Ch6 MultiArea OSPF Flashcards
1
Q
Three Issues of Single-Area OSPF
A
- Large Routing Table - no route summarization resulting in large tables.
- Large Link-State Database = results in larger memory requirements.
- Frequent SPF Algorithm Calculations = Larger networks = more changes = more calculations.
2
Q
Three Advantages of Multi-Area OSPF
A
- Smaller routing tables - routers within area summarize routes to other areas.
- Reduced link-state update overhead = fewer routers = fewer LSAs = less memory needs.
- Reduced frequency of SPF Calculations - Restricts topology changes to area within which the change occurs.
3
Q
Multi-Area OSPF Two-Layer Hierarchy
A
- Backbone (Transit) Area - Area 0
- Regular (Non-Backbone) Area
4
Q
Three Characteristics of Backbone Area
A
- Often connects other OSPF types.
- End users usually not found in this area.
- Usually called area 0.
5
Q
Two Characteristics of Regular Areas
A
- Set up along functional / geographic boundaries.
- Does not allow traffic from another area to use its links to reach other areas by default; all traffic from other areas must cross a transit area.
6
Q
Cisco Multi-Area OSPF Guidelines (3)
A
- Area should not have more than 50 routers.
- Router should not be in more than 3 areas.
- Any single router should not have more than 60 neighbors.
7
Q
Four Types of OSPF Routers
A
- Internal Router - All interfaces in same area.
- Backbone Router
- Area Border Router (ABR) - Interfaces attached to multiple areas.
- Autonomous System Boundary Router - Has at least one interface attached to an external internetwork.
8
Q
Five Characteristics of Area Border Routers
A
- Must maintain separate LSDB’s for each area connected to.
- Can route between areas.
- ABR’s are exit points for an areas.
- Can be configured to summarize routing information from LSDBs of their attached areas.
- Multiple ABRs can exist within one multi-area OSPF network.
9
Q
Route Redistribution
A
Process an ASBR uses to import non-OSPF network information to an OSPF network.
10
Q
Three Characteristics of OSPF LSA Types
A
- OSPF has 11 different LSA Types.
- Any OSPF implementation must use LSA Types : 1 to 5
- Each router link is identified by an LSA type.
11
Q
Six Characteristics of Type 1 LSAs
A
- aka Router Link Entries
- Used to advertise directly connected OSPF-enabled links.
- All routers generate Type 1s
- LSA Type 1s area flooded within an area , stay within that area.
- Type 1 LSA link-state ID is identified by the router ID of the originating router.
- Uses external bit (e-bit ) to identify ASBRs
12
Q
Six Characteristics of Type 2 LSAs
A
- aka Network Link Entries
- Contains Router ID and IP address of DR as well as Router ID of all other routers on the multiaccess segment.
- DRs generate Type 2 LSAs
- Do not extend beyond the area they originate in.\
- Identified by the DR Router ID.
- Only exist for multi-access and nonbroadcast-multi-access network that have DR and at least 2 routers on the multi-access segment.
13
Q
Five Characteristics of Type 3 LSAs
A
- Describes a network address learned by a Type 1 LSA
- Required for every subnet.
- ABRs flood Type 3s to other areas and are regenerated by other ABRs
- Identified by the network address.
- By default, not summarized ( but networks should be summarized manually)
14
Q
Four Characteristics of Type 4 LSAs
A
- Used to advertise route to an ASBR (Autonomous System Boundary Router) to other areas and provide route to ASBR
- ABRs generate Type 4 LSAs when an ASBR exists within the area.
- Generated by the originating ABR and regenerated by other ABRs
- Identified by Router ID of ASBR
15
Q
Five Characteristics of Type 5 LSAs
A
- aka Autonomous System External Entries
- Used to advertise external, non-OSPF network addresses.
- ASBRs generate Type 5’s for each external route it has.
- Type 5’s are flooded throughout area and regenerated by other ABRs
- By default, routes are not summarized (but should be)
16
Q
Three Types of OSPF Routing Table Entries
A
- O // Intra area route created by Type 1 and Type 2 LSAs
- O IA // Interarea routes created from Summary LSAs
- O E1 or O E2 // External LSAs appearing as Type 1 External (E1) or Type 2 External (E2)
17
Q
OSPF Route Calculation Order (3)
A
- Type 1 and 2 LSAs used to calculate best paths to intra-area destinations. ( O )
- Type 3 and 4 LSAs used to calculate best paths to inter-area destinations. (O IA )
- Type 5 LSAs used to calculate best baths to external autonmous destinations. (O E1 or O E2)
18
Q
Four Steps to Implementing OSPF
A
- Gather network requirements and parameters
- Define the OSPF parameters - select Single or Multiarea
- Configure OSPF on devices
- Verify OSPF on devices.
19
Q
Three Extra Considerations for MultiArea OSPF
A
- IP Addressing Plan - Detailed including subnets.
- OSPF Areas - Indentify ABRs and ASBRs as well as summarization and redistribution.
- Network Topology
20
Q
Configuring MultiArea OSPFv2 (IPv4) - 3 Areas
A
- router ospf process_id
- router-id router_id
- network network_id wildcard_mask area 1st_area_id
- network network_id wildcard_mask area 2nd_area_id
- network network_id wildcard_mask area 3rd_area_id
21
Q
Configuring MultiArea OSPFv3 (IPv6) - 2 Areas
A
- ipv6 router ospf process_id
- router-id router_id
- exit
- interface interface_id
- ipv6 ospf process_id area 1st_area_id
- interface interface_id
- ipv6 ospf process_id area 2nd_area_id
22
Q
What do ABRs use to Summarize Routes
A
- Type 3 LSAs used to summarize Interarea Routes
23
Q
What do ASBRs use to Summarize Routes
A
- Type 5 LSAs are used to summarize external routes.
24
Q
Command Used to Configure External Route Summarization on ASBR
A
summary-address network_summary_address subnet_mask
25
Calculating Summary Routes (3)
1. List all addresses in binary format.
2. Count number of far-left matching bits to determine the mask for the summary route.
3. Copy the matching bits and add zero bits to create the summarized network address.
26
Command Used to Configure Interarea Summary Routes on ABRs
**area** *area\_id\_to\_be\_summarized* **range** *network\_address subnet\_mask*
// Router Config Command
27
Three Commands Used to Verify Multiarea OSPF
1. **show ip ospf neighbor**
2. **show ip ospf**
3. **show ip ospf interface**
28
Four Commands Used to Verify Specific Multiarea Information
1. **show ip protocols**
2. **show ip ospf interface brief**
3. **show ip route ospf**
4. **show ip ospf database**
29
List Some of the Important Information from ***show ip protocols.*** (3)
1. Router ID
2. Number of areas in the router.
3. Networks included within the routing protocol configuration.
30
List Some of the Important Information from ***show ip ospf interface brief*** (3)
1. OSPF Process ID an interface is assigned to.
2. Area the interfaces are assigned to.
3. Cost of the Interfaces.
