Routing Protocols Flashcards

What is the default OSPF cost formula in Junos?

Answer: 100 Mbps / interface bandwidth (with minimum cost of 1)

Junos OSPF uses the standard cost formula: reference bandwidth (default 100 Mbps) divided by interface bandwidth, with a minimum of 1. A 100 Mbps interface has cost 1; a 10 Mbps interface has cost 10.

What is BGP 'communities' and how are they used?

Answer: BGP communities are route tags (32-bit values) attached to prefixes to signal routing policies across AS boundaries

BGP communities are optional transitive attributes that tag routes with 32-bit values. They are used to signal routing policies between ASes, such as which routes to accept, prefer, or not advertise further.

In Junos, what is the purpose of a routing policy 'term' order?

Answer: Terms are evaluated in order from first to last; the first matching term's action is applied and evaluation stops

In Junos routing policies, terms are evaluated sequentially. The first term that matches all conditions executes its action and stops further evaluation, making term order critical to policy correctness.

What does the Junos 'next-hop self' BGP configuration accomplish?

Answer: Forces the BGP router to advertise its own address as the next-hop for routes it reflects or re-advertises to iBGP peers

'Next-hop self' is configured on iBGP sessions to ensure that the router sets itself as the next-hop in BGP updates, solving the case where iBGP peers cannot resolve the original eBGP next-hop.

How does Junos handle ECMP (Equal-Cost Multi-Path) routing?

Answer: Installs multiple equal-cost routes and load-balances traffic across them using a hash of packet fields

Junos supports ECMP by installing multiple routes with equal cost into the forwarding table and distributing traffic across them using a hash of source IP, destination IP, protocol, and port numbers.

What is the significance of OSPF Router ID in Junos?

Answer: A unique 32-bit identifier for each OSPF router in the domain, used to identify LSAs and break DR/BDR election ties

The OSPF Router ID is a 32-bit value (written in dotted-decimal) that uniquely identifies each OSPF router. It is included in all LSAs and is used to break ties in DR/BDR elections when priorities are equal.