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BGP (Border Gateway Routing Protocol) BGP (Border Gateway Routing Protocol) is a standardized exterior gateway protocol designed to exchange routing and reachability information between autonomous systems (AS) on the Internet The Border Gateway Protocol makes routing decisions based on paths, network policies or rule-sets configured by a network administrator, and are involved in making core routing decisions BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing protocol employed on the Internet Figure Basic Topology of BGP Basics of BGP BGP is the path-vector protocol that provides routing information for autonomous systems on the Internet via its AS-Path attribute BGP is a Layer protocol that sits on top of TCP It is much simpler than OSPF, because it doesn’t have to worry about the things TCP will handle Peers that have been manually configured to exchange routing information will form a TCP connection and begin speaking BGP There is no discovery in BGP Medium-sized businesses usually get into BGP for the purpose of true multi-homing for their entire network BGP (Border Gateway Routing Protocol) An important aspect of BGP is that the AS-Path itself is an anti-loop mechanism Routers will not import any routes that contain themselves in the AS-Path Current Version The current version of BGP is version (BGP4) codified in RFC 4271 since 2006 Early versions of the protocol are widely considered obsolete and are rarely supported Types of BGP Figure Types of BGP There are different terms used when describing BGP These including: Internal BGP (iBGP) operates inside an autonomous System (AS) External BGP (eBGP), which is also known as an interdomain routing protocol, operates outside an AS and connects one AS to another These terms are just used to describe the same protocol just the area of operation is what differs BGP (Border Gateway Routing Protocol) Uses Most Internet service providers must use BGP to establish routing between one another (especially if they are multihomed) Compare this with Signaling System (SS7), which is the inter-provider core call setup protocol on the PSTN Very large private IP networks use BGP internally An example would be the joining of a number of large OSPF (Open Shortest Path First) networks where OSPF by itself would not scale to size Another reason to use BGP is multihoming a network for better redundancy, either to multiple access points of a single ISP or to multiple ISPs BGP AD EBGP-20 IBGP-200 Characteristics of BGP-4 The key features of BGP-4 include and not limited to these: It is an advanced distance-vector protocol BGP sends full routing updates at the start of the session, trigger updates are sent afterward BGP maintains connection by sending periodic keepalives It creates and maintains connections between peers, using TCP port 179 BGP sends a triggered update when a keepalive, an update, or a notification is not received It has its own routing table, although it is capable of both sharing and inquiring of the interior IP routing table BGP uses a very complex metric, and is the source of its strength The metric, referred to as attributes, allows great flexibility in path selection It selects the route based on the AS Path It selects that route, which provides a network with least AS 10 Max AS length 65535 11 to 64511 Public 12 64512 to 65535 Private 13 BGP doesn’t discover neighbour automatically In BGP we have to define neighbours They are called BGP peers 14 BGP supports only one type of authentication that is MD-5 BGP (Border Gateway Routing Protocol) How to advertise Routes in BGP? Using two ways Using Network Command Redistribution BGP Messages Open- BGP sends Open message using TCP Port no 179 Content of Open Message Version – (Right now we are using BGP V4) MY-AS – Itself AS number Router ID Hold Time – 180 sec (Default) Keep alive- BGP sends periodic keep alive Default time 60 sec Update- When two routers become BGP neighbour they send update message Contents of update a) Routes b) Route Attributes- These are those criteria’s which are used to select the best path Notification- When a neighbour is reset then it sends notification message It contains the cause of resetting BGP Tables Neighbour BGP Table Routing Table BGP States Idle- Searching for Neighbour Connect- TCP three way handshake done Open sent- Open