Networking Interview Question With Answer [Routing Protocol Theory]

Routing Protocols Primary Functions

1. Learn routing information from neighboring routers
2. Advertise routing information to neighboring routers
3. Best Route – If there’s more than one route to a subnet, use logic to pick the best route based on a metric
4. Convergence – if a topology change occurs (due to failure or addition of a new route), advertise the change and pick new best route.

IGP vs EGP

• IGP – Interior Gateway Protocols
• EGP – Exterios Gateway Protocols

IGP Routing Protocols Algorithm

• Distance Vector (Bellman-Ford)
• Link-State
• Balanced Hybrid

Metrics

• RIP – Hop Count
• OSPF – Cost
• EIGRP – Combination of bandwidth and delay

Administrative Distance

• A number that tells the “believability” of a route. The lower the number the better.
• Defaults:
  • Connected Routes = 0
  • Static = 1
  • BGP (external) = 20
  • EIGRP = 90
  • OSPF = 110
  • RIP = 120

Distance Vector Routing Protocol

• A router learns about the distance of a route based on hop count.
  • Distance – uses a metric to calculate the measurement of distance
  • Vector – the outgoing interface that points to the direction of the next-hop router.
• Sends periodic full routing updates. RIP uses a 30-second update interval.
• Full updates are sent by default.
• Split-horizon rules can stop full updates on a route to prevent looping.

Distance Vector Loop Prevention

Route Poisoning

Split Horizon

• A loop prevention mechanism that prevents a routing update to be advertised back to the original route that advertised it.
• It’s a good way of avoiding counting-to-infinity problem.
• on by default

Poison Reverse and Triggered Updates

• Triggered Update – Immediately sends a triggered update when a route fails, instead of waiting for the nest update period.
• Poison Reverse – To learn a failed route, suspend spli-horizon for that route so that the posisoned route can be advertised. It is a poison reverse only when the poisoned route is advertised back to the router from which the failed route was learned.

Holddown Process and Holddown Timer

• Counting to Infinity problems can still cause loops in redundant network despite split-horizon.
• Holddown can prevent loops caused by countin-to-infinity problems in redundant networks.
• The process workds like this: When a router hears of a failed route, start a holddown timer for that route. While the timer is effective, ignore any other information about the failed route. However, that router may accept information from the neighbor that originally advertised the working route before the timer expires.
• The holddown timer defaults to 180 seconds for RIP

Link State Routing Protocols*

• All routers learn complete information about each other including all subnets in the internetwork
• Link-state advertisements (LSAs) holds information about the routers. They are stored in RAM in a data structure called link-state database (LSBD)
• Routers flood LSAs when:
  • they are created
  • on a regular (but long) interval, if the LSAs don’t change over time
  • immediately when an LSA changes
• The LSDB does not contain routes but it contains all the information needed by the routing algorithm to calculate the best routes
• SPF algorithm runs in each router to calculate the best routes (defined by the lowest-cost and lowest metric) to reach each subnets
• Link-state protocols converge quickly by immediately reflooding changed LSAs and rerunning the SPF algorithm on each router.

*Mostly referring to OSPF