Useful BGP commands on Cisco Routers

When BGP is not behaving correctly, a “trick” to temporarily stop peering with a neighbor is to use the following command: router bgp 194 neighbor<ipaddress> password xxx Since the other router doesn’t have the same password, the two routers will stop talking to one another, without you having to do anything else. Later, when the [...]

BGP Quick notes

A transit AS is an AS that routes traffic from one external AS to another external AS   The “show ip bgp” command is used to display entries in the BGP routing table.   The AS-PATH attribute is used to prevent BGP routing loops. When receiving an BGP advertisement, the router checks the AS-PATH attribute, [...]

Memorising the BGP decision making process

1. Weight (Bigger is better) 2. Local preference (Bigger is better) 3. Self originated (Locally injected is better than iBGP/eBGP learned) 4. AS-Path (Smaller is better) 5. Origin 6. MED (Smaller is better) 7. External (Prefer eBGP over iBGP) 8. IGP cost (Smaller is better) 9. EBGP Peering (Older is better) 10. Router- ID

What is BGP backdoor?

In BGP it is a component that is used to modify the admin distance of eBGP in order for an interior portal routing protocol (IGP) to take precedence over an eBGP route. By default,External BGP (eBGP) has an admin distance value of 20. Administrative distance is the first criterion that a router uses to determine [...]

What are the types of BGP Routing table?

There are three types of BGP Routing tables: a) Adj-RIB-in b) Adj-RIB-out c) Loc-RIB Adj-RIB-in stores the unprocessed information received from its peers. Here the best path selection occurs as per BGP attributes and after conformation path is entered into the local bgp table i.e Loc-RIB. From the local RIB table it conform the next-hop [...]

BGP is IGP or EGP?

BGP is ia EGP(exterior gateway protocol) Interior Gateway Protocols (IGPs) handle routing within an Autonomous System (one routing domain) Exterior Gateway Protocols (EGPs) handle routing outside an Autonomous System and get you from your network, through your Internet provider’s network and onto any other network.

How are routing neighbours discovered in BGP

A BGP-speaking router does not discover another BGP-speaking device automatically. A network administrator usually manually configures the relationships between BGP-speaking routers. A BGP neighbor device is a BGP-speaking router that has an active TCP connection to another BGP-speaking device. This relationship between BGP devices is often referred to as a peer instead of neighbor because [...]

What is the concept of NLRI

BGP was designed to carry only unicast IPv4 routing information. BGP configuration used the Network Layer Reachability Information (NLRI) format CLI in Cisco IOS software. The NLRI format offers only limited support for multicast routing information and does not support multiple network layer protocols.

What are BGP path attribute

-Prefer the path with the highest WEIGHT. Note: WEIGHT is a Cisco-specific parameter. It is local to the router on which it is configured. -Prefer the path with the highest LOCAL_PREF. Note: A path without LOCAL_PREF is considered to have had the value set with the bgp default local-preferencecommand, or to have a value of [...]