Adjust and Troubleshoot Single Area OSPF, Multi-Area OSPF, Summarisation and BGP Flashcards
Learn the Qs (17 cards)
Why would some routers show a neighbour state of ‘2-WAY/DROTHER’?
This occurs on multi-access networks where a router is not the DR or BDR. ‘2-WAY’ means Hello packets were seen, and ‘DROTHER’ means the router will not exchange LSAs directly — only with the DR.
What advice would you give for decreasing OSPF timers to reduce convergence time?
Lower the Hello and Dead intervals (e.g., Hello = 1s, Dead = 4s). This speeds up failure detection, but both ends must match or adjacencies will fail. Avoid flapping by using on stable links.
Why and how should OSPF updates be authenticated?
To prevent malicious or accidental routing updates. Use MD5 or plain-text authentication. Enable it on interfaces and set keys using ip ospf authentication message-digest and ip ospf message-digest-key.
What are the main LSA types in Multi-Area OSPF?
Type 1: Router LSA, Type 2: Network LSA (from DR), Type 3: Summary LSA (from ABR), Type 4: ASBR Summary LSA, Type 5: External LSA (from ASBR)
Describe the OSPF two-layer hierarchy.
OSPF uses a backbone area (Area 0) and one or more regular areas. All inter-area traffic must go through Area 0. This supports scalability and isolates topology changes.
What are the benefits of hierarchical OSPF routing?
Reduces routing table size, limits LSA flooding to within areas, decreases SPF recalculations, simplifies troubleshooting and enhances scalability.
How is BGP relevant to an organisation’s Internet connectivity?
BGP connects different autonomous systems. It enables organisations to advertise public IP prefixes and control traffic flow in/out of their network using routing policies.
How does BGP compare to Interior Gateway Protocols (IGPs)?
BGP is policy-based and uses path attributes, not metrics. It operates over TCP, unlike IGPs which are faster but metric-based. BGP is suited for inter-AS routing.
Compare Internal BGP (iBGP) and External BGP (eBGP).
iBGP peers are in the same AS and do not need to be directly connected. eBGP peers are in different ASes and must be directly connected. iBGP needs full mesh or route reflectors.
In the context of BGP, what is a ‘path vector’?
A list of AS numbers that a route has passed through. It prevents routing loops and supports policy-based routing decisions.
What are IGPs and EGPs?
IGPs (e.g., OSPF, EIGRP) are used within an AS. EGPs (e.g., BGP) are used between ASes.
What does a route marked ‘O*E2’ in a routing table indicate?
It’s an OSPF external route learned from an ASBR, with a default external metric (E2 = cost does not increment across hops).
What is an autonomous system and how is it identified?
A group of routers under a single administrative domain. It is identified by an Autonomous System Number (ASN), assigned by IANA or an RIR.
What is multihoming in BGP?
Connecting to multiple ISPs for redundancy and load balancing. Requires BGP to advertise routes and select optimal paths.
What are the key steps in BGP path selection?
- Highest Local Preference, 2. Shortest AS Path, 3. Lowest origin type, 4. Lowest MED, 5. eBGP preferred over iBGP, 6. Lowest IGP metric to next hop, 7. Lowest Router ID
What should a network manager ask when meeting an ISP for BGP setup?
ASN allocation, IP prefix advertisement rules, default route provision, redundancy options, route filtering policies, and traffic engineering support.
When should a network manager avoid implementing BGP?
Only one ISP connection, limited router CPU/memory, lack of BGP knowledge, no need for traffic engineering or full routing table.
