Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Loop Avoidance

Distance vector uses split horizon and route poisoning for loop avoidance. What techniques are used in MPLS for loop avoidance?

Thanks,

John

3 REPLIES
Silver

Re: Loop Avoidance

MPLS uses good old IP TTL to avoid loops. MPLS works on top of routing protocols and is not a routing protocol in itself. It is a switching mechanism ( a general definition). loop avoidance stil depends on your routing protocol.

HTH

Bronze

Re: Loop Avoidance

Just like TTL in the IP header, MPLS header also has a 1-byte TTL

When an IP packet is first labelled, the (IP TTL -1) is copied to the MPLS TTL

When the label is removed, the MPLS TTL value of removed label is copied to the either MPLS TTL of inner label or IP TTL field (if no inner label), provided

MPLS TTL < IP TTL

And then, IP TTL is decremented

HTH

New Member

Re: Loop Avoidance

Frame Mode MPLS utilises the underlying IGP for loop avoidance. When populating the LFIB it is the label in the LIB chosen based on the next hop to the destination from the local routing table.

So really loop avoidance is under the control of SPF (OSPF or ISIS)

The TTL is there to bin a packet should a loop ensue.

Cell Mode MPLS is a little different.

879
Views
0
Helpful
3
Replies
CreatePlease to create content