message has been sent Open confirm- Open message has been received BGP (Border Gateway Routing Protocol) Established- Connection Done Figure BGP States BGP Terminology Next Hop Self- When a BGP edge router learns the external routes then it advertise those routes with default next hop to IBGP neighbour To solve this drawback we have a solution that is called next hop self This command tells a router give your own IP address as a next hop to your IBGP neighbor (Note: Always applied on edge routers.) Route Reflector Client- Normally an IBGP neighbour never exchanges the routes of one neighbour with another neighbour To solve this problem we have a solution that is called route reflector client This command tells a router exchange the routes of one neighbor with another neighbor BGP (Border Gateway Routing Protocol) EBGP-Multihop- When a BGP router wants to establish EBGP neighbourship then it set TTL value in open message If your neighbour is not directly connected, then you have to change the TTL Value using EBGP Multihop command Max-Path- By default BGP selects one best path using route attributes If you want to implement load balancing then you have to change max path value BGP- By default, the redistribution of iBGP into IGP isn’t allowed on Cisco IOS Update Source- When we want to establish neighbourship via loopback then we have to use update source command Aggregation- In BGP Summarization is called Aggregation BGP Attributes/Rich Metric/Path Attributes Weight- Cisco Proprietary Attributes Directly Connected – 32768 Indirectly Connected – Preferred – Directly Connected Local Preference Default 100 < 4.2 billion Preferred – Higher Self-Originate- a router gives preference to self-originate routes AS-Path- Lowest Path will be Preferred Origin- ebgp >ibgp >Redistributed MED- Multi Exit Discriminator By default – Lowest will be preferred eBGP > iBGP- eBGP will be preferred IGP Cost- Select Lowest Cost BGP (Border Gateway Routing Protocol) Ebgp Peer- Old neighbour will be preferred 10 Router ID- Lower Router ID will be preferred By default BGP selects single path, for load balancing we use maximum path command BGP Path Selection BGP could possibly receive multiple advertisements for the same route from multiple sources BGP selects only one path as the best path When the path is selected, BGP puts the selected path in the IP routing table and propagates the path to its neighbours BGP uses the following criteria, in the order presented, to select a path for a destination: If the path specifies a next hop that is inaccessible, drop the update Prefer the path with the largest weight If the weights are the same, prefer the path with the largest local preference If the local preferences are the same, prefer the path that was originated by BGP running on this router If no route was originated, prefer the route that has the shortest AS_path If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than incomplete) If the origin codes are the same, prefer the path with the lowest MED attribute If the paths have the same MED, prefer the external path over the internal path If the paths are still the same, prefer the path through the closest IGP neighbour 10 Prefer the path with the lowest IP address, as specified by the BGP router ID Message Header Format Figure Message Header Format Marker- Included for compatibility, must be set to all ones Length- Total length of the message in octets, including the header Type- Type of BGP message The following values are defined: a) Open (1) b) Update (2) c) Notification (3) BGP (Border Gateway Routing Protocol) d) KeepAlive (4) e) Route-Refresh (5) BGP Example Figure BGP Example Topology R1 (config) #int fa0/0 R1 (config-if) #ip add 172.168.101.1 255.255.255.0 R1 (config-if) #no shut R1 (config-if) #int s0/0 R1 (config-if) #ip add 192.168.1.1 255.255.255.0 R1 (config-if) #no shut R1 (config-if) #int s0/1 R1 (config-if) #ip add 192.168.4.2 255.255.255.0 R1 (config-if) #no shut R1 (config-if) #do sh ip int br BGP (Border Gateway Routing Protocol) R2 (config) #int fa0/0 R2 (config-if) #ip add 172.168.102.1 255.255.255.0 R2 (config-if) #no shut R2 (config-if) #int s0/0 R2 (config-if) #ip add 192.168.1.2 255.255.255.0 R2 (config-if) #no shut R2 (config-if) #int s0/1 R2 (config-if) #ip add 192.168.2.1 255.255.255.0 R2 (config-if) #no shut R2 (config-if) #do sh ip int br R3 (config) #int