2-2 - Troubleshoot Core IGP Flashcards
Why do most service providers tend to use OSPF or IS-IS as their IGP?
- EIGRP is Cisco proprietary and may hinder interoperability with other vendors.
- MPLS TE requires the help of a link-state protocol, and EIGRP is a hybrid distance vector protocol.
Describe the overall operation of OSPF.
- upon initialization, or due to a change in routing info, the router generates a link-state advertisement (LSA). This is a collection of all link states on the router.
- all routers exchange link states by flooding. Each router that receives a link-state update stores a copy in its link-state database (LSDB) and then propagates the update to other routers.
- After the database of each router is complete, the router calculates the shortest path tree to all destinations, using the Dijkstra algorithm. The destinations, associated cost, and next-hop to reach those destinations form the IP routing table
- If no changes in OSPF network occur, OSPF should be quiet.
T/F: All routers within an area have different LSDBs.
False
T/F: Flooding and calculation of Dijkstra’s algorithm on a router is limited to links within an area.
True
Why do modern networks typically use single-area designs?
Modern networks typically use single-area designs since they allow for simplified MPLS TE designs and newer hardware capabilities make CPU power and memory resources less of a concern than they were in the past.
Describe the following OSPF area type: backbone area
- carries all the routing info
- all other areas must directly connect to the backbone area
- contiguious area
Describe the following OSPF area type: regular area
- nonbackbone areas that directly connect to area 0
Describe the following OSPF area type: stubby area
- areas that do not originate or receive any external routes
Describe the following OSPF area type: totally stubby area
- areas do not originate (redistribution from other protocols) or receive any external routes or summaries (only the default route)
Describe the following OSPF area type: not-so-stubby area (NSSA)
- can originate external routes (redistribution from other protocols), but they do not receive them from other OSPF areas
Describe the following OSPF area type: totally not-so-stubby area (Totally NSSA)
- can originate external routes (redistribution from other protocols), but they do not receive them from other areas
- these areas also do not receive summaries (only the default route)
Describe the following OSPF router type: internal router
- routers with all their interfaces configured in the same area
Describe the following OSPF router type: backbone router
- internal routers within area 0
Describe the following OSPF router type: ABR
- area border router
- belongs to multiple areas and connects these areas to the backbone area.
- ideal design: each ABR connect to two areas only. the backbone area and another area (with 3 areas being the upper limit)
Describe the following OSPF router type: ASBR
- autonomous system boundary router
- connects the OSPF area to a different routing administration (e.g. BGP, EIGRP, etc.)
- the point where external routes can be redistributed into OSPF
IOS-XR command:
router ospf [process ID]
- enters OSPF router configuration mode
- process ID is only locally significant
IOS-XR command:
router ospf [process ID] area [area #] interface [interface]
- enters the area subconfig and enables an interface for OSPF in that area
IOS/IOS-XE command:
router ospf [process ID]
- enters OSPF router configuration mode
- process ID is only locally significant
IOS/IOS-XE command:
network [network address] [wildcard mask] area [area #]
- within the OSPF subconfig, this command is used to match on networks in order to enable associated interfaces for OSPF within the specified area
- this is a legacy method for configuring this
IOS/IOS-XE command:
ip ospf [OSPF process id] area [area #]
- within interface subconfig, this command is used to enable OSPF on the interface in the specified area
- this is a new way of configuring it, introduced in IOS 12.3(11)T and all IOS-XE releases
Describe what this OSPF route abbreviation in the RIB represents:
O
- OSPF intraarea route
- networks within the are of the router
- advertised by means of type 1 (router) and type 2 (network) LSAs
Describe what this OSPF route abbreviation in the RIB represents:
O IA
- OSPF interarea route (summary)
- networks outside of the area of the router but within the OSPF autonomous system
- advertised by means of summary (type 3) and summary ASBR (type 4) LSAs
Describe what this OSPF route abbreviation in the RIB represents:
O E1
- OSPF external type 1 route
- networks outside of the OSPF autonomous system of the router
- advertised by means of external LSAs (LSA type 5)
- adds the costs on links inside the OSPF network to the cost of the external link (more accurate)
Describe what this OSPF route abbreviation in the RIB represents:
O E2
- OSPF external type 2 route
- networks outside of the OSPF autonomous system of the router
- advertised by means of external LSAs (LSA type 5)
- has the cost of the external link no matter how many links it passes through in the internal network (less accurate)
IOS-XR Command:
show ospf
- verify the OSPF routing protocol
IOS-XR Command:
show ospf interface
- verify OSPF interface information