CartFor many service provider networks, the only addresses that need to be in the route table are those of the loopback addresses for the P and PE devices. IS-IS makes this policy very easy to implement with the use of the “advertise passive-only” command under the router isis configuration mode.
Consider a network of 3 devices: PE1, P1 and PE2. PE1 has the following configuration:
router isis
net 49.0100.0000.0000.0001.00
advertise passive-only
log-adjacency-changes
passive-interface Loopback0
Each of the other devices has the loopback interface advertised with a passive-interface command and the advertise passive-only command configured as well. The route table of PE1 looks as follows:
PE1#sho ip route isis
10.0.0.0/32 is subnetted, 3 subnets
i L1 10.1.1.2 [115/10] via 192.168.0.2, Serial1/0
i L1 10.1.1.3 [115/20] via 192.168.0.2, Serial1/0
PE1#
Only the loopbacks appear, the connected network between P1 and PE2 does not. Let’s look at the ISIS database before and after the command. The connected route no longer shows up because it is not advertised with the passive-interface command.
Level-1 PE1 LSP Before:
PE1#sho isis database PE1.00-00 de l1
IS-IS Level-1 LSP PE1.00-00
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
PE1.00-00 * 0×00000005 0×3384 1153 0/0/0
Area Address: 49.0100
NLPID: 0xCC
Hostname: PE1
IP Address: 10.1.1.1
Metric: 10 IP 192.168.0.0 255.255.255.252
Metric: 0 IP 10.1.1.1 255.255.255.255
Metric: 10 IS P1.00
Level-1 PE1 LSP After:
PE1#sho isis database PE1.00-00 de l1
IS-IS Level-1 LSP PE1.00-00
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
PE1.00-00 * 0×00000006 0×4371 1196 0/0/0
Area Address: 49.0100
NLPID: 0xCC
Hostname: PE1
IP Address: 10.1.1.1
Metric: 0 IP 10.1.1.1 255.255.255.255
Metric: 10 IS P1.00
Regards, Bryan (Post by Bryan Bartik)
Tags: ccie lab, CCIE lab training, CCIE Service Provider, CCIE SP Lab, Cisco IS-IS, r&s
