© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Configuring OSPF Link-State Advertisements

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Issues with Maintaining a Large OSPF Network

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v The Solution: OSPF Hierarchical Routing Consists of areas and autonomous systems Minimizes routing update traffic

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Types of OSPF Routers

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Virtual links are used to connect a discontiguous area to area 0. A logical connection is built between router A and router B. Virtual links are recommended for backup or temporary connections. Defining Virtual Links

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Configuring Virtual Links area area-id virtual-link router-id [authentication [message-digest | null]] [hello-interval seconds] [retransmit-interval seconds] [transmit-delay seconds] [dead-interval seconds] [[authentication- key key] | [message-digest-key key-id md5 key]] Router(config-router)# Creates a virtual link remoterouter#sh ip ospf Routing Process "ospf 1000" with ID Supports only single TOS(TOS0) routes Supports opaque LSA Supports Link-local Signaling (LLS) Supports area transit capability It is an area border router

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF Virtual Link Configuration Example

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v RouterA#sh ip ospf virtual-links Virtual Link OSPF_VL0 to router is up Run as demand circuit DoNotAge LSA allowed. Transit area 1, via interface Serial0/0/1, Cost of using 781 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:07 Adjacency State FULL (Hello suppressed) Index 1/2, retransmission queue length 0, number of retransmission 1 First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0) Last retransmission scan length is 1, maximum is 1 Last retransmission scan time is 0 msec, maximum is 0 msec RouterA# The show ip ospf virtual-links Command

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA TypeDescription 1Router LSAs 2Network LSAs 3 or 4Summary LSAs 5 Autonomous system external LSAs 6Multicast OSPF LSA 7Defined for not-so-stubby areas 8 External attributes LSA for Border Gateway Protocol (BGP) 9, 10, 11Opaque LSAs LSA Types

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA Type 1: Router LSA One router LSA (type 1) for every router in an area –Includes list of directly attached links –Each link identified by IP prefix assigned to link and link type Identified by the router ID of the originating router Floods within its area only; does not cross ABR

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA Type 2: Network LSA One network (type 2) LSA for each transit broadcast or NBMA network in an area –Includes list of attached routers on the transit link –Includes subnet mask of link Advertised by the DR of the broadcast network Floods within its area only; does not cross ABR

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA Type 3: Summary LSA Type 3 LSAs are used to flood network information to areas outside the originating area (interarea) –Describes network number and mask of link. Advertised by the ABR of originating area. Regenerated by subsequent ABRs to flood throughout the autonomous system. By default, routes are not summarized, and type 3 LSA is advertised for every subnet.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA Type 4: Summary LSA Summary (type 4) LSAs are used to advertise an ASBR to all other areas in the autonomous system. They are generated by the ABR of the originating area. They are regenerated by all subsequent ABRs to flood throughout the autonomous system. Type 4 LSAs contain the router ID of the ASBR.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA Type 5: External LSA External (type 5) LSAs are used to advertise networks from other autonomous systems. Type 5 LSAs are advertised and owned by the originating ASBR. Type 5 LSAs flood throughout the entire autonomous system. The advertising router ID (ASBR) is unchanged throughout the autonomous system. Type 4 LSA is needed to find the ASBR. By default, routes are not summarized.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Interpreting the OSPF Database RouterA#show ip ospf database OSPF Router with ID ( ) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count x x00401A x x003A1B x800002D7 0x00EEA9 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum x x004EC9 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum x x00FB x x00F516

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Interpreting the Routing Table: Types of Routes Router DesignatorDescription O OSPF intra-area (router LSA) and network LSA Networks from within the area of the router Advertised by way of router LSAs and network LSA O IA OSPF interarea (summary LSA) Networks from outside the area of the router, but within the OSPF autonomous system Advertised by way of summary LSAs O E1Type 1 external routes Networks outside of the autonomous system of the router Advertised by way of external LSAs O E2Type 2 external routes

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Calculating Costs for E1 and E2 Routes

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v The show ip route Command RouterB>show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set /24 is subnetted, 2 subnets O IA [110/1563] via , 00:12:35, FastEthernet0/0 O IA [110/782] via , 00:12:35, FastEthernet0/ /8 is variably subnetted, 6 subnets, 2 masks C /32 is directly connected, Loopback0 C /24 is directly connected, Serial0/0/0 O /24 [110/782] via , 00:12:35, Serial0/0/0 C /24 is directly connected, FastEthernet0/0 O /24 [110/782] via , 00:12:37, FastEthernet0/0 O E /24 [110/50] via , 00:12:37, FastEthernet0/0

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF LSDB Overload Protection Excessive LSAs generated by other routers can drain local router resources. This feature can limit the processing of non-self-generated LSAs for a defined OSPF process. max-lsa maximum-number [threshold-percentage] [warning- only] [ignore-time minutes] [ignore-count count-number] [reset-time minutes] Router(config-router)#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v ip ospf cost interface-cost Overrides the default cost calculation. Values from 1 to can be defined. Changing the Cost Metric auto-cost reference-bandwidth ref-bw Sets the reference bandwidth to values other than 100 Mbps (legal values range from 1 to 4,294,967 in megabits per second). Dijkstras algorithm determines the best path by adding all link costs along a path. The cost, or metric, is an indication of the overhead to send packets over an interface. Default = (100 Mbps) / (bandwidth in Mbps). RouterA(config-if)# RouterA(config-router)#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary There are four OSPF router types: internal routers, backbone routers, ABRs, and ASBRs. A virtual link allows discontiguous area 0 to be connected, or a disconnected area to be connected to area 0, via a transit area. Virtual links should be used only for temporary connections or backup after a failure, not as a primary backbone design feature. There are 11 OSPF LSA types. The first five are the most commonly used: –Type 1 router –Type 2 network –Type 3 and 4 summary –Type 5 external

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary (Cont.) In the IP routing table, OSPF routes are classified as either intra-area, interarea, or external; external routes are subdivided into E1 and E2. OSPF LSDB overload protection limits the processing of non- self-generated LSAs. The OSPF cost defaults to (100 Mbps) / (bandwidth in megabits per second). The cost can be changed on a per- interface basis, and the reference bandwidth (100 Mbps) can also be changed.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v