I'm studying for the bsci, and I'm doing some IS-IS tests. My topology is very simple: there are three routers (ra, rb, and rc), connected via serial lines, each router has a LAN attached.
ra --conn1-- rb --conn2-- rc | | | LANa LANb LANc
LANa: 10.0.0.0/24 LANb: 10.0.1.0/24 LANc: 10.0.2.0/24 conn1: 172.16.0.0/24 (ra is .2, rb is .1) conn2: 172.17.0.0/24 (rb is .1, rc is .2)
IS-IS is enabled on all routers on all interfaces, with the default settings (ie, each router is a l1-l2). All routers are in the same area.
Now, I do a "sh isis database detail" on router ra. The l1 database looks as expected: each router advertises one LSP, containing IP reachability info for the connected IP networks. I expected the l2 database to contain the same info. But instead, the l2 database shows more entries for each LSP:
ra#show isis database detail [cut] IS-IS Level-2 Link State Database: LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL ra.00-00 * 0x00000004 0x4234 442 0/0/0 Area Address: 49.0001 NLPID: 0xCC Hostname: ra IP Address: 172.16.0.2 Metric: 10 IS rb.00 Metric: 10 IP 10.0.0.0 255.255.255.0 Metric: 20 IP 10.0.1.0 255.255.255.0 Metric: 30 IP 10.0.2.0 255.255.255.0 Metric: 10 IP 172.16.0.0 255.255.255.0 Metric: 20 IP 172.17.0.0 255.255.255.0 [cut]
why does ra advertise reachability for networks that are not connected (eg, 10.0.2.0 or 172.17.0.0)?
And, how is the address shown in the line "IP Address:" chosen for routers that have more than one IP address? For example, for rb LSP it's 172.17.0.1. Why is that one chosen, instead of 172.16.0.1 or 10.0.1.x? Is this the same "highest IP address" rule used for eg ospf router id?
Thanks for any help.