ISIS Link Overload

It is useful for routers in IS-IS routing domain to be able to advertise a link being in overload state to indicate impending maintenance activity on the link. This document provides mechanisms to advertise link overload state in the Link attributes TLV as defined in

The link-attribute sub-TLV is carried within the TLV 22 and has a format identical to the sub-TLV format used by the Traffic Engineering Extensions for IS-IS (): 1 octet of sub-type, 1 octet of length of the value field of the sub-TLV followed by the value field -- in this case, a 16 bit flags field. The following bit represents the Link in overload. Link Overload: 0x04 When set, this indicates that the link is overloaded.

The Link attributes sub TLV with link-overload bit set indicates that the Link which carries the sub TLV is overloaded. The node that has the link going for maintenance, sets metric of the link to MAX-METRIC and re-originates the LSP. The metric in the reverse direction also need to change to divert the traffic from reverse direction. The node SHOULD originate Link attributes sub TLV and set the overload bit and originate the LSP and flood it in the respective IS-IS level. When the originator of the Link attributes sub TLV, purges the LSP or re-originates it without the Link Overload bit set, the metric on the remote node SHOULD be changed back to the original value. Based on the link type of the overloaded link, actions listed below MAY be taken by the receiver.

When a link attributes sub TLV with link overload bit set is received for a point-to-point link the receiver SHOULD identify the local link which corresponds to the overloaded link and set the metric to MAX-METRIC. Receiver node MUST re-originate the LSP with the changed metric and flood into the ISIS level.

Broadcast networks in ISIS are represented by a star topology where the Designated Intermediate System (DIS) is the central point to which all other routers on the broadcast network connect. As a result, routers on the broadcast network advertise only their adjacency to the pseudo-node.As a result, routers on the broadcast network advertise only their adjacency to the pseudo- node. Routers that do not act as DIS do not advertise adjacencies with each other. DIS originates pseudo-node which contains adjacenices with all the neighbors. For the Broadcast links, the MAX-METRIC on the outgoing link cannot be changed since all the adjacencies are on same link. Setting the link cost to MAX- METRIC would impact paths going via all neighbors. When a link-attributes sub TLV with link-overload bit set is received by the remote end for a broadcast link. - If it’s non DIS for that link, SHOULD not take any action. - If receiving node is DIS for the link, it MUST set the metric from the pseudo-node to the originator of the link overload bit to MAX-METRIC and MUST re-originate the pseudo-node LSP and flood into the ISIS Level.

The mechanism described in the document is fully backward compatible.It is required that the originator and receiver of link-overload bit understand the extensions defined in this document and in case of broadcast links the originator and the DR need to understand the extensions. Other nodes in the network compute based on increased metric and hence the feature is backward compatible.

This document does not introduce any further security issues other than those discussed in and .

This specification updates one ISIS registry: ISIS Link attributes Sub TLV i) 0x04 - Link overload bit