fa0/0 R3 (config-if) #int 172.168.103.1 255.255.2550 R3 (config-if) #no shut R3 (config-if) #int s0/0 R3 (config-if) #ip add 192.168.2.2 255.255.255.0 R3 (config-if) #no shut R3 (config-if) #int s0/1 R3 (config-if) #ip add 192.168.3.1 255.255.255.0 R3 (config-if) #no shut On Router 3, we will create some loopback also R3 (config-if) #int lo R3 (config-if) #ip add 172.30.1.1 255.255.255.0 R3 (config-if) #int lo R3 (config-if) #ip add 172.30.2.1 255.255.255.0 R3 (config-if) #int lo R3 (config-if) #ip add 172.30.3.1 255.255.255.0 R3 (config-if) #int lo R3 (config-if) #ip add 172.30.4.1 255.255.255.0 R3 (config-if) #int lo R3 (config-if) #ip add 172.30.5.1 255.255.255.0 R3 (config-if) #do sh ip int br R4 (config) #int fa0/0 R4 (config-if) #ip add 172.168.104.1 255.255.255.0 R4 (config-if) #no shut R4 (config-if) #int s0/0 R4 (config-if) #ip add 192.168.3.2 255.255.255.0 R4 (config-if) #no shut R4 (config-if) #int s0/1 BGP (Border Gateway Routing Protocol) R4 (config-if) #ip add 192.168.4.1 255.255.255.0 R4 (config-if) #no shut R4 (config-if) #do sh ip int br Here we will Perform BGP Routing R1 (config) #router bgp 100 R1 (config-router) #network 192.168.1.0 R1 (config-router) #network 192.168.4.0 R1 (config-router) #network 172.168.101.0 mask 255.255.255.0 R1 (config-router) #neighbour 192.168.1.2 remote-as 100 R1 (config-router) #neighbour 192.168.4.1 remote-as 100 R2 (config) #router bgp 100 R2 (config-router) #network 192.168.1.0 R2 (config-router) #network 192.68.2.0 R2 (config-router) #network 172.168.102.0 mask 255.255.255.0 R2 (config-router) #neighbour 192.168.1.1 remote-as 100 R2 (config-router) #neighbour 192.168.2.2 remote-as 200 R3 (config) #router bgp 200 R3 (config-router) #network 192.168.2.0 R3 (config-router) #network 192.68.3.0 R3 (config-router) #network 172.168.103.0 mask 255.255.255.0 R3 (config-router) #network 172.30.1.0 mask 255.255.255.0 R3 (config-router) #network 172.30.2.0 mask 255.255.255.0 R3 (config-router) #network 172.30.3.0 mask 255.255.255.0 R3 (config-router) #network 172.30.4.0 mask 255.255.255.0 R3 (config-router) #network 172.30.5.0 mask 255.255.255.0 R3 (config-router) #neighbour 192.168.2.1 remote-as 100 R3 (config-router) #neighbour 192.168.3.2 remote-as 100 R4 (config) #router bgp 100 R4 (config-router) #network 192.168.3.0 R4 (config-router) #network 192.68.4.0 R4 (config-router) #network 172.168.104.0 mask 255.255.255.0 R4 (config-router) #neighbour 192.168.3.1 remote-as 200 R4 (config-router) #neighbour 192.168.4.2 remote-as 100 BGP Creates Three Tables Neighbour Table BGP (Border Gateway Routing Protocol) R1#sh ip bgp neighbours R1#sh ip bgp neighbours 192.68.1.2 R1#sh ip bgp neighbours 192.68.4.1 BGP Table R1#sh ip bgp Routing Table R1#sh ip route bgp Now if we talk about the network 172.30.0.0, we can see this network on R1 via 2.2 Because it advertises routes with default next hop which is 2.2 Now on edge router we run a command next hop self Here edge router is R2 & R4 R2 (config) #router bgp 100 R2 (config-router) #neighbour 192.168.1.1 next-hop-self R4 (config) #router bgp 100 R4 (config-router) #neighbour 192.168.4.2 next-hop-self Here we tell router, provide your own IP add as a next hop In BGP a router will share the information with the directly connected router only Router would not share the information with the Router For that here we will run route reflector client command R2 #sh ip route bgp We won’t see any routes of 104 networks R4 #sh ip route bgp We won’t see any routes of 102 networks R1 (config) #router bgp 100 R1 (config-router) #neighbour 192.168.1.2 rout-reflector-client R1 (config-router) #neighbour 192.168.4.1 rout-reflector-client R4#sh ip route bgp Now we can see here 102 routes R2#sh ip route bgp BGP (Border Gateway Routing Protocol) We can see 104 routes here R1#sh ip bgp First rich metric is weight For indirectly connected Next local preference 100 Always prefer higher Next AS path 200 Next self-originate MED is by default We have two ID 4.1 & 1.2 1.2 will prefer (lower will prefer) R1#sh ip route bgp Here we can see 172.30.0.0 route learn via 192.168.1.2 R1#traceroute 172.30.1.1 via 1.2 R1#sh ip protocols By default it selects only one path If we want to implement load balance then we need to change max path value R1 (Config) #router bgp 100 R1 (config-router) #maximum-paths? to 16
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