In my last ISIS tutorial, I mentioned that while ISIS and OSPF are both link state protocols, their actual operation differs greatly. To pass the BSCI exam and earn your CCNP, you'll need to know these differences! Today, we'll take a look at ISIS Hello types and the adjacency types that form through the use of these Hellos.
Hello packets have been mentioned several times with ISIS, and with good reason. Hello packets are the heartbeat of OSPF and ISIS when heartbeats are no longer heard from a neighbor, that adjacency will be dropped. A major difference between OSPF and ISIS is that OSPF has one type of Hello packet, where ISIS actually has three!
An ES Hello (ESH) is send by all End Systems, and all IS devices listen for this Hello. This is how a router (IS) discovers a host (ES).
An IS Hello (ISH) announces the presence of an IS. An IS Hello is sent by all IS devices, and End Systems listen for these hellos.
An IS-to-IS Hello (IIH) is used by an IS to discover other ISes and to form adjacencies with them.
An interesting side note: A router will send an IIH to another router on the link to form or maintain an adjacency, but it will still send an ISH as well in case there are end systems located on that segment.
ISIS and OSPF both create and maintain adjacencies with the Hello packet. Let's take a look at the rules regarding ISIS adjacencies as well as the adjacency types.
L1 and L2 Hellos are different messages, so an L1 router must exchange Hellos with another L1 router to form an adjacency, just as L2 routers form adjacencies with L2 routers. L1 routers can only form an adjacency with an L2 router if one of the two routers involved is actually an L1/L2 router.
L1 routers must be in the same area in order to form an adjacency. The Hello timers, as well as the MTU, must match between the interfaces used to form the adjacency.
That's a lot of L1, L2, and L1/L2, isn't it? Let's review the adjacencies each router type can form:
L1: Can form adjacency with any L1 in the same area and any L1/L2 in the same area.
L2: Can form adjacency with any L2 in any area, and with an L1/L2 in any area.
L1/L2: Can form adjacency with any L1 in the same area, L1/L2 in any area, and L2 in any area.
Knowing the similarities and differences regarding ISIS and OSPF is vital for CCNP exam success. Take your time, master the fundamentals, and before long the magic letters “CCNP” are behind your name and on your resume!
Showing posts with label L1/L2. Show all posts
Showing posts with label L1/L2. Show all posts
Thursday, December 25, 2008
Cisco CCNP / BSCI Exam Tutorial: ISIS Router Types
To pass the BSCI exam and earn your CCNP, you've got to know ISIS inside and out. There are many similarities between ISIS and OSPF, but one major difference is that ISIS has three different types of routers - Level 1 (L1), Level 2 (L2), and L1/L2.
L1 routers are contained in a single area, and are connected to other areas by an L1/L2 router. The L1 uses the L1/L2 router as a default gateway to reach destinations contained in other areas, much like an OSPF stub router uses the ABR as a default gateway.
L1 routers have no specific routing table entries regarding any destination outside their own area; they will use an L1/L2 router as a default gateway to reach any external networks. ISIS L1 routers in the same area must synchronize their databases with each other.
Just as we have L1 routers, we also have L2 routers. Anytime we're routing between areas (inter-area routing), an L2 or L1/L2 router must be involved. All L2 routers will have synchronized databases as well.
Both L1 and L2 routers send out their own hellos. As with OSPF, hello packets allow ISIS routers to form adjacencies. The key difference here is that L1 routers send out L1 hellos, and L2 routers send out L2 hellos. If you have an L1 router and an L2 router on the same link, they will not form an adjacency.
An ISIS router can act as an L1 and an L2 router at the same time; these routers are L1/L2 routers. An L1/L2 router can have neighbors in separate ISIS areas. The L1/L2 router will have two separate databases, though - one for L1 routes and another for L2 routes. L1/L2 is the default setting for Cisco routers running ISIS. The L1/L2 router is the router that makes it possible for an L1 router to send data to another area.
In the next part of my ISIS tutorial, we'll take a more detailed look at those ISIS hellos!
L1 routers are contained in a single area, and are connected to other areas by an L1/L2 router. The L1 uses the L1/L2 router as a default gateway to reach destinations contained in other areas, much like an OSPF stub router uses the ABR as a default gateway.
L1 routers have no specific routing table entries regarding any destination outside their own area; they will use an L1/L2 router as a default gateway to reach any external networks. ISIS L1 routers in the same area must synchronize their databases with each other.
Just as we have L1 routers, we also have L2 routers. Anytime we're routing between areas (inter-area routing), an L2 or L1/L2 router must be involved. All L2 routers will have synchronized databases as well.
Both L1 and L2 routers send out their own hellos. As with OSPF, hello packets allow ISIS routers to form adjacencies. The key difference here is that L1 routers send out L1 hellos, and L2 routers send out L2 hellos. If you have an L1 router and an L2 router on the same link, they will not form an adjacency.
An ISIS router can act as an L1 and an L2 router at the same time; these routers are L1/L2 routers. An L1/L2 router can have neighbors in separate ISIS areas. The L1/L2 router will have two separate databases, though - one for L1 routes and another for L2 routes. L1/L2 is the default setting for Cisco routers running ISIS. The L1/L2 router is the router that makes it possible for an L1 router to send data to another area.
In the next part of my ISIS tutorial, we'll take a more detailed look at those ISIS hellos!
Subscribe to:
Posts (Atom)
