Loop Avoidance

[jrgarrigues]

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

Thanks,

John

[attrgautam]

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

[romccallum]

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

[scollinson]

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.