Nov 29 2008
Traffic Engineering – IGP extensions (IS-IS)
Some extensions were added to ISIS to provide resource information to the TE process.
IS-IS Extenstions added to support TE
TLVs have been defined to extend IS-IS functionality to include it amongst the protocols of choice for TE.
Informational RFC 3784 details the TLVs which have been added To IS-IS to support TE. RFC 3874 also introduces the concept of sub-TLVs.
Sub-TLVs have been added to IS-IS. Sub-TLVs are identical to regular TLVs in their structure and layout. They differ however with respect to their location. Sub-TLVs are found within regular TLVs which in turn are found in IS-IS packets.
Sub-TLVs are used to add additional information to the parent TLV.
RFC 3784 extends IS-IS with the following TLVs
1-The Extended IS reachability TLV (TLV type 22). (see section 3 RFC 3784)
2-The Extended IP Reachability TLV (TLV type 135). (see section 4 RFC 3784)
3-The Traffic Engineering Router ID TLV (TLV type 134). (see section 5 RFC 3784)
The Extended IS reachability TLV (TLV type 22)
TLV22 contains many sub-TLVs, the key sub-TLVs required for TE are
Sub-TLV 9: Maximum link bandwidth (section 3.4)
Sub-TLV 10: Maximum reservable link bandwidth (section 3.5)
sub-TLV 11: Unreserved bandwidth (section 3.6)
The Extended IP Reachability TLV (TLV type 135)
TLV135 removes the previous restriction which only allowed a metric range of 0-63. TLV135 provides a 32 bit.
TLV 135 also added one bit(up/down but) to indicate a prefix has been redistributed from level-2 to level-1. This bit is required to prevent routing loops. When a prefix is avertised from level-2 to level-1 the up/down bit is set to 1. Prefixes with the up/down bit set to 1 cannot be advertised to level-2. This is used for loop prevention.
The Traffic Engineering Router ID TLV (TLV type 134)
The router ID TLV contains the 4 octet router ID of the router.
An Important note:- If the loopback address is used as the TE router ID and it is advertised in the IGP as well as BGP then the BGP router ID should be identical to the TE router ID.
