External BGP Flashcards
What is the only Exterior Gateway protocol in use today?
BGP
What is meant by “reachability”
What prefixes are available?
Define NLRI
Network Layer Reachability information
What is the difference between and EGP and IGP
Exterior gateway protocols exchange routing information between autonomous systems. Interior gateway protocols exchange routing information within an autonomous system
What is the purpose of the ASN with regard to BGP?
Used for best path selection and loop prevention
What does BGP use in place of a metric?
Path Attributes
What is the purpose of the AS-PATH path attribute?
Lists ASNs in the route, on hop-by-hop basis
Describe a single-homed Internet Edge design
Single link connected to a single ISP
Describe a dual-homed Internet Edge design
Two or more links connected to a single ISP
Describe a multi-homed Internet Edge design
Connections to two or more ISPs, each by a single link
Describe a dual multi-homed Internet Edge design
Connections to two or more ISPs, each by two or more links for redundancy
What is meant by redistribution
advertising externally learned routes into the rest of an organization via an IGP
Does BGP have any concept of being enabled on an interface?
no
Does BGP have any concept of dynamically discovering neighbors?
No. Neighbors are pre-defined
BGP advertises about prefixes added to the BGP table. By what methods can prefixes be added to the BGP table?
via network statements
route redistribution
prefixes learned from peers
How does BGP exchange update messages
via TCP 179
What command verifies that BGP peers are actively exchanging information?
show tcp brief
What commands are used to configure BGP (generic answer—looking for basic config)
router bgp $my-asn
neighbor $neighbor_ip remote-as $their-asn
network $net-id [mask $subnet-mask]
What can be verified by the command show ip bgp summary
remote ASN local ASN Peer IP Local RID neighbor state
How is the BGP router ID determined?
by explicit config (bgp router-id $RID)
highest loopback
highest working interface IP
show ip bgp summary reveals a potential neighbor in the IDLE state. What can be inferred by this?
Neighbor’s bgp process in admin-disabled (shutdown)
show ip bgp summary reveals a potential neighbor in the CONNECT state. What can be inferred by this?
Connection between peers is attempted, but no completed.
show ip bgp summary reveals a potential neighbor in the ACTIVE state. What can be inferred by this?
Routers are connected, but no messages have been exchanged yet
show ip bgp summary reveals a potential neighbor in the OPENSENT state. What can be inferred by this?
Router has sent the first message to establish a neighbor (peer) relationship, a BGPOPEN message
show ip bgp summary reveals a potential neighbor in the OPENCONFIRM state. What can be inferred by this?
A full TCP connection exists between the two routers. The local router has received OPEN messages
show ip bgp summary reveals a potential neighbor in the ESTABLISHED state. What can be inferred by this?
Routers are now peers and can exchange UPDATE messages
Assume that a router has two connected networks, 192.0.2.0/29 and 192.0.2.128/26. The router is configured to advertise 192.0.2.0/24 via BGP. Will neighbor routers receive routes for the subnetted networks?
No. Routes configured for BGP advertisement are only added to the BGP table if a corresponding route exists in the routing table. Since a summary advertisement would be preferred, a static discard route for 192.0.2.0/24 should be created on the advertising router.
ip route 192.0.2.0 255.255.255.0 null0
An ASN can be how many bits long?
16 or 32
BGP is a routing protocol
path-vector
When is it most appropriate to use BGP?
With multi-homed designs
