In this article I am going to cover a nice tool for enhancing IS-IS convergence, I am really amazed by the ideas that the guys out there pop up. Inventing such wonderful tools requires intellectual open minds.
Practically speaking in most Service Providers’ backbones the IGP is required to make the loopback IP addresses of the providers’ routers reachable to each other in order to be able to have all the MPLS stuff operational, no other IP addresses information is required to be exchanged by the IGP in the global routing table for things to work out (if the Service Provider is providing internet through the global routing table, this kind of routes are carried via BGP and not the IGP).
IS-IS has a characteristic that has always made it more flexible and scalable (at least in the single area designs) than OSPF, and that is it separates the neighbors and the IP addresses information. It uses IS Neighbors TLVs (or Extended IS Reachability TLVs with wide metrics) and IP Internal/External Reachability Information TLVs (or Extended IP Reachability TLVs with wide metrics).
This well engineered data-structure of IS-IS has leveraged it huge benefits when it came to Partial SPF calculation (AKA Partial Route Computation - PRC) , since it was easy for a router to understand whether their is a topology change that needs full SPF calculation, or is it just IP addresses reachability that would just require a partial SPF calculation.
This well engineered data-structure keeps providing IS-IS with flexibility that features like the one that I am going to talk about today leverages as well. Cisco names the feature “IS-IS Mechanism to Exclude Connected IP Prefix from LSP Advertisements”. It is all about minimizing the database that is required to be flooded, synchronized, stored and processed on all the network routers and thus enhancing the convergence process.
Back to the second paragraph, since in most service providers’ backbones only the looback IP addresses reachability is required for proper operation (I am of course talking about a MPLS backbone), thus why not only maintain such information and discard any other useless information.
Cisco has introduced two mechanisms for this feature (check Cisco’s documentation):
The logic behind this feature is extremely simple; “don’t advertise useless IP addresses reachability information” by not advertising their IP reachability TLVs in the LSPs.
This feature affects only connected interfaces running IS-IS, it has nothing to do with redistribution, and accordingly any redistributed IP information is never affected by this information.
Find below the lab illustration of this feature - this is my fast lab with six inline routers: CE1(R5) <–> PE1(R1) <–> P1(R2) <–> P2(R3) <–> PE2(R4) <–> CE2(R6).
Note that in the first output both the connected IP information (12.12.12.0/24) and the redistributed IP information (111.111.111.111/32) are received by P2, while in the second output, after configuring this feature on PE1 only the connected IP information disappears, while the redistributed IP information persists, since this command excludes the connected IP prefixes only.
Before configuring the feature on PE1
P2(R3)#sh isis database verbose
IS-IS Level-2 Link State Database:
LSPID
LSP Seq Num
LSP Checksum
LSP Holdtime
ATT/P/OL
PE1(R1).00-00
0x000002FD
0x6CF8
1191
0/0/0
Area Address:
49.0001
NLPID:
0xCC
Hostname:
PE1(R1)
IP Address:
1.1.1.1
Metric: 10
IS-Extended P1(R2).00
Metric: 0
IP 1.1.1.1/32
Metric: 10
IP 12.12.12.0/24
Metric: 0
IP 111.111.111.111/32
P1(R2).00-00
0×000002FE
0×5509
515
0/0/0
Area Address:
49.0001
NLPID:
0xCC
IP Address:
2.2.2.2
Metric: 10
IS-Extended PE1(R1).00
Metric: 10
IS-Extended P2(R3).01
Metric: 0
IP 2.2.2.2/32
Metric: 10
IP 12.12.12.0/24
Metric: 10
IP 23.23.23.0/24
After configuring the feature on PE1
P2(R3)#sh isis database verbose
IS-IS Level-2 Link State Database:
LSPID
LSP Seq Num
LSP Checksum
LSP Holdtime
ATT/P/OL
PE1(R1).00-00
0x000002FE
0xB6FB
1183
0/0/0
Area Address:
49.0001
NLPID:
0xCC
Hostname:
PE1(R1)
IP Address:
1.1.1.1
Metric: 10
IS-Extended P1(R2).00
Metric: 0
IP 1.1.1.1/32
Metric: 0
IP 111.111.111.111/24
P1(R2).00-00
0×000002FE
0×5509
474
0/0/0
Area Address:
49.0001
NLPID:
0xCC
Hostname:;
P1(R2)
IP Address:
2.2.2.2
Metric: 10
IS-Extended PE1(R1).00
Metric: 10
IS-Extended P2(R3).01
Metric: 0
IP 2.2.2.2/32
Metric: 10
IP 12.12.12.0/24
Metric: 10
IP 23.23.23.0/24
I hope that I’ve been informative.
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