted hello remains through other path and keeps the LDP session up.
B) Please, see:
https://community.cisco.com/t5/mpls/ldp-session-flap-on-a-7600-router/td-p/1926870
- increase the hold queue size on the interface
- also recommend you to configure mpls ldp sync
https://community.cisco.com/t5/mpls/bfd-with-ldp/td-p/2264164
In short, you could save your traffic from a flapping link by using a holddown timer.
Alternatively, you could use IP dampening on the interface which will give it a penalty on every successive flap and keep it down.
But, this will keep the interface completely down rather that the LDP-IGP Sync funda.
...
The holddown timer is used to ensure that IGP waits for LDP to be up for the specified interval before it stops advertizing the maximum metric and makes the route usable.
The question: "Which two solutions are used to overcome a flapping link that causes a frequent label binding exchange between MPLS routers?" implies you want to fix something really good. In my opinion increase hold time feels like a band aid, if the link is unstable having a longer hold down time would not save the day. It might prevent many LDP sessions to be recreated, but it does not feel like something you would take to production. The link dampening seems to result in a more stable environment without any need for much LDP sessions to be re-created as a flapping link is taken out-of-service anyway. Summarizing, I agree with E, but my hunch says go with D (Create link dampening on links to protect the session.), so D+E seems best in production scenarios.
To avoid having to rebuild the LDP session altogether, you can protect it. When the LDP session between two directly connected LSRs is protected, a targeted LDP session is built between the two LSRs. When the directly connected link does go down between the two LSRs, the targeted LDP session is kept up as long as an alternative path exists between the two LSRs.
For the protection to work, you need to enable it on both the LSRs. If this is not possible, you can enable it on one LSR, and the other LSR can accept the targeted LDP Hellos by configuring the command mpls ldp discovery targeted-hello accept.
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